Geology of the Molo Area

CONTENTS(.IOXTENTS PAGEP .x L.‘ [-

Abstract.-'\3.‘.\.".1'1t£.‘i

I-Introduction.l-—-1n‘.r0du::iia'm...

II-PhysiographyH I"h}\fi'\1gl'ii1'!]'f}' 3 III-PreviousIll—Pz‘cx'imls CicofogjculGeologicalWork 6 IV-Summaryl\-' SLJnn'nnr} of0:70:31)Geology 7

V-Details\J—Dutnilvt mof G's-3103'Geology. . 8

1.1. Tertiary'I'CrliaLr} 9 (1)I' i) KerichoKcrlchu phonolitesgﬁizmzuiéicx 9

1 '. -.\'-- -- —-.. .. i '-. ’...'n. . l', _ (2)f._ JanBasalts:.u.. a intercalated.. HL‘LlIi.- . :: c x. inI’.u Kerichok-I1., .-.. lullm--. phonolites‘-. |\-:=.£.fll x. 10il] (3)f3] PhonoliticPIiL.)I“.O.|iI‘.L.‘ nephelinitesnephcli.’ ...... 10in (4)l4].\c;\.‘.g.|r1:tu. Nephelinites‘Kl' '.\..and.111..... melanephelinites:.1c...:.cp.1;.1. '. I. ' of(11‘ Nyando\'_-..I.'.1.iu Valley\"'.;.E-.:'-. ] 1 (5)['5] 'I'LTuffsfl‘S 02‘of thelite MauMam 3.11;:and MountMOLIT‘L LondianiLomilam; ]2 (a)rm Tuffs12m} of theLhc south-west.50.1LI'1—xx . 13 (b)N}: Pink fineFm“ gmirsdgrained eutaxitic;-'..:.-.\|:j.‘. tuffs'..:.":k ]3 (c).‘H PalePatio greygm; cut-An:eutaxitic crystal tuffs[Lji'z'n I3 (6)m) Turrieir}Tertiary pyroclastic.§.'J'm:fastic rocks"0c 0:"of the UpperL j‘pcr MauHim 14 1 . (a){.11 GreyCir-C} and bleached‘ifiicaiuhcfi. agglomerates, tuffs"2} andan. sediments.s-cgiin‘cz‘w . 15 (b)"3"?! BlackB15»; ashes.mlics of E]burgon 17 (c)H'] Eutaxitici-.L.!l.L\i'.iu welded\Aclulcd tuffs-.-.. 18 (7).51 SedimentsSeaiirncr-H of LondianiLon. and Maji Mazuri ]9 (8)[BL-\IixA]kali basalts.‘nulzx'nf\Eu'-..~:..-.1-c.a.1of Molo plateau 20 [‘)J(9) BasaltsBit‘iklllx 01'of MLLjiMaji Mazuri.\-1.L/!;:'i .. 20 (10)IE{}|T1I‘.;5LIZ'(':Tinderet phonolitesphcwim‘dtux . . 20 (1])-;I1_\'y..Nyando.n Valley\ l;i.C_\ phonolitesPin-1' 21 (12)_: TrachytesTrudiylm and quw‘lzquartz trachytesmac-:1}. !L‘.\' ul'of Londiani!.-;1:1d'.:=.|‘.f and KilombeKiioltwim‘ .. 22 (a):.«.-IT:';=.L.-':1}t'c

2....r Pleistocenel’lcis'. murm- 24 (1)!. E II MaficMaui; basalts,5* basanites and limburgites of Londiani 24 -:I(2) Mau3!”... LLashes with basal tuff 26 (3)I'FJ 'I‘LEI‘ZK'Tuffs su‘.and lI'IC-iashes UI‘of MogotioMngnié-ﬂ 26 (4)=—+il Ashes.-\<':“.c< ;\"T'§:1\ici'=-"of Tinderet . . 27 (5)52 KilelwaKi':'.-.: Hill": phonolites.“ii-nan" . 28 (6) Trachytes of the north-east 28 (7) Eutaxitic welded tuffs of the north-east 28

. -- CONTENTS-( Con/d.) PAGE 29 3. Quaternary 29 (1) Black ashes of Rongai plain and Njoro 30 (2) Pyroclastics and sediments of Rongai plain and Mau slope 31 (3) Menengai pumice ...... 31 (4) Sediments of Rift Valley floor and Kilombe . . 32 (5) Sediments of Mau Summit 32 VI-Structure 34 VII-Economic Geology 38 VIII-References. . ILLUSTRATIONS 3 Fig. I-Physiography of the Molo area 5 Fig. 2-Average annual rainfall in the Molo Area

MAP\1.\P at end '\ \.1’_'- Geological map of the Molo-.‘,L‘ area\L'L‘, (Degree'DJLJI'L‘C sheet\Ik‘L“. 42,Jr: N.E.) .. '"

ABSTRACfABSTRA Cl‘ The report describes an area of approximately 1,200 square miles in the Rift Valley region west of Nakuru, bounded by the eqmvtor and latitude OO30'S and longitudes 35°30'EL ;andm1 36°00'E. The greater part of the area to the west and south consists of forested highlandsahﬂ and farmland on the western shoulder of the rift, while the north- easterncan (gunkquadrant includes the Rongai Plain on the floor of the Rift Valley. This area in- cludesgum much:tmch of 1‘the highest ground to the west of the rift and can be considered as the U, r‘li a» m centreL C of0; themg, volcanicn? LJT‘HL‘activity for the region.

...\\ ﬂ , ‘ w. r. ,, nv H ‘ ‘ The rocksTUghn- v'. encounteredLLkI LA, u are limited toA the Tertiary. A, 7‘ and\L‘ Quaternaryk A volcanicw suite, , , 1 V of centr:al Kenya and are largely pyroclastic in ori~in. A wide range‘ ¥ ofA lava11x; types have been recognized and described, ranging from mdilite and perovskite melanephelinites and basanites, through varieties of basalt, phonolite and trachyteKr to the‘g‘ .4quartz trachytes of the Londiani region. Many of the pyroclastic rocks are attributed to an ash-flow origin, and this concept js briefly described.

The structure of the area is largely obscured by late-stage\L, pyroclastic‘ ‘, x ¥ activity‘L and little has been added to present knowJedge of Rift Valley tectonics. GEOLOGY OF THE MOLONIOLO AREA

I-INTRODUcnONI—INTR()I)L'C.'1"I()N

During the1111: ped'Odperiod February16131111113: 19621%: ta10. July11111 1962 a:1 gealagical‘ reconnaissance was carried:CL aut.1111 in1:1 an.111 areaawn ta:11 the1115 west11:5: 'Of1'15 Nakuru,.\'..111:‘ induding‘1L1L111'1 the westernWC. ' ‘11 scarp\C" 11P- 'Of the Rift VaHey,‘...1C1_1'_ Mount Londiani1011111111 and1111111115:the northernL1C‘1r‘111c1‘11. Mau.\‘1; The survey0.1111: coveredcmVcrcd same31011113 1,2001. square miles‘1“:r. boundedbuLdd by3 the:11: 1:C111;11111equator 1111::and latitude1113.11.11.11]? oo30'S11.1118 and longitudes.1.1.11;111111;~5 35'3.”35°30'E and 36°00'E.' ‘111‘ 1". The111: area111611 faUs1:111. within111111111 the111C RiftR111 Valley\'.111Cj. Pravince,P111111“: 1.1.1131with the greatergrc. 11:1 partp.11: administered1111111111516 Cd by1“-. the:11: District C'Ommissianer,C1’m11113i51111101‘. Nakuru.3111111111, A smaller51.11;? 1:1 part111.171 in111 the111C north-east,:1011117011x‘1. 11311111152111belanging ta the1‘11: Baringa113111131) District,1)1~‘.PC‘I. is1.1 administeredL111"'11f1‘1~.1C:‘:‘C1 fr'Om111'111‘1 Eldama1.11.1111; Ra'V'ine,RSL1.11‘.C. and..1‘C1 areas.11‘C‘gm in11 the5:11" north-“1171.11 west~51 and sauth-west~11uLh-WC51 are1.1'6 administered.11111‘111115131‘131'1 from11:11:11 Kerioh'O.c1C‘11o

Land Utilization.-The1131111111. 11:: Mola\10111 a,rea can~ 1 be1 divided: 1' "1 int'O1.111 two11.111 major111.1jc1' parts,:.."‘1\. the:larger,111: 1:1":zer. same\1’1111; 56 per coo,t. 'Of1111111:the wh'Ole,11111:. being' ranch, plantation and farmland, while1:111:1C theL11: smaller,1111;111:1‘. same~111111C‘ 42 per cent,‘ ' . is1\ caveredC131 L‘“L.‘\1 by3‘). natural and planted farests. The remaining 2 per cent in the :narth falls[71.11% within11111.1 the Lembus land unit and is grazed by the cattle and gaats 'Of the Tugen and“111011 Njemps\I‘C‘f1‘1m tribesmen. TheT11C‘ extremeC1111: 11c n{)!fth-east1111 1.1-c.15 'Ofof the111: area,.11C.1_ consisting0111111111115 'Of111 11111.:that partD1111. at:11 lawest11.1'1VGSi altitude:1 111C111C (belQw13:11.1“ 5,500$51111 ft.):11 and11111 with1.1.111 IQwest111‘.‘\':"\I rainfall15111111 (avemging:1u.cr:11111ig: ,less15.. 17.1111.than 35715 inches1.1111161 per11:" annum)1;:111L1111'1 is-. cxtcnexten- sively.21 used1:11:11 far11:11 sisal9;.11 plantatians.111.;7111111’1m. A»\ factary15.111111. far1111‘ extracting31.11.1111” ,the‘11: fibre1'1b is‘11 sited~1tC‘L1 anon the M11111M'OI'O RiverRi‘uC‘r‘ 'One1.111: mile111.1: south\11LL'."1 1.1.1'Of Mogatia,\11.’1_1C1..'1T‘1.1. and411.1 411.1111:anather 7111‘just 'Outside the:31c aTea.1:‘C.. ta111 the111.“ 11111i1'1—‘C‘1'LR‘L.naDth-east.

TheT1: RangaiRangu Plain,.1111. whioh111c11 extendsemcmk fram1:11:11. Kampi1\.1:1'111. yaj. .1. Mata.\1111.1 in{11 the131: narth11111111 (5,500 ft.) tQ south 'Of the Nakuru-Mau11:11 .\1‘.'LL1 Summit31111111111: RaadR11.1L1 (6,500111.5111 .£.t.),11.1. and.11111 ,is11 flanked11.1:‘111511 ta1C1 the:11: west by the rising land at the faat1111-1 'Of111‘ the1111; Rift Valley 'sc'arp, is~1. utilizedL:1:112C‘C1 mainly1111111113 for11:11:11C111‘1T11L1C‘13the productian 'Of maize,11.111; beef1)::1 and dairy11.11“}. cattle.C;LLL1C. At311 a:1 slightly higher altitude‘1.11.11i: (6,500-7,50011151111351111 ft.)111.1 theT113 rolling hills.1111. and.1;1C1 plains111:1111~ in511 the111C Njara\111.“1.1 area‘.."C:. carry maize and C1111.cattle, alsa.11\1\ wheat,111113.11. market1111mm g;1711Cf1garden produce11:1 .11.:.';‘ and111111 peas.11111» L A belt 'Of1'1.1‘ highly1111:1111 praductive1‘ 1111L1‘C‘Y1‘.C‘ smallx.‘ mixed farms..11"’1’\ extendsC‘\1L'1111\ alang.: the. well-watered‘1‘1C11:‘~'1.11.C': 1'11 lawer1.51‘1'x‘C'" slapes\ 11:13. 'Of1.17111131111f1h»1:.1\1the north-east Mau.\11L11.’1:C‘1.1:‘:11(be.tween 7,500.5111111‘10.and 8,50031.51111 £t.)111 frDm111.1111 Njoro\f1.\'1.1 through111111111511 ElburganElburg 'and. .. Turi. .. 1 ta:11 Mala.311111: Mixed\ \CC1 farming1‘11" is1. alsa.111 carried11:11 aut1.111 an1.111 the111: plain$1.111". narth[11.17111 'Of Londiami1: C1 . (7,500-8,000‘.<1‘111-5_1’11'111 ft.).f:_1. On0:1 the111: 111211high gr'Ound sauth>.1.1L1‘.11 'Of1.11 Turi,111.1. in111 the'regiDn111: of Mau\Igu Summit911:111:111 1111and particularly.‘L:CL11.111\ within11:111111 the:hC‘ large1.11.91;- arable.1..1:1.C enclave811C1(L\L‘ in1:1 theThe Mau311.11 Forest1111\1 which11111.11 acoupies1VJCLJP1C< the111C 51.1111.sauth- 1 'west quadrant1'1C1111. 'Of1.1 the area, cultivation is carried aut1.11 ta1.1.1 altitudes12111111CLW 'Of11. 9,00011.111111 ft.11 Wheat and pyrethrum,:‘1 '..-.r11’.11‘1.. cattle.111: and.111C1 sheep01:1:11 are.1 the majar'1‘ .111." praducts.:11‘11L11C‘m.

SmallV1 specialist\p‘C‘C‘11111\1 prajects11:01:?“ have beensec-11 develaped1EC\C11111C‘-51 in111 cannexianC‘1’11111e\31'1;1 with,1.11111. 'Or1.1“ as11‘ part1111.1: 'Of1'11 the1.1:: lacal agricultural'.C‘L11‘1.L.1‘... industry.111111»: Peas are canned1111:11 rat.11 NjDro.\:1:‘111 and:111C1 meat111m: praductsyrsduch at Turi,' and a butchery 'at Landiani,.. ..1111, transparting by rail,7:111. pr'Ovides11:. regular111111;: meat117C111 supplies14:11111Cs :1»as far away as Na]Quru ,and1:111 Kisumu.1\ 711:1‘11 A large paultry farm at Landiani suppLies eggs ta the Nakuru and Nairobi\.11:1‘11j1m.j"1..‘C"markets and.:1.1 crapsL'11111\ 'Of1. strawberries1i:.1’11.11:1“1‘1:1~ fram Njoro and Mala and111 peaches1‘: i:C11-‘ fram111 1 Londiani1.11:. 1.1.1.11 and KedawaK:-.;111\ .1. are.11: flawn:1.:1.\ 1 ta:11 Eurape11111111; at:1: certainC:1111117 seasansof\C‘.1<1.1n.\ 11: the‘1 . year.71C12'. Dairy1):11r\ pra-117.1. duceC111CC is processed by a:1 KenyaKcnyg Ca-operativeCU‘UPCRHUC Creameries(1154111311115 factary111011111 an11.1 the:11: railway..111.1 .111' at:1: Mom.Mala. IixpcrinwnuvkExperimental plots;:101~ and.11111 researchrC‘\:.11‘1:11 statians51:1:11'1115 dealingCiC‘..T11‘.g with1.11111 a:1 variety1:112:61. 'Of111' cropsC7111; are[TC main-111.1111. tainedI. .b\by the111: Agriculture.1\‘L11IL11D:‘1..\.1Department.:111C‘:1L. Improved111111r‘1‘1'1CC1 varieties...11‘.C‘11C"\ r101'Of pyrethrum11} 1C1112L1111 areL121? develapedL1C‘1C1111CC1 two..111 miles111168 north-we&tnorth-was: 'Of111 Mala,3111111. and.1111} thethe problems1110;11:1111‘ o,f1.1?111C‘.the maintenance11C‘11.1111.C‘C 'Ofof high-aiLtitude1111111— '1 1:116 grasshlinds.1111: rare1;:‘C investigated1 1’111‘11CC'1 six\1.\ miles111110 .sauth-west~11111h-11.:‘\'C 'Of111” Mala.X10111 ResearchRC‘ScdlC11 inta1111 strains11.111.111.11of wheat1111CL‘1‘. resistant'x..=.r1[ ta111 rust diseases is1~ carriedCa“:‘1C‘1.1

Forests.-The bigher slopes are extensively forested withi'I ;.a naturalI]=.'.lif‘.i' “Rag;association of -.1cedar,; pada, olive and bamboo., and sawmills have -‘L‘L".been (~i.i‘f‘..~_"‘|L'\|establisbed taexplo.itL: L'\}1 \' timber. The: "k sawmill situated faur miles sautb-west af Njara specializes‘DSL'H .'Q‘ in,3. the'Ir‘lc production‘-"-'.' afll: cedar‘3; slats, exparted far use in tbe manufacture af pencils, and cedar ail extracted from the sawdust and sbavings.

i' ll-PHYSIOGRAPHYH—PHYSIOGRAPHY The13m area.mca cangm: be'm subdivided\Ubdix {dcxi into:10 six{x main areasarms (Fig.\Fi_::. 1)in“as follows:J‘uIlovx's:

\ «LJ 7 fiffi‘fimrw L" \I)

36‘0’.‘

.. SCALE 0g 5 10w: 15\SWLESMilES L 1L 1 4J

Fig.Fig lrI-PhysiographyPhsx'iugruphv of {htthe Huh:Molo ureaarea 1.1 The[ﬁg Rongai Plain, slopmg ,gently north-eaJstt fromfwm 7,0007mm ft.f; min thethy .5CLIThSO'llth [muand (.5i6,500 ft.H, in the west and occupying some 35H250 squatsquare milesmjiex mof the Rift \‘gHCj.Valley floorﬂoor in thethe north-east.maria—amt.

2. Kilombettmbc Hill,Hm. a pump;young, well-preservedMd -;:'m;<:1x'cd mks:volcano’ withmm 4a craterQ" a.“ plI‘Lyparty infilledinﬁllc‘d with lakeMk: sediments.\cdimcma hIt ri,sesI'3\:‘> from{rpm the11“.; RongaiRnngai PlainRisa north-eastnorth-cast of HauntMount Londianli.LmdinnL The billhill cm'cmcoveIis :1a bibsbase alfea4:3; Hfof sameaomc 4H40 «qua:square“e milesmifm at mow6,000 ft. ,risingrising into :zhou;about 7,000HIM ft.ft‘ at.u theLhc :rim,‘im. with iocal70ml pcszlwpeaks exceedingmending 7,500“SUD ft.

3. The.The majurmajor topograpru'cal:opogruphickil Axisaxis runnin,grunning 11L'>L‘[}‘1~S‘\)Ulhnorth-south throughthrvugh the:hc ccmrccentre of ha?the area,:‘rcu. formedfur-mm by'9} thei'orestedme :‘ly‘cstsq mountain maJssesmiiéstu afof the MauMm mto the.the southscum, and Mount Londianilondiani tom the north,not. with high gruundground extending voto the narth-westnorth-west anon to the{he Churneastern ﬂflankank of 'l'irtdcrci.Tinderet.

3DJ 4..44 The LondianiLondi~DJi Plain,Plain. which cansistsconsists of a depression lyingly,iDJgbetween 7,500 ft. and 8.0008,000 It.ft. ﬂflankedanks? to the north and Wes:west by spursspW"s of TindercLTinderet, and to the east by Mount Lnndlanl.LondiaDi.

5.U1 The MoloMalo Plateau (2501250 square miles) which occupiesoccuplcs mos:most of the south—westernsouth-western quarter 1.11"'Of 1h:the areaea and carries an:1 almastalmost '1':campletemplcts 111margin‘ 11:"of 1mmforest.. Ri;.._Rising from 7.6007,600 11.111ft. in the extreme south—sauth-westWest the gr.11112ntgradient is uniformunifarm and 11nl}"only 51'slightly sit‘cpsrsteeper than the Slop:slape 'Ofof tltheﬁ. 11underlying1111111111131 rock»rocks. The highexthighest pfxrtx‘parts 115Crise .11‘111‘1:above 9,000 ft. 111to ,thethe northnarth 11ndand east. 6. The highly dlfucctcddissected cumincountry abaut1.720111 KedawaKEIQ'UWJ aIIldand ta111 the west which1111-311 1111:1115farms par:part of thcthe NyahdoNyando River lcy.valley. Rainfall.-ARam1'11lt.vAvcrverage;1 rairufallI'ltlilf.‘lll1figures11gt11m extractedcxtrhctcd iJ.irom1111111 the.111: publixhsdpublished records1‘2’1‘11'.‘118 'Of01 the 319.0-Mete- orologualarological Dep;‘1r.11.ct1:Department .11”of the E151East AfriAfrican1n ComrnunlyCammunity and shown 111.1gr‘111111111‘1tica111.diagrammatically in Fig. 32 jndlcatcindicate a:1 variation11111.1111111 fram11111111110111abaut 35 inchesincho in the 0.131east and{11111 north-111191north-east to111 1111011160above 60 inches in111 the south-west,so1~uth west. withwith marked increase1111‘.“ " m: in111 precipitationnrcgipimtlon. with increaseincremc in1n altitude.11111111111. Th:The llatter(111:: factor1:1:1t‘111r1sis well1.1 011 11.211101151111011demonstrated by the Youthrecords 'Of1.11 .1a nurthiadtnarth-east to south-westxeuth-xx‘cst 111113line 111of stationsstations. anon the slopesdupes of'Of 1h1‘:the Mau,M u. 8:151east 111of Elburgon, where 1111an increamincrease in :1lzitudealtitude of'Of 01:61over 3.312'02,200 11.111ft. in nine miles is1.: rereflectedﬂecled in 1111an increase in average annual rainfall fromfram 35 inches Itato 51 inches.

Rainfall is1S markedly111:11’11'1'1113' seasonalSC£1§DDLll with1151.11 precipitatianpreclpltgtion peaks inin April-May.r\;111h.\1;13‘ and July-J1‘lyv AugustAugL‘ut and113111 the driest112151 lSpellwell inin December1111:31111211 ,and411111 January..l.’11111:11‘y. Rain for1111 the11.3 majoritymajor? of111?.hcthe 3111.5area is 123111611carried an1111 thcthe pmprevailing411111; 1:111'111-gz1xtc1'1j.north-easteTly winds,1.11111 but thcthe highl1 fainfallraimall expenenucdexperienced in:11 thetho extremec111 113111; south-west3011 .1—‘ncx: is15 largelyl;1r‘_ " non-s,easonal,111311—9:;1\11111;l. and.316. is calrried1211:1311 byn. afternoon.1jt.;»m1m: thundeT-:hnnderr storms.w'LOITl’]: from the west and south.sauth.

Drainage.-The river1:15;" 11.1.courses\c~ 'Of1:3' the1h: region (Fig.(F111. 1)11 display111;;11111' a crudely radial1:101:11 pattern111116111 and contribute@1111 111L11c to111 1.111four 111:1major lakes1.1.1.0 in1n Eastlfmt Africa'sif :11. RBIvast area311:1 'Of01 inland111111111 drainage.111111 -. 1111-The drainagedrama-g6 systems5ta of111 the northern111711116121 and':11 eastern11:1x1c1‘11 slapes5(13' :11; of01 the mountains11111111131118 cells.colleot in'11 thethc NjOTO,N11111:». Rongai,R.. 7 Malo,311.1. Bi'ssoi,1111.113. Narasura31111511111 and11:11.1 Maji\lall Maz:uri\1111111‘1 rivers,131;.5. and.11111 contribute€1.111111h11'; a£1 majar111111111 11.11101part 'Ofthe1116 supply511.3p to:11 the 11.17111.permanentncnt rivers1‘1\'&1\ flowing11011111; in the 110111flaor of111' the RiftRi‘t \Valley..1 lc}. TheTh: Njora.\j'1'1r11 River,R1131 flowing1111:111‘111" east,amt 11rdrains111x inta11111.1 the111:. saline«1.11; Lake1.11}..- Nakuru. The'lhc Malo\ln‘o and(111.1 RongaiRnnggi rivers,1‘11'1‘1‘9. 'Originating(11" '11:.11ng on11:1 1'11:the northeTn111.11‘1l1em slopes\lnpcs 'Ofof the1c Mau,.1.1u_ traverse11.»: C15: the RongaiRxljgdl Plain.Fla“: Th:The BissaiB13015 River,R'V-cf. dr‘a1111111:draining the north-eastem111111111“: ~ 1111: slope\lcpe of111' Mount Londiani,lund1.1‘1.. is:~ aA majar111.1j111‘ tributarytribui: ‘_\' 1:11of 1316the Molo3111l11 River,R521”. joining11m 11111 it1‘. '0n1: 11 the111: plains111113» south-eastx11111h7€a

Thellic 1'1”crivers 111111191;draining 5132111113111.southern T{“1;is.“et.Tinderet, nectcmwestern Mount.'\I.111".t Londiani1.1‘1r1dl. and Hi;the :1north- western1111511rn Mau1\1 1.11 11.11111contribute111111." 111to the Nyando.\" 11"11.) Rim-1‘River 11‘F11chwhich flows111mg westwards fram Kedowa down the1 LumbwaLut‘nhzxt Valley\: ‘1 and.11111 enters:1. ‘5 theLie KavirondoK.111‘11r11‘l1‘1 CullGulf 111‘of Lake Victoria south of Kisumu. The system 'Of nearly parallel consequent1113111111 streams flowing south-west on the MoIo Plateau emerge from. the.1. forest to combine with the rivers of the Kericho region, iand drain into the Kavirondo"".1r‘1l Gulf(ml via the Sondu River. Those ,streams flowing south- west'1‘. 1:51 over the southern”:11 boundary5011113113. between1111:1111 35°40'£ and 35°50'E contribute to the Mara River which dra'ins into Lake Victoria south of the Kenya-Tanzania border. Those few streams draining south-east from the peak of ,the Mau, crossing ,the southern 'boundary east of longitude 35 °50'E, form the headwaters 'Of the Narok and Uaso Ngiro rivers, draining via the latter southwards into Lake Natron.

43,.I‘. . 41

.35B55

. 0 40 KAMPfKAMP! YAw: MOTOMO? .44 C) 44 41 t, .... .35 . LOND/ANI. . . 37

35. 45-.

""~ 0'" W") . 36 \ 40 . '"\ .\ NJORO \ \ 045 ci;3~\N \51 '\j \, \ \\ '\ "\ '\

SCALE

0 5Ln 10 15 MILES L 1- j J

Average annual rainfall in incheswwhes - lsohyets at 5 inch Intervals Fig.73:. 2-Average27 \wrage annualLEIZJ'LLISH rainfallrainﬁﬂl inin the1m“ Mojo‘ylvia areaarc-.1

5'-Jl In-PREVIOUSIII—PREVIOUS GEOLOGICAL-GEOWGICAL WORJ(WORK TheT111: atlas41111; published by the Survey of-L Kenya (1959, p. 34*) suggests the earliest travel- lers1:111 to111 enter the area were9.53: Capt.(an: FrederickF551.- Lugard (later Lord Lugard), heading for Kabarnet from Lake NaIroru, who passed near Kampi ya Moto in 1889, and F. J. Jackson.flL'kmn who11111111".in theL119 same13m: yearf-C'ii' pioneered1:111n'cur'L-11 a'-4 route11111:. between11L" Nakuru and Sotik which passedp:1.~..s'scd nearnear Njoro. In 18911891 JosephJoaeph Martin3111:1111 found a:1 direct routemuic on0:1 to1:1113:1.)the “IsinuMau at:11 about21.111114: fuis11151; latiwde:11t1tud1: and in111 1891-2. 391;" Capt.(41111.J. R. L. MacDonald\1:1L'D-..1:1:1111 made1113.113: a:1 preliminary survey of the[he route‘."1‘.'..'11..‘ for1111' a:-. railway211111-41 to1.11 Uganda.1.. 1 Sir811' George{11:11 L Whitehouse' huLhe was119x responsible1' for a more comprehensive:111'1p1L-‘11 1111\L-:'1\1:'-.c—1L1.1c3route-'Survey for the UgandaL. 41:11:: RailwayR411“ 43' in 1898.1898.

The first.'=\; 1:\pE1m:1'explorer to111 show«111111 aa. specificallyc‘fﬁ-Luliy geological interest1111.516: m:rl the:11: RiftR1111 Valley‘..—.:i-:} of11 "1 Kenya4 was11:15 J. W.\\ Gregory(irtlm‘ .' (1894,11591 p.13.3116}306) whose“111151: expeditioncKpLx‘IitEL’m of01‘ 1892-31692—3. reachedruched the1h: eastemeastern margin11 111 of111' the1h.) area:11'161 when11-11311 tr,avelling1: (xv-1:11:11fromrum Lake1.2111;- NakuI1U\;11\L:1'11 to11') Lake1.411: Baringo.3:1rfng'11 He observed11b~L r'xL-d the‘116 relatively.L‘1:111\'L11' gentlegen-11¢ slopelap-911111.:of the riftr11: wall'41! on11.1 the{11: western‘k-L‘\. c 11 sidesiuc oppositeOPP-1N1: Nakuru,514111.114 attribut- ing1:1; this11115 to11:1 the1371: structurealructuru of11.? the1h: valley' thereabouts,"51:1. I». which“huh he111: visualized‘»-.'11:111/L111 as41 derived from' a faulted14111311 syncline,x}. :1L .1.-s. with“17.11.11.115walls formed. L11 by11'» dip slopes11119:» interrupted by numerousn1:"111-.1111~ cliffsLi. Ls originating4.1117151 from a:. series~er1cs of parallelp::..'1elleI ‘faults.4.11115 E. E. Walker (1903,110113. F.p. 6)(11 a1'1 Government(111' 'fﬂlI‘lCI‘:I geologist,51311111; «1. L'JITICLIcarri~d out1111: 4a number 111'of 1.011111111-country- wide1511i: traverses‘11".11L‘rscs in111 1902113112 and3.1111 1903]:|.‘13 to10 assess'4s'x-L'“ the111-: scan-4:114economic mineralminL‘nJ potential111113111141 ofL15 Ken)”.Kenya. InIn the:11: 1:11.115»:course of11- his travels he climbed1111bcd. Kilombe.K111111113; HeH: roeflred to the volcano' as Eldalat, describing it as a ,good example of a volcanic cone,9111:. dissected14‘s~L'L'1‘LL: by a stream which has cut a ravine in the eastern flank.

G. T. PrimP1'11'1' published1111111i \I1-L'1. :1a report13121:: (1903, pp. 239 and4:111 250):93): (1:1on the.31.- petrology1‘.L‘.:':‘-1-.1g‘;. of various1.111143) rock specimens\F‘U'CIIIICIIS collectedL'11JiL-L'1-L‘d by111. Gregory:'j and" officers' ”r ofc“ the Ug,anda' Survey. From among the latterLi." PriorI 1.. described' a phonolitic-nephelinite with aegirine-augite and sphene from the111:; lower!.:--.\cr Kedowahcdn'xm River,R311“: 411'and melilite'11331'11-3 and.mL' perovskite-bearing}‘-L‘T1'-'-. nephelinites from the lower'-L":' Kedowa[(611114 and4:14 Nyando.\'_'-1.'11‘1-.1.1"1'-L':rivers.

I'll 1905-6 H. B. Maufe\T' of the British Geological1.1-- Survey. traversed the country along the:1 length"1 £111 of the1111': newlynew.- - Lansinh'lcdconstructed railway2111-1141 fromIn. m Mombasa\111.I1D:1\;1 to11.1 KisumuI\ 1:11.11 and:111d in his report (Maufe, 1908,111111 pp. 45-48) described those1:1{1sc partsDLLI'.s L"..~1'.east and.1116 west'.'1L‘\I of Mau Summit as the "western flank of the rift" and the "Nandi Slope" respectively. He noted the great ,thicknesses of red derived volcanic soils on the east-,facing slopes and commented on the presence of a massive flow of streaky rhyolite south-west of Molo "which contains so many inclusions it resembles an agglomerate." This is shown on the map accompanying this report a:s.1N.lL‘L11.lI\|‘111L'a eutaxitic welded'1‘1-‘1k1L'11 tuff of tuff-flow origin. To the west of Mau Summit the railway descends rapidly1" p114,»v.7 over7.1. .7 a succession of rocks which dip less steeply to' the West. Maufe distinguished twO'lava types between Mau Summit and Londiani, referring :to' the first as a basic phonalite and yaunger varieties as quartz-bearing tr.achytes. Between Kedowa and Lumbwa, MaJUfe collected a suite of racks including trachyte, phanalite, limburgite, nephelinite, agglomerates and tuff.

Gregary revisited Keny:a in 1919, and W:aJSagain impressed by the gentle slope of 1111:'11-’11. 'L'..1‘;i.-: ‘31: the western Rift Valley wall ail:this latitude. He suggested as a contributory cause-1. theI" readily weathered soft volcanic tuffs composing the slope (Gregory, 1920, p. 22). I'll his later volume on East African geology Gregory (1921, pp. 120-129) made several observations on the Molo-Londiani district. He related the pyraclasticL ".x'a»rocks banked‘-..“!:r.'L'L1 against.1131 the1he'11.1:stcrnwestern scarpECLlI—P-wall\1: 111 to111 an:111 origin at.1: the Menengai CalderaL'cz'4 11:11':and L'1's1‘nmem-ccammented11 anon the probablep.1nbub‘1L faulted141112;'1-.11rorigin of the' steep~1L‘L‘p hill11'! i slope between311 the111; Mola.1310 and.1111] BissoiB:s~-.11 T1‘x-L-:'s.rivers. Gregory alsoL11\11 noted{11"L'L‘L .aL1 succession of lav,as1:11;» with intercalated tuffs as he ascended the Nyando River valley from the westi to':0 Mau.\!:111 Summit. The basal member is a phonolite; intermediate flaws are of nephelinite and21:10*- phonalitic3111‘: nephelinite and the uppermost

*" ReferencesRuicrmm: '41":are L;1_;11L'1iquoted on4:171.p. 55.38.

6 -r flows have the composition of phonolitic trachyte. He sug.gested that the phonolitic 'trachytes .are mid-Pliocene in age, and were succeeded in the late Pliocene by the upper Tinderet valea'Dic rocks. He visualized the easterly extensian 'Of the Kavirondo Rift faulting succeeding most 'Of the volcanic activity, but preceding the extrusian 'Of the upper Tinderet and Lumbwa rocks. Bailey WilJlls (1936, pp. 64, 265, 270) in his study 'Of East African Rift Valley structures also stressed the unU1>ualnature of the rift wall in this region,. and insisted that no true escarpment exists, but that structurally the rift wall isis a:=. manodinali-‘I:.' .'1IL'L“.=.: flexure, i15form being partly masked by subsequent volcanic activity. E. C. Bul'la'rd in. 1933-34 ca'I'Iied aut a series of gravity measurements to'.L- eLucidate'-..'.L the structureattuczurc at.11; depthL10. 2. beneath Rift Valleys and pravide evidence 'Oftheir1h." mode10. L of. formation.L'.I.-n1-.1:ion {13L.1.'.-..='L;‘.(Bullard, 1936,19.130. pp. 509-518)..— :II. In 1952-3 G. J. H. McCall, then a gealogist 'Of the HydrolagJical Section.‘I‘ 'Ofc" the‘.."-L' Minis-‘31 1.5.»- try 'Of Works, mapped the Nakuru area as part of a detailed investigation inta graund- water conditians in the northern part 'Of the111L'1:.:'11.1.i.:1farming area within the1:10. RiftLiii'. Valley. The111; reportICELJI'L (McCall,[\iC 1.1 1957B)19. includes the Rongai, Njoro.‘L'inm and Kampi1'\a‘-m[‘-| ya':=. MotoX1010 parts of01' the1110 present‘ area.1311.101One of McCall's main gealagical interests is the Menengai Caldera, situated'ft-uLZCﬁ. n'Orth 'OfHUN:NakuI1l,k'. with which he dealt in a. paper presented to the Internatianal. Geolo-C10.) L:— gical (_'.1‘.1_:.'L‘~;\Congress inii: 1956I' :- (McCall, 1957A). In both publications". .1 McCall's. L‘_a."|'.~'. L1hw:'\':>.:i<.3ns.observations, like_

After joining the11“.: Mines.‘IEir‘L: and'.I.r:L'i Geological£9". Department1piirtit1m'. McCallNHL... 1 extended his mapping of0.." the NakuruN.'1.kuru areaarc-.1 ta7.0 include1111:51l3 the1310 quarter-degree0.41103?—dc:4. cc sheetsﬂeets immediatelyi:1.;11ed..-.::._V ta10 the eastc and narth-east:10:"L11—enst of0f111cthe present{3.1350L111 areaarLu (McCall,{ML C.:-.1| 1967).296:1.J.\K'211.\.h111:1ppedthcJ. Walsh mapped the EldamaE1d:‘:m;3 Ravine areaare-.1 to the1113 nO!1th11...:3r11 in111 1959-60f95961‘. (Walsh,..\\-.'ilsf1. 1969)3969'- and;mcl to:‘0 the:11: south«001.11 L.1.-. A. J.I. Williams\'\"111i;'u‘r1.u (report{report in.111. preparatian) mapped the sauthernSOLIE1L’TI‘. Mau.\1 L1 inin 1962.19.63 TheT‘1e. KerichaKeriLJ-‘J area:-.:'c;'=. ta1.0 the we:SJtwas: wastrue mapped by F. W. Binge in 1949'..nd;':1Land the repal1t1'6 0. ‘ publishedSUI..-“ in 1962.

IV-SUMMARY OF GEOLOGY The entire areaLil'cL'L'l is'\ covered by valeanic rocks, ranging in .age fram Tertiary to Recent, and lacustrine"in: and fluviatile[?L:1.i; sediments. .dc.'ivr:L'1derived directlydirLL. from them. The bulk of the rocks fiarm an:111. interleaved1"Ict'ic ”3.1 successionNLICCC.~>1I.‘-l‘-. 'Of0." pyroclastics,03.10013. apparently derived from several different saurces, which cover an extraordinary range of variatian in colaur, texture, !fragment size and degree of welding. Lava flaws are less camman but as they normally : .. possess' greaterII resistance to erosion they outcr'Op moreI-H'Ix persistently_ where they occur. Broad13r0;-.L'1 tracts1.1.01.4 'Of thickly forested country and a copiousL's-031mm; soil5.011 coverL'.'.1.\'Lr derivedderivu; fram the weathering of a mantle of recent ash combine ta obscure outcrops011370515 of0:” bedrock.11':e Some difficulty was thus experienced in relating different parts 'Of the successian isolated by these natural barriers.

In two small areas the exposure is exceptianally good; in the west in the regian of Kedowa and the Nyando Valley and in the centre, where the main road climbs the Mau scarp west 'Of Rongai. In both cases rugged terrain combined with a camplex stratigraphical sequence of small units of contrasting rock.IOC'1\ typesIV;.-L‘% rendersI€DL1€FS mapping[11'l 1'" an0-3 a3 reconnaissance scale difficult and generalizations have been'.':-.:. inevitable.i:.c1.'II.:'.!31-.

'7 Apa!l1tApart

Phonolites.-Phonolizr.\‘.—ThcThe lowest member of the successiQnsuccession .ofof the MolQMold Plateau ix“.is a.a distinctive biatite1oiotite phonolite which 'Outcropsoutcrops extensivelyextcnxiV‘ely inin the south-westsouth-nest and in the scarpscnrp S'Outh-muth— eastczist .ofof Kedowa. It is correlated with the Kericho phQnolitephonolitc which outcrops farther west.West. YQungerYounger phonolites .occuroccur as small Widelywidely dispersed ﬂflowsows at different heights in the successi'Onsuccesdon and phonoIiticphonolitic plugs we,rewere ,recognizedI'C“t)nZc.d at Mount BlackettBlaekett and in the Nyando Valley.

'Basalts.-Olivine‘Basalts. OliVine basalts are locaJlylL *nll} intercalatedintercalated m.in thethe. .olderolder phonQlites,phonolites. and also occur near MailMaji Mazuri and westVVcst ofot‘ LandiMri.londiani. Alkali basaltshasalti occuroccur locally on the MolQMolo Plateau.

TrachytesTrat'lri'tcs and quartzquart: (rarity:trachytes.-Trachytic.77‘rztghytic Javas,litV.1\. locaHyloudly carryingCnrrjcin smallsmaJ.I amountsamounJts of free quartz.quartz, form the lowerin“ or slopesslcvpe< 'Ofof MauntMount Landianil ondiani andar‘d extend from west of Landi-l undi- aninni tQwnshiptownship t'Oto the northern boundarybound 1rv of the map between MajiMaii MazuriMnmri and11nd KilQmbe.Kilotnbe Tuffs,fruit; «grittmiw‘u1‘:.V‘agglomerates and sediments.-Much.V't'nfs'iinuizrt‘. riMuch of'Of the MattMau and Mount Londiani an;are com»cam- posedposed of variableVariable pyroclasticpyroqlastic rocksrock.» among which manytnn“ can behe recognizedreciignized as.1~1 derived by an ash-flowash-ﬂow rather 'thanthan an ash—iallash-fall mechanism.n‘iechaniVm. Many .ofof thesetheie rock:rocks are welded and cQntainglassycontain ghmy discoidal compressedCL‘H‘l})i'C\\CCl pumicemimic: fragmentst :men‘s and:"ld numerous erraticCl'TliilC pzir—par- ticlesticlss .ofof lava.lava Outcrops extendextendtfromruin Maj.i\l.1ji Mazuri\11211‘1 1 andrnd Tinderetl nd: to Kedowa,lit-dam '1 MQlo\lolo and houndu'} oi the :‘1tszt. Owrlyirig the n

Little can behe added to generalgnieral knowledgelx:1uVVled': .ofof thehe age and mechanicsmechanic; ol‘of RiftR1" ValleyValle} f.ormatianformation £roml‘rom evidence in thethe area sincexirice much .ofof the 'latelat: stage\tatre extrusionextrux‘ion and pyro-pyrry clasticelastic activity post-datespost—dates the faulting,faulting. 'Obliteratingobliterating the structure with lilVéllava ﬂflowsows and :1a blanket of ash.(uh.

The areanot provedp:odied to contain noft.» rocksrouts older thanth‘..r. Tertiary,Tertiary. and apart.mart from horizonshari/onK .of01‘ building stone\lt no and1nd pozzualanicpt //t1t1..m c materialiltL.‘.Ctl[‘.l. littleiittl e 'Ofof econamicecononiit value‘.1l‘.1; wasV\:1\ faund.tound

V-DETAILSV—DETAIIS OF GEOLOGY Thelhe rockslUCks ofat the area range in age from Miocene\lioecne to Recent. The'l he phont.i:ephonolites\ o:of the MolaMol 0 Plateau are cQrrelatedcorrelated wiJthwith the KerichaKerieho phanolitesphonolitei further west,west, whichwfhc overliemerlic directlydirect the basalba lTinderctTinderet agglomeratesZiZgl-tf‘t‘3"t‘.lc> and ,tuffistuft) associatedassociimd with the KoruKorn sediments«amend .ofof provenprinton MioceneMineehc age (Binge,tBingc. 1962).lWiZi.

Petrol'OgicallyPctrologically the rocksrock\‘ of the area1:133 mngerange framfrom TertiaryTertint‘V‘ lavaslaws .ofof ext:extreme basic composition to:0 quartzUtiiirtztr‘trachytes.lCh‘ELS. Thel ic ,0Ultcropstuterwtm of1‘1 undersaturateda,.1.1.-*l .tl‘lLl .1“"tl rocksml 4 are c'OncentratedC i: inin thethe west,west particularlyparticuulae' abautabout the Nyando\V ndr1\;;llcx1dValley, and arezire apparently:tpjitrcri. lv associatedzismci 1t with a3. sourceVon cc at:1: Tinderet.Tine-e: t The basanites,bauiiits». and meliliteandme :litc .‘md perovskitepCTt‘V>l{ll£ melanephelin,iter'l .m-ph....t:t: mem- .bersbers are comparablecompar. .‘z with thosethciie foundtotirid in the MteteiMtetc‘i Valley and about NabkoiN..hl§of and LessosleRsm on thethr‘ north-western:ioi‘. h .Vestern flanktumult ofat Tinderet'l i:‘.de“e: (Jennings,i‘Jtrnrik 1964).PM»1 1,

TmchytesTrachytes were extruded from Mount Londianilondinni and Kilombe,Kilomhe. but it is only the exten- sivex’i'; flowsFons to the west“xi-cat fromfirm the formerlifirn'ier sourcesaurce whichhigh contain.‘ontdin free tiimrtz.quartz, particularlyI“.tttilarl‘; the outcropsoui'cmp,» about Landiani.l.t‘3'l{il‘d.l’ll.

8 Thethe succession of lavalav,a types anon the western”.VL‘l‘iTl shoulder 'Ofof the rift in this are;area appears to:0 be:-be :— PleistocenePleistoeene BatsaltsBasalts and basanitests‘Ltnites West of Mt. Londiani

TrctLTrachyteshfies and quartz triL‘testrachytes Mt..‘Vit. LondianiLLmLiiitni and KilombeKilontbe Basal.Basalt MajiMitii Mazuri\lutri Pliocene tPhonolitesPhonolites NyandoNV‘Lm‘L’lt’V Valley,\‘tley. Sorget.Sorget, etc. t‘ BasicBasie nephelinitesnephelinitcs Nyandol\ udo ValleyV’LtllcV‘ { Phonoliticl’ltonolitie nephelinitesnenhelinites Molo\ olm Plateau;Plateau: scar“;scarp SE.S.E. of l Kedowa;Keilowu: Nyando\V':tntl(.1 Valley.\".iilc_V'. Miocene\Iiocene Phonolites Molo\iolo Plateau;Plateau: sent'pscarp SE.S.E. of . Kedowa.KCLlU'V'VZi. 1. Tertiary (1)(It KERICHO PHONOLTIESPHtjmjitiTEs The phonolite lavaITLV’a flowsflOVV‘s outeroppingOIUtcropping cxtensiV‘elyextensively inh the extremeexitCttte south-westsouth—west are apparently.ippdrchtly the oldest roe-itsrocks of the area.area, and aretire oorrelatedCorrelated with the Kericho phonolitesphont‘lites toin the nestwest (Binge,tBinL'e.1962.1962, p.13. 25>.25). Theyhey constitute the basal member of the sequence forming' 1rmif‘LL' the MoloMohV Plateau,P L 3:111. andLind theirTitClI' upper surfacesiit'fL'ice slopessl ones in ait .generallygenerailVl 'south-westerl~soutl —.\€~[erl- direction-tl;rection at some 60 feet per miIe.mile. Since thisthis is only0a slightlysh L'ht' shallowershni lOVVer than the gradientgradient of. . the :iV‘erriver beds the outcrops appear usas elongated stripsstripstfingeringt1 nL'3riinL' up the valleys.mile ,‘s The height.tCtE‘ of the upperupper surface of theseihC\L‘ flowsflows risesrises fromfrom 8,000R (MU IT.1t . inin the'.1L souvh-werstsouth-“est to (thou:above 8,6008130“ ft.:1, three milesn1: lcs ntuth-north~westVVest ofot KeringetKerinuet ()titeeOffice, andnd neanearlyrl‘ ,thatthztt height fourfour miles 'west-south-west“est—stinh-must ofor SimbaSim; :t SWamp.SVVIm. A thickness ofor 500-600Stithfitfl feetteet of phonolitephonhlite is:s exposede\pnsed in thethe scumscarp two miIesmiles south of Kedowa.Kedouu. The phonolitephonoiite isIs aL1 fissileiivilc porphyriticpL‘L‘pt‘ laval:t\:t withVVith aLt finetine grainrigrained groundmass.gt'oundntass. TypicallyI'V; _ 1t‘t farmsforms rounded billowshillou's ,inin valleyV'tiliL’ flfloors,oors. and:tnd :rapidsrL'tpiLls and11nd platformsplu‘i‘hrms in s

*’ Numbers 42(988,4199“ etc.,. referretry to specimens in the regional Lt‘tlIeL‘titmcollection atof 111;the Mines and Geological Department,DL‘prtr night. Nairobi.

9 (2):21 BASALTS8.15.1115; INTERCALATEDlNTERClAl.ATED IN1N KERICHOKLRIL‘HO PHONOLITESPHUNOLITFS Basalts132511115 0141131011111outcrop in theme stream bed four£11m and six miles south-south-eaJst5L1uth~seuth-east :11“of Ndmnet.Ndoinet. Thel‘he rock of Ithethe nor,therlynortherly exposure 142‘(42/952)952)iis a:1 fine-grainedfine-grained black lava with 2mm2mm. diameterd1ameter vesicles and occasional:hcasionnl microphenocrysts.microphenoer:515 Small erystulgcrystals of multiple—tmultiple-twinnedfinned basic‘ ic labradoritelab ad‘oritet(Anos)An are 3»:accompaniedeompanied bvby subhedral augite and olivine phenocrysts1111:1111" alttaining"1‘31. lengthsleneths of lri1‘t11..Imm., in an1111 1nde1erntinahleindeterminable chloriticchlorine. base peppered1111;311:151 with Havel.black jron. ore.ere \Vugsup are largelylargelv chlorite-filled.ehlnrite- fil lL‘d. The[be fine11.11: grained,grained hackly,haekly.b‘1:1ckblack basalt1135111110111from twotwo miles1111‘ He fartherf‘aithez‘ sooth5011 11:112‘(42/953)95., containsL‘C1nta1115 plagioclase1l:';:1'1‘e1;15e and olivine microphenocrystsm:‘L‘rnphennen ts (theIthL‘ latter largely‘arrgeLél‘ altered:1l ‘1: e‘d to1e serpentine)5erne1111‘11c1 in1:1 a gToundmassgr‘otundmass .ofof plagioclaseplagivelase lathsl:11‘t15 and grains of111 pyroxenep3r11‘x’en and black1.111.511 iron ore. A compact b1sicbasic 1:111:115pcein1enlava (specimen 42942/983)S3} exposed by faulting in the rixcrriver four miles south-east5outl1~efi51 ofOt‘ FiveFire M.ile\lile PostP1151 is1‘5 of111‘ limitedl3n1‘teLl outcrop,nine 11:1 ""111and L‘on5i5t5consists of111‘ :;a shelf (1:1‘11‘1'11Ll‘.OIl]!which et the11': river1‘3\L‘. course1‘11111‘5L‘ 1‘5is only .lightlygh 9]}Ll etched.L‘hL‘11. The[he fault-line.‘ELLIJ t— 1: me 11:defines‘t‘inL‘5 the edgeedze of the111:- 5hLshelf,l and the notchmatch eroded at.11 the waterfall‘.‘-:1111 Lzlllis negligible..31 le OlivineCl“ 1‘:‘nL phenocrystsph‘1111L11515, 3mm.31111.11 in length are partly :1l‘eredaltered to L‘hlatL‘:chlorite and1111 serpentine51V.1_11L‘n ‘111L minerals,111iner:1l5 11:11}and are L-11r1131‘71edicontained in. a1 l“:fine-grained base11:15:: 131‘of pyroxenepyr’xene grains,' 1n5. 111.11;elarger enhL‘LIraleuhedral blue}:black 11‘1111iron oreOTC? cr3‘313l5‘crystals Lll'ld‘and 5mallsmall pooh311'1L1l5 111‘of zeoliticxenlitie 111115:1.-‘Lmosaic in a base11:15: 01‘of interstitialinter5titinl felspar.l‘el5p3r.

(3)131 PHONOLITICP110501 111:: Ntt‘11111x1115NEPHELINITES TheseTtellavas11:15 outcrnpoutcrop extensivelytLnsiyely in theIhe\y:1nd0Nyando RiverRixer valleyvnlle‘» at.11 altitudesC111itudes belowbelowx7,200 it. and on0!] the We.5twest of1111‘ thethe Molo\111:11’[11L111Plateau betweenb LI‘LA ‘Ln 7,300 eft.tr and11111 8,50015’ ‘i‘ ft. It11 is15 suggested511g2e5 that. the heighthe‘ght discrepancyd15et‘ep3ne3' resultsre511lt5 fromtrem north~eastnorth-ext toto. south-west5:1nih-1LC5t foolting1:11:11n alongLlung thethL thatfoot of111‘ !the‘Lhe scarp5L :1rp easte:15t od)111‘ the KerichoKeriehn road.103.01. It is,15 appa;rentdeparent fromfrom outcropsnutL‘rop5 mapped ,and.11111 petrolo-petrulni gical3 descriptions’ ’ how that the111: lavas11135 of111‘ the Nyando Valley\‘.1lTL-j1 'extendL‘\ICT‘1(l to11.1 the west into the KerichoKet‘iclln area,area where'Vl1e." theythL‘V wereWere mapped as15‘ phonoliiticphur‘olitie nepheHnitesnephelinitn overlyingLi‘lerlymg a:1 great thickness of{blotbiotit~bea:ringte ‘11:.1t‘ing phono1itephunuliiL (Binge1‘3 11 1e 1962,196.12. p.31 . 32).321.

Cooling-fraatmesr 1 fracture», in the phonoliticphonulitie nephelinite flowst‘lmrs are widelyWidely 531331].spaced, 5‘11so the bunkboul- dersder5‘ derivedde11\ ed by spheroidal5pl1er‘oidal weathering are.1rL‘ large and11nd I'ounded.rt)11ndecl.\\’ithin:heWithin the outcropouterOp of theseﬁnes: ~avas1:11:15 cliffeliﬂ‘ or cragemg features are rare;rare: more1110 re usuallyusuallv steep51ee31 slopes51(1pe5 are paved with giantginnt cobbles,L‘ohhleu5 and:ntl roundedrounded hillshill5 carryLill'l'V a:1 111.1121float of oblatelyoblael; spheroidal531hem1‘ti‘11l b11111lde2‘5.bouldeTs, often several5e1‘err1l feetl‘eet inin djameteT.111 unetet‘. Auto-breccias:‘XlllO‘blé‘CClllS are:1rL‘ l11LL1llylocally presentpre5ent near the base1.1.151: 11t‘1heof the flows.1311115.

l.:1\‘.‘15‘Lavas of1211‘ this{his group show511011“ wide \‘:1ri:~.ti0n.n1.‘1inl\‘variation, mainly in t\‘petype :111dand proportion of pheno-3111:7111 C1151;crysts, and examples,examples with :1a selection of wen-formedWeilltormed L‘tV"."l§crystals make an intere5tinginteresting petrologicalpetrolopienl study. A ,typicaltypical member appear;appears in hand specimen as.15 :1a greenish grey rock with E1a fifine-grainednei ncd gro'Undmass2 .111111‘ 5111155 carrying3:11‘1‘1i21g numerousn1;n‘.e“11‘.15 large clustersL‘lu5tL‘1‘5 of11f euhedralcuhedrfl nepheline‘11l1elft1e L‘ry5111l5'.crystals, Indi‘riLhtnll}individually reachingruehing 3mm.3111111. ,across,1131155. occasional11111153111111 t‘el53‘nrfelspar 31:15.15prisms :1111land micro-lathsmacro-l ‘13 of p1‘r11xene.Pl1enoL‘r1‘51pyroxene. Phenocryst content 15is extrrordinarilyextraordinarily variable.\‘ i.1hle. SometimesSometimes. the nephelinesaheli 11L5 are close-packedL‘ln5L-p‘1eked to the exclusion:111‘i 1511 11 111‘of other1.1: her varieties;.111‘ietir5; inn others11111915 large conspi-3111:1361, 1.1111125cuous feJspars1el5“.1:5 11313‘r1111happroach nepheline11e11hLl111: in11‘. amount,111111.‘11nt. while1111i]; pyroxenepj:“1.1.‘:;::‘;e prisms315151115. :11;are best dc‘1el‘.13e11developed in \:11‘.e:ie5varieties :1l111n5t‘1'leV11iLlalmost devoid 111‘of leucocraticlL‘LlL‘L‘L‘1‘J11‘.e phenocrysts.1:131L‘1111L‘r35t5. Numerous 531e:“‘1111_‘n5specimens 111‘of these1he5L‘ 1:21.15lavas were oonected,1‘1‘.L‘l:te111‘1‘.L‘l11L1'in‘:_including examplesCX1111r1l1‘5‘ from11:01.1 the1he NyandoNynnio REV-erRiver valley on the ‘11e51e7nwestern boundary,henndnry. half11‘:1lf:1a mile1 11L 51111111south of111 ,the‘hL road-rail.11:1d —1‘:‘.1l crossingL‘mis 5112 (42/828);142 32M: fromt‘ron‘. thethe hillsl11‘l‘15' one-and-a-half1’1ne-C1n11- -h:1l.‘" miles1113l1‘5 and three miles:1‘ile5 south-west511.11: -‘1E :51 ofU i‘ Mount‘11 4n Blackett,Blaeket. and11nd. the \‘L1lle3'valley three 111ile5miles 51111:?1—5‘1'111th-1-1e51south~south-west 11—12(42/832;1‘32; 4242/845;54. 42/865);‘ high onOTI the scarp5:11:11 111:)two miles111235 south51111‘h 1.1?of R70Kedowa:1 (42/850);1—13 \‘P ‘: and.1n:i 1111on the:h: MoloMule l’lg.Phliteau2:111 three1h :‘L‘e milesmin-.5 5011111south of Kerisoi F111‘L‘5tForest Station 1.12(42/935).“13:171. FelsparFEW" appears::3131L“.‘1r5 to{1‘ liebe absent:1lfi5en‘. fromt"“r"1 somemime l:1‘.':‘15lavas t—i“(42/935)l. and. occurs“5 only in:‘1 the groundmass211111111111315'5 11fof othersethers 142(42/832).R321. In1:1 5mmsome 1—12(42/8655’15 113111and 4242/845)8451 fe/spar155:3." phenocrystspliL‘nt‘L‘Tf-K‘EQ occurnee-11: in‘211 an:1‘1 indeterminable,:‘111‘11'1111‘11‘11‘11‘. 1“e1:-1‘51“1‘t;’11“iL‘near-isotropic base,l while in{:1 othersOthers well de1el11pcddeveloped felsparfel511.1r occurs11L‘eur5 both as.15 phenoorystsphenoeryst5 and in111 the :groundmassgr'1.111rdz11:125 142(42/828).5‘28“. FelsprurFe‘5;1‘:‘:1‘ phenocrysts,pl1er10::_15:.5 often111cm s.1ighty51?:TIlff‘

~ ~

rounded,raunded, are usually simply twinned, occasionallyoccasianally vaguely cross-hatched,crass-hatched, and in one example showedshawed incipient multiple twinning. Biaxial negativenegaltive ﬁfiguresgures with moderate or small optic-axial angles suggest anarthoclase.anorthoclase. The pyroxene is usualiyusually highly zonedz'Oned aegirine—aegirine-augiteaugite in euhedral or subhedral prismsprisma 2-3mm.2—3mm. long,long. with outer fringes 'Ofof aegirine.aegirine Rarelyarell} the pyraxenepyroxene :sis augite,augite zanedzoned VVithwith neutralneut :11 cares,cores. and 'Outerouter marginsm rgins weaklyVVeakly pleochraicpleochroic in pale shades 'Ofof brownbrawn and greenish broVVnbrawn (4(42/832).23‘832l . SometimesSometJimes aegirine-augiteaegirine augite carriescarizes a pinkish augitic borbordertier (42/850).(4?. 850). Aegirine-augite, baldlyboldly pleochroicpleochraic in shades of'Of green, is often partly resarbedresorbed and the zoning, parallel tatheto the terminal pyramid faces, produces a bold chevranchevron pattern across the entire crystal. Occasional sphene euhedra.euhedra, 'Oftenoften exceeding 0.5mm.O.5mm. in length.length, occur in all rocks,rocks, 'Oftenoften associated with small VV'ell~formedwell-farmed or skeletal black iraniron oreare grains,grains. which also occur separately.

(4) NEPHEHNITFSNEPHELINITES AND MELANEPHELINI'I‘ESMELANEPHELINITES OF'OF NYANDONyANDO VALLEY LavaLaVa flawsflows 'Ofof unusual campasitioncomposition and limited distributi'Ondistribution occur near or'Or at the toptap of the phonDlincphonolitic nephelinite sequence in the NyaodDNyando Valley,VaiNey,'Onon the flflanksanks of'Ofa prominentpromment hilIhill one mile north-east 'Ofof the road-rail crossing on'Onthe western boundary. The rock capping the western spur 'Ofof the hilIhill (423‘822)(42/822) is a black glassy lava dotteddDtted with waxy micraphenocrysts,micmphenocrysts. and cut by a reticuiatereticulate pattern of'Of reddish fractures. Mic—Mic- roscapicroscopic examinatianexamination showedshawed the isotropic glassy base tato be locally devitridevitrified,fied, and to contain abundant well-preserved prisms of melilitemelllite up to O.5mm.in0.5mm. in lengthJengtJh and smallsmaLl nepheline euhedraeuhedrn forming olustersclusters up tota 1mm.hnm. in diameter. PeravskitePerovskite euhedra with diameters of 0.25mm.025111111. are present and 'Occasionallyoccasionally attain a diameter of 1mm.Imm. Rare small aegirine-augiteaegirine—augite pJ'\ismsprisms occur, preserved within crystals of melilite.me1iHte. Specimens of lavalaw. framfrom west (423‘(42/823),82 3.) nDrth-westnorth VVest and north—narth-easteast (42(42/825)3'9825) of'Ofthe hill are all characterized by partly glassy, partly altered turbid indeterminate bases. Nephe- line phenocrysts are abundant singly orDr in clusters.clusters, and the groundmass is locallyIDcally crammed with smallsmaN nepheline euhedra. Melilite is poorly preserved but lathlath-likelike pseudo—pseudomorphs attain a length 'Ofof Imm.1mm in specimen 42/825.423 825. Aegirine-AegiJ'line-augite-augite grainsgmins, peppered with included nepheline, exceed O.5mm.0.5mm. in length and include alsoalsa smaIlsmall perOVskiteperovskite euhedra, which are also commoncomman in theIthe groundmassgraundma'ss of 423’823.42/823. Large perovskiteperavskite crystals are rare, usually occurring'Occurring singly,singly. or occasionallyoccasi'Onally associated Withwith large skeletal crystals of'Ofblack iraniron are,ore, 4mm. in diameter.diameter, and prisms of apatite. The rocks capping hills one mile north—eastnarth-east and one'One mile southsauth-easteast of the roadroad-railrail :crossingcros~ing onan the Westernwestern baundaryboundary are maremore basic lavas. The floflaws“3 are limited in extent and their outcrops are signisignificantlyficantly aligned alongalang a possiblepOssible north-sauthnorth-south fault zone. The heaVvheavy hacklybhackly blacklack rock fr'Omthefrom the I\V3andoNyandD ValleyVaNey southsauth 'Ofof the roadraad (specimen 423‘42/820)820) carries abundant 2mm,2mm. oliVine'Olivine phenocrysts. Occurring singly or'Orin clusters, they are traVersedtraversed by tracts of'Of opaque oliV'e—greenalive-green serpentine minerals developeddevel'Oped along bold fractures. SingleSingle, zoned,zaned, bufiacolouredbuff-colaured augite euhedra attain lengths of'Of 1.5mm.1.5mm., and atzgfteaugite also.1lso 0:31'Occurs‘i in groupsgraups of smaller elongate prisms formingfarming roughlyraughly radial clu~1ers.clusters, associated with black ironiran oreare. lnves:igationInvestigatian of'Of the fine—fine-grainedgrained groundmassgraundmass revealsreveals a predominance of granular p).pyroxene,oxene subordingcsubordinate blackh..l;‘1 l; iron ore.ore,occasionaloccasional interinterstitialst;tial poalspools or'Of analcincanalcime and :tirerare suLhedr:subhooralBill grainsrains of'Of nephelinenepheline, but no recognirecognizable:3l3: fc‘fe1spar.soar The rock is thus classiclassifiedfied as an extremely basic melanephelinitemelanephelinite. A xenolithic patepatchch of coor:3—-r_v1aincdooarse-grained Llldll’—iClSpEiI‘qu

The abundant olivine'Olivine and zonedzaned augite phenocrysts in a ﬁfine-grainedne-grained blue-black lava9V3. fromfrom thethe hilltaphilltop north 'Ofol‘ the LumbwalumVa raad,road. specimen 4242/824,‘824. displaydismay similar size and habit. The rackrock base is maremore leucacraticleucocrntic than the previausprevious example; poolspoals 'Ofof zeo-zea- lite, largely analcime,analcime. are larger and maremore common and the !includedincluded grains of pyroxene

11ll

'" - - --'" and black iron ore are smaller. Search revealed no indubitable nepheline, but a few micrlo-Iaths,micro—laths. apparently of plagioclase feispar.felspar. The rock is closely reia:cdrelated to the previous basic melanephelinite,melanephelinite. but shows in thethe presence of plagioclase and lack of nepheline an affinity with analanalcimeime basanites.

Exposures in the strcarnhedstreambed and as rocky knolls in the valley floor one-and-a‘quarterone-and-a-quarter miles north of Kedowa were interpreted as a dyke (the more northerly exposure) and .smallsmall intrusive bodies in the country rock of tolltuff and ash. The age of the bleached tutistuffs and Lig'glL‘lTCI'liiCSagglomerates into which these are intruded cannot be clearly demonstrated,demonstrated. but these minor intrusions areai"e probably younge.ryounger than the lava flows justjus

The dyke,dike. a fine-grainedline—grained black rocltrock in which only nephelinencphcline phenocrysts coqcould hebe recognized in hand specimen 423‘EU7,42/807, waswars found to carry highly altered lath-likelath—like patches in the groundmass which could equally represent altered felspar or altered meIiIite.melilitc. The ncpl‘fincnepheline phenocrysts,phcnocr}sts. often cuhcc'raleuhedral but occasionally largelylargely resorbed.resorbed, occur with\xith perforated laths ot‘of aegirine-augite,aegirine—augite. in a base largely of analcimc.analcime, with acgirinc-augitc.aegirine-augi.te, black iron ore and granules of peroysltitc,perovskite, the latter indicating a suitable environment for original meIilite.melilite.

The rock of the knolls (42/810)(-12 "810‘; is composed of more normal phonolitic nephclinitcnephelinite with phenocrysts of nepheline,nepheiine. anort‘ncclasc.anorthoclase, sphene and zoned augitc.augite, all showing resorption or reaction to varying\arying degrees,degrees. in a background of {)ccasionaloccasional flakesﬂakes of cos~cog.. syrite, laths of felspar and -aegirine-augiteacgirincougite with a ﬁfinelynely crystalline.crystalline, probably felspathic base.

(5){5; TLTH‘STUFFS OFor THE MAI;MAU AND MOUNT\lOL‘Nl' LONDIAN!Loxorm The bulk of the Tertiary tui‘fstuffs in the \ioloMolo area areaTe demonstrably of ash—ash-flowflow origin, as contrasted to tuilstuffs of ash—fallash-fall (airborne) origin. Ash-Ash-flowflow :utfstuffs are ,rocksro:l(s which resultresult from the consolidation of high-temperature mixtures {)fof pyroclastic material and gas.gas, ejected explosively from a crater or from a fifissure,ssure. which have travelled rapidly down the craterorater slopes or along the ground. Ash~fallAsh-faU tuftstuffs result fromfrom the compaction of pyroclastic material which has fallen to the ground from airborne clouds.

Ash-Ash-flowflow tuffs move in much the same Wayway as molten lavas.lavas, following existing valleys.valleys and flflowingowing round obstacles. They are typically tens of feet thick, unsorted and unbedded.ded, showing varying degrees of compaction and welding by their own heat.heat, and often showing columnar jointing developed by shrinking on cooling. Ash-fall tutfs.tuffs, in contrast.contrast, mantlemantIe the ground irrespective of topography.topography, and frequently show marked stratistratificationfication due to sorting of the components into size units.

A characteristic of ash-flow tuffstolls is.is their eutaxitic texture,texture. resulting from the flatten-Flatten- ing of component fragmentsfragn‘ents with their longlon.g axes lying parallel in a horizontal plane,plane. which appear as streaks and lenseslenses in the solidisolidifiedﬁed rock. \‘s‘ithWith increase inin the degree 0f{)f compaction pumice fragments shtm'show stages from almost equidimensional vesicular lapillilapilIi to completely Faitcncdflattened streakystrcalty glass. FutaxitfeEutaxitic Weldedwelded tuftstuffs commonly form satisfacorysatisfactory building stones.stones, and throughout the areaa,rea theirtheir outcrOpsoutcrops are marked by lines of quarries.

Crystal inclusions,inclusions. particularly prismatic and platy fragments of sanidine and sanidine-anorthoclase,sanidinc-anorthoclase. a.reare common inin all the tutuffsﬁ‘s of the Mo‘oMojo area,area. but only when the crystal inclusions occur abundantly are the rocks referred to as crystal tuffs.tufts.

DevitriticationDevitrification and tr:icrocrystallizetionmicrocrystallization within glass fragments occurs both in cem—compressed pumice lapiIlilapillf and in the groundmass.grcundmass. Inin the pumice lapillilapilLi sheetssheafs of"of microlitesmicro!ites arere cemetimessometimes disposed in crude spherular aggregates,aggregates. but more commonly are arranged parallel or sub-parallelsub—parallel to the boundaries of the lensoidleuSQid compressed pumice fragments.

.. (a) T115;Tuffs of the south-westsouth—west The oldest tuffstufts exposed in the MoloR'lolo area are found in valley sides at two locations in theteh south-west,south—west. 1hethe fifirstrst four miles north—north-westwest of Ndoinet\idoinet and the other three miles north of Ndoinet. Inin each place they directly underlie ,thethe phonolitic nephelinite already described. The exposurecxposu:'e north-westnor-‘-h .1'e.st oiof Ndoinet.\1l.1ir1et shoWISsho‘:11 30 to[-1 4040ffeettofof brillbr!illiantlyian ly colou;coloureded mot- tledticd clays derivedderi1ed [111111from the weathering\1-‘eai1ering of tuffstufts and fig-1511agglomerates,1merates- with occasional fragmentsfrarrrzients' of11'; relativelyrelativle1 unalteredunalterec pyroclasticc}.'r11cla~tic material.111::1e:'3:1l.AtAt thetch second localitylocalitj. north of NdoinetNils-'71:: thehe ‘.1'.:1tuff appears to be much less de:;:l;.deeply weathered,1.1.2.1111 ere-J. but exposures there are few and small. (b)[bi Pink fine-grainedfine—grained eutaxiticemoxirit‘ tuffsinf}: The relationship betweenbeiween these tuffstull's (TvfsfTVf: on map) and the pale grey eutaxitic crystal tuffstufis (Tvfs){Tyfg} is not clear. At a quarry exposure four miles south-eastsouth—east of Kedowa TvfsTyfa overlies Tvf..Tyfy. whereas a mile to the north-west the situation is reversed. Three miles south of MounltMount Blackett Tvf2Tyf. directly overlies phonolitic nephelinite,nephelinite. and is separated from the higher TvfsTyf3 by 2002'00 feet of agglomerate identical to Tvf"Ty'fv described later.lain: It1!. appears likelylikely that Tvf2T‘r'f; and TvfsTyf- (and possiblypossibly also Tvf4)'l'y h) are in fact different1!:li'erent zonesrunes of11f the same5-1m: ash-flowash-flow unit. Comparison of11? the altitudesaititudcs of the differentdillcrent units in different parts of the area proves of no value in determining stratigraphic relationships as it is:s' clear that the tuffs111111 were1.1-ere extrudedcs;:‘1ided over111'e1 an already:1]: ead1 irregular- jnlar topography which was greatly modifiedmodified by faulting subsequent to tufftuft deposition. A typical specimen of pink,pink. fine-grainedtine—grained eutaxiticeutaxftie tuff,toll. 42/93242 .932 from the Molo Plateau 'fourTour miles south-east of Kerisoi,Kerisoi. is a lightly welded,11'e!ded. sparsely felspathicfclspathic tufftuii with an orange pink base and thin eutaxitic fragments of mottled purple and white colour. Three miles farther south—eastsouth-east from Kerisoi (42/931)(42.931) the dull pink base is crammed with grey tasselatedtasselatcd lenses up to 5cm.Sent. by EC2cm.cm. 1.11'thwith occasional elongated central cavities. In this sectionsemion the devitriticationdevitrifica :ion of compressedcompressedlfragmentsragments has not who11ywholly obscured !relicrelic pumice structure,structure. and the originaloriginnal pore spaces are nowr1011- seenseen as thin dark streaks across 'thethe aggregaltesaggregates of microlites, para11elparallel to the elongation of the fragments. Inln specimen 42/80642,3806 from the same tufflull north of Kedowa a fewfe11' fragments of felsparieispar lare1': presentpresent in the slide,slide. and many of the grey pumice fragments display limitedlimited com-corn: pressionpjes s: 111. with11ith hollow cores partly filledfilled with yellowyello‘ts alteration products.products. (c)(1}Pa-cPale grey contain":eutaxitic ('rjsvi‘olcrystal tuffsInf}: Specimens of these rocks were collectedco11ected from oneone mile south of Limutet (specimen 42/874),4215'“; twotoo miles south-westsouth-11'est11fof Londiani (42/861),{428611. four miles south of Mount Blackett (42/737),1'11. the forestforest road two-and-a-halftwo—zind—a-halt miles south-eastsouil -east of Kedowaludoa .1 (42/839)(—13.5839) and the quarry one mile east of KedowaK111311111 (42/878).1:42 3.731. Thelite rocks are genera11ygenera-.ly pale grey to off-oil- white11.h'.te in colourcolcur with elongated eutaxitic fragmentsliag ‘-ci1ts commcommonly1:111 betweenbet'oeen 2cm.EC111. and 3cm. 'in'in length. Such inclusions are not conspicuous,conspicuous. beingbezng very similarsimilar in texture and colour to the matrix,mat‘ix. as are mostmo 1 of thet e rare included lava fragments. The'l'he eutaxitic lenses can be seen to:11 bifurcaterilu' cate and display tasselated1-..s.s:l;=.:ed ends.ends, and occasionally to wrap around included crystal and lava53m fragments.fragniea. Broken surfacessurzaccs varyvary in texture from porce11anousporeelianous to hackly.hackiy. Inln specimens:pccfrttens 42/73742,-?3? and 42/839425839 external moulds of twigs and leaves are common. Inin thin sectionsect'on occasional included{needed lavalava pe11ets,pellets, 2mm.2mm, across.across, of trachyte or phono-phone- liteliie and blocks of streakyStreaky glass are seen to accompany abundant partly resorbed sanidinesaniciine crystals,cry tals. often simply twinned.t11in:ed. CompressedCon‘presseci pumice lenseslenses attain lengthslengths, of lOmm.117mm. The thinner lenses are entirely of brownish glass.glass, preservinlgpres-crying faint traces of the original pumice pores asas darkdar" streaks parallel to the elongation,elongation and under crossedcrcssed nicolsnicois partial devitriticationdevizj'iiication is seen,see . with microlites arranged in crudely spherularSpherular pat- terns. Thicker lensesleases display a broador cad marginmarg'n ofot devitrifiedde1'itrified glass and haveare hollow

13 centres. Such hollow centres are often infilledinﬁlled with vapour-phase minerals growing radially inwardsinwards from the cavity walls. Minerals.Nlinerals recognized in such cavities are aegirine and felspar.telspai. The[he less well compacted varieties,Varieties such as 41387542/878, >hoshowV a lesser degreedeg ee of compaction of includedincluded fragmentslragments set inain a powderyponder} base withVVith onlyonl_. a fewten felspar relics.1 1 These varic-varieties make .goodgood building stones and smansmall quarries follow s'everalseveral miles of 1Jbethe outcropOutcrOp in the Kedowa area.

(6)to) TERTIARY PYROCLASTICPYROC‘I.ASTIC ROCKSnorss OFor THErun UPPER MAUMM; Rocks overlying the biotite phonolites and crystal tuffstufts of the western Molo Plateau forming the high ground of the Kerisoi-Five Mile.N‘lile Post ridge and the forestedtorested highlands of the Upper Mau, are almost exclusively pyroclastic in origin. Predominating in the lower part of the sequence highly compressed, weldedVVelded eutaxiticeutaxitie ash-flowash-ﬂow tuffs are com-com— monly exposed on the MoloNlolo Plateau and south of Keringet,Keringet while scant exposures on the higher ground and in the eastern MauNlau Forest suggest that uncompressed tuftstuffs and agglomerates form the majority of .thethe upper part of the sequence. Poor ex—exposure over much of the area in the south is caused by a thick mantle of soil derived from a sporadic cover of recent ashes.

The rock succession is composed of highly varied units.units, with lateral and vertical variation too rapid to obtain a precise stratigraphical sequence from the dispropondispropor- tionately poor exposure available. Rocks overlying,overlying. underlying and probably inter-inter— calatadcalated with the outaxiticeutaxitic welded tuftstuffs to the west of the eastern Mau Forest are commonly bleached fragmental rocks of agglomerate-grade with equidimensional hollow pumice fragments and smaller angular chips of lava and black glass dispersed or con—con- centrated in an ash-ash-gradegrade matrix. A few exposures display wewell-definedell-deﬁned bedding and :cancan be referred to an ash-fallash-fall origin.origin, but the majority are of unbedded inhomogeneous material probably representing uncompressed, poorly welded, ash-flow tuft”.tuff.

Descending the slope into the Rift Valley, within the eastern Mau Forest towards the south of the area,area. the highest exposure located is of well-bedded ashes,ashes. material of relatively recent origin from a source in the Rift Valley, possibly Menengai.Mencngai. Elsewhere .thethe highest exposure is of coarse-grainedcoarse— grained bleached agglomerate,agglomerate. comparable to that ,immediatelyimmediately underlying the ashes too the west.nest. UnderlyingI ride:lVing this,this high in the scarp.scarp, is a substantial band of pale eutaxitic welded-tuff,welded-tut? similar,si m:lar, but not identical to those to the west of the forest already described. The persistentpeflSistent exposure of this tut?tuff in the upper iP3rtpart of thethe scarp,scarp followed by a considerableconsiderable steep descent withoutVVithout rock exposure,exposure :suggestssuggests that the scarp slope is a significantsign: ticant fault zone,zone with.Vith the !inferenceinterence that the welded tufftuft forming the bold feature at the top was1V 1\ emplaced beforehe to e final faulfaultingting

Thelhe porous black ash which occurs widely at the foot of the scarp in the south outcropsout crops over a wideWide area inin the countrycounttV about:1. out Elburgon,Eli: uryon wherewere itit isis seenseen persistentlypc 5 .e~t or V in stream-strcEm~ beds to heights up toto 8,800 ft.ft inin otherwise poorly exposed country.countrV. The topographytopoguphy of the countrysidecount: \side about ElburgonElburuou is ofoi a£1 gentlygcl billowingbloVirr forestedt11.e~t:d slopesope rising1i~ng some 2,0002.000 feet in 10 miles,miles from thethe margin of ,thethe RongaJ.Rongai Plain toto thethe Upper Mau.Man. This 'slopeslope replaces the more precisep. eci so scarpso 1 .rp whichVVhich boundsho 1nds thetie Rift ValleyVa .le» farther:‘:1 her north,no.1‘n and on itsits eastern margin at thisthis latitude.

Inln the innerlower part of the valley of the MoleMolo River,River. where it descends the Rit‘:Rift Valley wall in thethe centre of the area,area bothboth eutaxiticcutaxitic weldedVVelded tufftuft and porous black ash are wellVVell displayed. Here the blackhi1 ck ashash:floorsloo: 5 the river bed,hed. and the eutaxf'ceutaxitic member forms bold features in the valley walls.VValls. Both apparently dip in a generally north-easterlynorth—easterly direction at some 10°.

~ (a)(0) Grey and bleached agglomerates,agglon-zerares, tuffsmﬂs and sediments Bleached yellow,yellow. buff and pale grey agglomerates, v4Tvf, on the map, appear to com- pose the majority 'Ofof the MauMan succession. Usually soft and readily eroded, they do not normally form conspicuous expQSures.exposures. In the upper part of the succession ash-fall rocks are more common, distinguished by narrow banding of contrasting types, but ash-flowash-ﬂow types alsDalso occur. Extensive pyroclastic deposition, and removal, reworking and redeposition by water, occurred contemporaneously. Outcrops of lake deposits were recognized,recognized. and exposures of pebble beds and cross-beddedcross—bedded tutfstuffs provide evidence of sedimentary origin.

Thethe. agglomerates consist of a fine-grainedfine—grained ashy:Lihy base with included rock fragments and generally uncompresseduncompremed pumice,pumice varying in size and proportion, closecIose-packedpacked or dispersed.perSed They occur firstfirst in1n the succession beneath the lowestlowast ash-flowash-flow tuff and persistently recur. Their origin is doubtful,doubtful. but certainly many sources were involved. The'The majority were probably derived fromIrom the MauMan and Mount Londiani, but those in the north-westnorth-west were from Tinderet,Tinderet. and those immediately underlying the latest ashes came from the Rift Valley.

I'llIn the Kedowa district bleached agglomeratesagglomeratcs occur (i)--underlying(if—underlying and intercalated with pale crystal ash-flowash-ﬂow tuffstufis (TvfJ;[Tyfyjn (ii)-sDuth(iiHsouth of the Nyando Valley,Valley. apparently overlyingoverlying both ba'saltsbasalts and phonolites,phonolitcs. and (iii)-in(iiiliin the Kedowa River valley where pale agglomerates probably formfDrm a valley-fillvalley-ﬁll deposi

(a) The south-facingsouth—facing scarp of the Kedowa Plateau (capped by phonolitic trachyte) two-and-a-halftwo-and—a-half miles north-west of Kedowa is composed of variable,variable. friable, bleached,bleached. off-white agglomerates and

(b) A series 'Ofof minor cliff features are visible on either side of the Kericho road between two and four miles south-west of Kedowa. These are composed of coarse agglomerate, locally quarried as a poor-quality building stone. West of the road the outcrop dips gently 202. to 303‘” north-east.

In the quarry three miles west-south-west\y est—south—west of Kedowa the rock is composed of a pale grey base with 2-15cm.Z-lfcm. pumice bombs, locallylocaliy slightly compressed. In the quarry two~two- and-a-balfand—a—half miles west of Kedowa the agglomerate (42/818)(42,0818) carries widely spaced 6-lOcm.6—l0cm. uncompressed black pumice bombs accompanied by smaller fragments, in a powdery grey base with angula:rangular 'Obsidianobsidian chips, Javalava fragments and felspar crystals. Farther west the rock is bleached.bleached, and the texture preserved in shades of cream.cream, yellow and off-white.oft-white.

(c){(1 The possiblepossible valley-fillvalley—fill deposit dsis exposed in madstoneroadstone quarries near the Kericho 11000,read. both north-east and south-westsouth—west of Kedowa.Kedoua, ItIt is variable,variable. but one mile north-eastnorth—east ~f{of Kedowacou'a appears as a lapilli tufftut‘l with an off-whiteoff-uhlte fine..grainedfinegraincd base carrying 1-3em.i-3c1n. ostriatedstriated yellow pumice fragments.fragments To the east of the road this overlies agglomerate wjthwith a grey base and included 8em.8cm. charcoal-blackcharcoal—black bombs and 2-5cm.2—59m. striated yellow pumice fragments.

. j. r

On the Molo.V‘lolo Plateau a hard bandhand within thethe pale tuffsThis and agglomeratesagglonierates is locallylocally quarried for building stone.stone. Itsits outcrop caps ridges and oftenofzen forms aC. minor cliffcliﬁ" feature. Samples were taken from threethree miles north-eastnorth—east (42/929)(—11 seal and six-and-a-halfsix-:.nd-a-hall' milesmiies east-(335(- north-eastnorth—east of Ndoinet Foresti-‘orc‘t Camp,C..n“j:. (42/947),i‘—'“ will. and six miles west-south-west" ' l..t-"" of' FiveT." Mile.‘v‘liic Post. Inin each case:a-e the:he basei. '- is yellow' or buff-grey,' and slightly compressed grey and charcoal-black pi-mieepumice fragmentsl" are commonly about 20cm. long. Felspar crystals and angularannular glassy chipsch occur in the base. The presence of a pebble band through one exposureexposur suggestscsts a sedimentary horizon.ho

At many exposures of bleached tuftstuffs banding and dedefiniteﬁnite sedimentary structures are absent. DefiniteDeﬁnite sediments were recognized in several isolatedi

A large quarry on the Kerisoi-FiveKerisoi—Five Mile Post road.road, ﬁfive-and-a-halfve—and—a—half miles east of KerisoiKcrisoi Forest Station, exposes sediments overlying coa,rsecoarse bleached agglomerates. Dips are «regular,irregular, from horirontalhorizontal to 30°300 south and east. The uppermost unit is a 30—foot30-foo1 band of powdery buff ash with abundant whole and broken concentrically layered, spherical nodules of 1cm.lcm. diameter. The underlying chalky,chalky. cream-colouredCream—coloured ash carries rare nodules and hollow, yellow, weathered pumice ﬂflecks.ecks. This in turn overlies a 4-foot pebble bed (42/867;(42l867; 42/868).42 868).

Similar fifine...gra.ined,ne-grained. buff.buff, nodular tuli‘stuffs are exposed ininaa road-stoneroad~stone quarry fifive-and-ve—and— a-half miles south-Westsouth-west of Five Mile Post. There the exposed succession includes brown and ochreous clay horizons and coloured tutufffi bands, the well dedefinedfined bedding being locaD.ylocally contorted due to slumping (429590.(42/959). Only tracesteams of bedding remain in a swirled and slumped,slumped. gritty.g11itty. nodular sediment from six miles south—wes:south-west of Five Mile Post (42/942). Ripple marks in fifinene ash.ash, and undulating and graded bedding in finefine tuffs,tufis. are well preserved in specimen 42l96842/968 from the river valley fivefive miles west—northwestwest-north-west of Keringet,Keringet. while alternation of grain size in narrow bands is displayed inain a finelyfinely lamin- ,atedated white clay and grit sediment from the river bed two-and-a-halftwo-and--a-half miles south of Molo (42/753).(42“753). Sections through sedimentary horizons are common in the deeply incised river valleys south.south, south—eastsouth-east and east of Molo. It is unlikely that any strict stratigraphical signisignificanceficance can be attached to these exposures of Sediment.sediment. Rather they indicate that the prolonged period of sporadic ash-failash-fall pyroelasticpyroclastic activity was also a period of considerable rainfall. and numerous small lakes were formed on the undulating terrain or:of unconsolidated fragmental debris.debris, material readily amenable to transport and redeposition by small streams. Bodily movement of laJrgelarge quantities of wa.terloggedwaterlogged detritus as mud—flowsmud-flows down gentle gradients is also envisaged. North of the londianiLondiani Plain. in the region of Sorget and Makutsno,Makutano, and towards Maji .‘ilazuriMazuri and the northern boundary.boundary, pale coarse wastuffs and agglomerates with associated sedimentsseddments outcrop on the south—easternsouth-eastern flflankank of Tindcrc:Tinderet and the ridge joining it to Mount Londiani. To the west they overlie fifinene grey and oil-whiteoff-white crystal eutaxitic tuffs.tuffs, and farther east the coarser darker eutaxitic tuffs of thethe M010Molo and Maji Mazuri area. Although probably derived from Tinderet and Mount Londiani, these rocks cannot be di~tinguisheddistinguished from the similar association of bleached agglomerates and pale bedded sediments on the Mau and Molo Plateau.Plateau, with which they are correlated,correlated.

Well‘beddedWell-bedded sequences.sequences, mainly of undoubted watcrlainwaterlain origin from MaliMaji Mazuri and farther east, and a major development of lacustrine sediments from a lake which extendedextended from Londiani to Sorget and Makutano,Mal-tutano. have been delineLeddelineated. These are considered to post—datepost-date the tuftstuffs and agglomerates.agglomerates, and to be largely composed of

r- reworkedrearo. Red detritusde: Fitti's from them. Some justification dsis felt for :hthe(i.- correlation of sediments- occurring along the northern boundary east of Maji Mazuri, along the railway' south!South 1‘5of Maji Mazuri, and on the pla,in north of Londiani, where in each case they are immediatelyi1\ .m . overlain by trachyte lava, with fine..grained banded tuffs of aeolian or lacustrine origin in the road cuttings two miles north-eastpith—Ins of Molo,lo. and indubitable ,gediments in the steep river gully' four-eur miles north-east:1 :1—e1st o:of M'Olo,\felc bothhe- 11 eexposureso: again 1, ._ » overlain by3' “4‘15.lava. The1.: correlati'On.- ion may13}: be furtherrutncr extended to includeac.‘udc sedimentaryse... ni-etitary'

horizons_ . s high in the sesequenceLzuence onen the MoloHolo Plateau,l’l...... _1u including those L‘Knoiedexposed six- )1 miles east of Kerisoi lorestForest StationStation, and four miles south—south-south-westsouth—west of Five Mile Post.Post, both outcroppingouteropping at about 8,9008900 ft. These sediments were deposited in didifferentﬁerent lakes and ponds,p1 nos at differentdiffered: altitudes,aitizudes. buttut appazentlxapparently at much the Samesame time.

(13)(b) Black ashes of Elburgon Th:seThese roclLsrocks occur widely on'On the gentle slope of the Rift Valley Wallwall in the southern halfhall of the areaarea, where they are banked against a terraced scarp.scarp, 'Overlyingoverlying :hethe grey eutaxiticeutaK1tic welded tuffstutis and yellow tuftstuffs and agglomerntcsa~I'Omerates of which the Mau‘lau is composed.co mposed TheirThe} ho..1ogc..city.lhomogeneity, lack of bedding and columnar jointing suggest a:an ash-flowash-1‘1a orig-in.origin, theteh lloflowsOWS probablypr'Obably emanating from the scarp faults. Previously (McCall.(McCall, 1957A.1957A, p. 61) theythe' ' had been ascribed to ana. ash-tallash-fall origin from a sourcesource at Menengai.

Due to their soft,soft. readily weatheredWeathered nature and overburden of thick soil andand more !recentrecent ashes.ashes, they are generallyge-ncrallj.‘ seen only in streamstream beds, in which the stream is often incisedincised in a small gorge. They are easily w'Orkedworked and are often cut into blocks for useuse as poor-gradepoor—grade building stone.

Specimens “erewere examined fromfrom a:1 quarry near the road bridge on the M'OloMolo River three miles south-west of Rongai (42/761),t42,-"T6]i, k'Omfrom beneath a lava flawﬂow in a tributary to thethe Molo‘x‘lulo River six mi1esmiles south.-westsoufﬂe-est of Rongai (42/801),142-8131). from the RangaiRongai River bed at Elburgon Forest Station (42/984),{4239841. four miles north—eastnorth-east of Elburgon (42/991),(42.3991), from the quarry at Nessuit Forest Station (42.51004),(42/1004), from the Njoro RiyerRiver two miles south- east of Nessuit (42/1009)[42'1011191 and six miles south 'Ofof Nessuit (42/1012).(42 it‘ll)

Due to their porous texture and the nature of their exposure in rivL-rriver beds these roerockss usually appear in the ﬁfieldeld as saoty-blacksooty—black waterlogged material. The ash is occasionally sculptured-CLilpTUted by ri'ic.’river actionaction, but maremore oftenotter: occurs as billowingtillowine platforms with rare columnar jointing producing a poly:-polygonalnal pattern on the surfacesurface, with a cell diameter ofor about 25cm.25cm., well displayeddispl 1 ed in the Molo\tolo River four miles southsouth-westwest of RongaiRongai, and the .NjoroNjoro River near Nessuit.Nessuit, Small unterfallswaterfalls are common in the rivers,riycrs, and locally the stiastrataLa apnm'entlyapparently dip north-east at about 10':10°.

On drying out thesethat: rocks display a dark grey or purple friable, gritty base, usually fleckedtleeked with small rock pelletspellets and abundant glassy felspar crystals often attaining a lengthlength of 4mm. Conspicuous but rare included pumice fragments have roundedrounded irregular shapes,shape . commonly lcm.tern. in diameterdiameter, withAith a largelarge-scalescale vesicularyesicula.' szrstructure.more. V1Weldingelding in the matrixmatrix is just sufficientsuﬁiclent to cause the pumice fragmentstracments to be bisectedbfseeied as the rock breaks.breaks. inIn unweldedunneided ashes breaksbreaks occur roundtour (:1 rather than through included pumice fragments.tragments.

fuin thin sectiansection partial devitrificationdevitritication of larger pumice fragmentsfragments can be recogni2',ed.recognized. and some showShow limited compression. The groundmass is composed almost entirely of well dedefined,ﬁned. uncompressed glassy shardsshards, and many pore spaces carry minute c;crystalsystals of vapour phase minerals, aegirine-aegirine-augiteaugite and felspar. Uncompressed ovoid pumice patches occasionally includeinclude felspar phenocrysts, which like those of thethe base are whole or fragmentedfragmented sanid.inesanidine crystals, and fine-grainedﬁne-grained peHetspellets of trachy.tictract-Lyric lava Meare common.

- (c) EutaxiticEuraxz'tie welded tutuffsfis Under this heading are grouped a series of welded pyroclastics of varying aspect of colour, texture, pumice content, degree of compaction and devitrification.devitrification. In no single exposure is a large enough cross-sectioncross—section seen to enable complete correlation to 'be'be made with other exposures, since the rocks appear to vary progressively both later- ally and ver:tica1'ly.vertically. 11N-eeThree main types were recognized-(i)recognized—(i) Coarse-grainedCoarse‘grained eutaxitic welded 1Juffs;tufts; (ii) Fine-grained 'greygrey tuffs;tufis; (iii) iMediium-grainedMedium-grained blue and purple tuffs.tufts.

(i) Coarse-grained eutaxiticeutaxi'ti'c welded tuffstuﬁs Rocks of this group were found in the valley of the MaloMolo River four miles north- north-east of Molo, at several localities in the Molo-Turi area at varying altitudes to about 8,600 ft.,ft, in a quarry 'One-and-a~halfone—and—a-half miles south-west:south-west 'Ofof MalaMolo at 8.4008,400 ft.,ft. four miles westWest of Rongai at 6,700 ft. and at varying altitudes in the scarp above the last mentioned exposure. The ash-flow clearly had its source or sources in the Molo area,area. and flowedﬂowed generally eastwards. In 11hethe Mol'OMolo River valley it directly overlies the black ashes of Elburgon. Almost identical types are exposed on the westWest flank of Mount Londiani and in the Makutano-Maji Mazuri region in the north-west of the area, and around Nessuit in the south-east.

The Molo River tuff,ruff, specimen 43/802,43l802. carries devitrifieddevitrified pumice fragments up to 30mm. long varied in streaks 'Ofof pale and dark grey along their lengths, often with elongated central cavities. Exotic green, grey and yellow lava inclusions, partly rounded,rounded. reach diameters 'Ofof 15mm. The ashy matrix, pale 'Oror khaki-grey in colour,colour. is crammed with red, yellow and white grit particles. In thin section the compressed pumice shows axiolitic devitrification,devitrification, with microlites up to 2mm. in length, and interstitial cavities have mingesfringes of mioro-crysta1smicro-crystals of aegirine and cossyrite. Cry&talCrystal inclusionsinclusions include eu-eu‘ hedral sacidinesanidine and skeletal grains of opaque j;roniron ore. The matrix is mainly glassy, with shards and contorted devitrifieddevitrified lenticles.Ienticles.

Rocks from the northern exposure,exposure. specimens 42/90842;“908 from the roadside half-a-mile south-westsouthmes; of Maji Mazuri and 42/899423899 from a small quarry six miles west of that place, are pale green-grey in colour with darker elongate bifurcating lenseslenses up to 40mm. long and included lava frngmentsfragments up to 15mm. across. The glassy matrix carries lathsIaths of simplysimply twinned felspar. Both the Maji MazunMazuri and the M'OuntMount Londiani tuffs, which are 'Overlainoverlain directly by Londiani trachytes,trachytes. have a thin, interbeddedinterhedded layer of waterlain ashes and tuffs.tuils.

(ii) Fine-grainedFine—grained grey tutuffsﬂs This variety,variety. a relatively thin unit near the base of the welded tufttuff succession.succession, out-aut- crapscrops widelyWidely due to its resistance to erasian.erosion. Itit was mapped in a series 'Ofof exposures around the MolaMole Plateau and the MauMatt Farest.Forest.

A typicaltypi~l specimen, 4294642/946 from four miles south-west of Five Mile Post.Post, has a fine-grainedFine-grained grey basebare packed with grit-sized particles of'Of included phonolitic and trachytic lava, occasianaloccasional irregular chipschip; of black glass.glass, and pumice fragments. The lava inclusions have an exterior alterationaIteration colouring of'Of yellow.yellaw, brown or'Or grey. Rare chips of black glass are roughly equidimensional up to 3mm. in diameter.diameter, and are frequently set obliquely'Obliquely to the general compressional fabric of'Ofthe rock.

In well-compressed specimens.specimens, suc'hassuch as 42.597442/974 collected ﬁfiveve miles south-west of'OfKeri- ngct.nget, collapsed pumice formsforms glass}glassy streaks up to 15mm. long, but only lmm.Imm. thick. OccasianalOccasional included felsparlfelspar crystals are seen, and finelyﬁnely striated hollows in the ground- massrnass appear tato be impressions 'Ofof plant stems.

18l8

~ -' Specimen 42/97642,1'976 from the road—riverroad-river crossing two miles south of Kiptunga shows in thin section a completely glassy base packed with pellets of rock up to 2mm. in diameter, mainly of phonolite and trachyte, though one such peYetpellet shows a multiple-twinnedmultiplewtwinned crystal of labradorite.labradorite, Abundant angular chips of glass occur, and compressed devitri-devitri« fiedtied pumice lenses are accompanied by small felspar fragments and occasional minute prisms believed to be aegirine.

(iii) .Medium-grainedMedium-grained blue and purple rutuffsﬁs Attractive blue and purple building stones are exposed at several localities among the rounded hills north-west of MOlo.Molo. Specimen 429698,42/698, from four miles north—westnorth-west of Molo, is of a purple grey colour, withMth a medium grained matrix with rounded pellets of lava sometimes exceeding IOmm.10mm. in diameter,diameter. compressed pumice lenses up to 30mm. long, occasional felspar crystals, and a scatter of yellow, white and red inclusions. lnIn 'thin section the matrix is seen to consist of closely packed devitrified shards and an abundant scatter of small prisms of green aegirine. Compressed pumice lenses display tasselated ends, and devitridevitrificationﬁcation has led to the development of sheaves of microlite—smicrolites lying perpendicular to the margins of the lenses. Such lenses sometimes have hollow cores with vapour-phase crystals of aegirine and felspar.

(7)(Ti SEDIMENTSSroisinxrrs OFor LONDIANILONDIAN] AND MAlI.VlAJI MAZUR!MAZURI The rivers incised on the Londiani Plain expose bedded sediments,sediments. which include bandshands several feet thick of compact chalky tuffstufis and claystones, some carrying nodules up to IOmm.10mm. in diameter,diameter. white laminated claystones, friable khaki and ochreous soils,soils. and well-bedded sandstones, grits and gravel deposits,deposits. white, buff,buﬁ, yellow and orange in colour.

A compact nodular chalky clay forms the diminutive hillhilI on the plain four miles north of Londiani, and bedded sandstone, grit and gravel-beds form a series of rapids in the river five miles nonthnorth of Londiani. Current-bedding is visible in the sandstones at the road-river crossing two miles north of Londiani. In ,thethe bed of the river draining an embayment in the lava boundary four miles north-east of Londiani the forest track crossescrOsses an exposure of some 40 feet of bedded sandstones, including bands sufsufficientlyﬁciently compact to form a 16—foot16-foot waterfall in the river.

Natural exposures in the sediments of the Londiani Plain are poor, but the road leading frlOmfrom Makutano Sawmill to the forestforest station follows the line of a disused fuel siding, dating back to the time railway engines burned wood fuel. The rail-cuttings display wcll—exposedwell-exposed sections of bedded sandstones and clays, nodular milstuffs and poorly sorted agglomerates, some with highly contorted bedding resulting from slumping (42/883).{41!883).

1nIn several localities ritersrivers flowing off the lava-covered slopes of Mount Londiani plunge as waterfalls on to underlying sedimentssediments. Inin such an exposure, west of Kilombe and fivefive miles east—northeasteast-north-east of Sabatia, the river falls some 60 feet. The river, already wellweil incised in a lava flow, cuts through some 30 feetfeet of lava,lava. 10 feet of ropeyropcy contam-contam— inated lava (the(1Jhesolesale of the flow), 10 feet of terra cotta soil (the baked upper tuffs)tufts) and 10 feet of poorly sorted yellow tutuffaceousflaceous sediments,sedirnenis. after which the river bed.bed, still descending rapidly,rapidly. is lost beneath rubblerubble of fallen lava blocks. Rocks from the valley three-and-a-half miles east of Sabatia where aa north—north-flowingflowing river etches a fault line, consist of bedded sandstones with occasional harder water-lainwater—lain tuff horizons. Lensing bands of sandstone,sandstone. grit and gravel are well displayed. Another inlier of sediments three miles cast—northeasteast-north-east of MajiMaii Mazuri consists of a variety of welil-beddedwell-bedded 'grits,grits. sandsmnes.sandstones, pebbles and laminated clays, some contorted by slumping.

- "

Buff, yellow and orange bedded sandstones.sandstones, ciavsclays and pebble-bedspebble—beds outcrop in thethe r-ail-cuttingsfflii-Lllllings antiand 5:1‘Ami3lsawmill Ittracks"i s :.tat Maji.N‘fii'ii Mazuri.,‘\f,=:‘::rf. Inin the track pavingpassing beneithbeneath the.he rstil-railway half-a-milehalf—a-mile sauthsouth of Maji.‘Vlaii Mazuri,.‘vlazuri. erratic one-:tnd-zi—haifone-and-a-half and two-foottwo—foot boulders are included in the Iinelyfinely bedded sequence. Resulting from epO‘onsexplasions which preceded the lava flflows,ows. the block's.blocks droppedsped on tota the unconsolidated 55L.“sedimentstents and sank into them.them, distorting the bedding tgito a depth of'Of:hreethree fset.feet. The junction between phanolitic-tmchytephonolitic—trachyte lava fl'OWSflows and underlying sediments is seen again in the rail cuttingscut‘ is. between MakutanoMam-tend and MajiMafi Mazuri where the rail aIignmentalignment foHowsi‘oilmxs the lavalam boundary.lituntlary. At the bend two miIesmiles south-westsouthwest of Maji\leii MazuriRfazuri 15 feet 'Ofof poorlypoarly s'Ortedsorted sediments withwi,th occasi'OnaJoccasional erratic boulders is overlain by fifiveve feet of'OfWell-beddedwell-bedded sandstone and capped by :i':i:hyte'trachyte lava. On the western boundary three-and-a-halfthree—and-a«half miles south-west of'OfS'OrgetSorget a deeply incised riverriver flflowsows down a gently terraced bed cut in finely laminated Whitewhite clays at 7.830.7,800 ft. No rock exposures were seen in the steep valley walls. Similar aeolian or lacustrine,liicuitrine. .... fifinelynely banded deposits were encounteredencauntered in a glade 'Onon the ridge a:at :1a height of 8.0008,000 ft. three miles south-west of Sorget.S'Orget. NcNo fossil rema.insremains were foundf'Ound in any of'Ofthe sediments mapped.

(S)(8) :‘xixsmALKALI BAsALTSBASALTS OFor MoroMOLO PLATEAUPLATEAL‘ A flfl'OWow of'Ofblue grey non-porphyriticnan-porphyritic alkaline lava overlies'Overliesa tuffaceous horizonhcrizon on'Onthe interfluveinterfluve between the NdoinctNdainet and SongonSong'On rivers three miles east of'Of Ndoinet.Ndainet. Down- stream 'Ofof the road bridge 'Overover the Ndoinet River one-and-a-halfone—and-a-half miles farther east.east, 'thethe granular lav'alava has a vesicular texture and a silvery sheen. A porphyritic alkaline lava was C'Ollectedcollected fromfr'Om the dam spillway fivefive miles n'Orth-westnorth-west of'Of Keringet. In hrindhand specimen the lava fr'Omfrom the first-mentionedfirst—mentioned locality, 42,42/948,948. felsparfclspar phenocrysts up tato 4mm. longlang occur. In thin secti'Onsection small anhedral olivine'Olivine phenocrysts are associatedass'Ociated with grains of'Ofblack iraniron ore;ore, which alscalso occurs as isisolatedalated euhedral and skeletal crystals. Occasi'OnalOccasional prisms oi'Of augite attain sizes of'Of up tot'O 1.5mm. and large fclsparfelspar phenccrystsphenocrysts with carlsbad and fifinene cross-hatchcrass-hatch twinning provepr'Ove tot'O be anorthoclasean'Orthoclase with a rather high optic—axial'Optic-axial angle. The groundmassgr'Oundmass carries abundant small lathslaths of'Of sodic plagicclase,plagioclase, and the interstitial material includes pyroxene,pyrcxene, soda—amphiboles,soda-amphiboles, calcite and prcbablyprobably analcime. AncrthoclaseAnorthoclase phenocrysts in a pale, granular, silvery grey lava frcmfrom the second locality (42/950)(43950; occur in clusters. They display res'Orbedresorhed subhedralsubhcdral shapes,shapes. fine-grainedtine«grained cross-hatchcrossehatch twinning and a rather high 'Optic-axialoptic—axial angle. N'ONo 'Olivineolivine was seen in this example, but hlackblack iraniron areore and prismatic augite phenccrystsphenocrysts are present in a:1 base of'Of cl'Oselypackedclosely packed sodic-plagioclasesodic-plngioclase laths,laths. granular pyrcxene,pyroxene. iron 'Oreore and sodic amphiboles.holes. (9)(9| BASALTSB.\s.ii.rs OFor MAIlMoi MAZURI A small lcbelobe 'Ofof basalt lava was loca.tedlocated in ,thethe forest 'Oneone mile southS'Outh'Ofof Maji Mazuri. The lava 'Overliesoverlies waterlain tuffstufts and ashes displayed in tracks and railway cuttings, and is directly 'Overlainoverlain by trachyte t'Oto the east and south. This black lava‘lava, with ovoid vesicles aOmm.20mm. in diameter.diameter, contains pyroxene crystals 3mm. in length and less commoncammcn glassy green 'Olivine.olivine. In thin sectionsecticn 42,“42/907907 the ferro-ferra- magnesians are accompaniedacc'Ompanied by labrad'Oritelabradorite phen'Ocrystsphenocrysts in 2.5mm. laths.laths, set in a groundmass 'Ofof plagioclase, pyroxene and black iron are.ore.

(10) TINDERET PHONOLITESPHONOLUES Not all 'Ofof the phonoiitesphonolites shown as Tinderet phonoliteph'On'Olite tTvpé)(TVP2) in the accompanyingaccampanying map can be shown to have been extruded fr'Omfrom Tinderet Volcano,V'Olcano, and tot'O fall within the originalgrcuporiginal group 'asas defineddeﬁned by Binge (1962).(1963. Other CXposuresexposures at Mount Blackett,

~ south-east 'Of Tinderet, and in the s'Outh-east and north-east corners 'Of the map are includedinc here for cornconvenience, since they are very similar to the true Tinderet phonolites,lies. and possiblypCT-Slhlju' of the same age.

A typical specimen 'Ofof this rockrose from the slopes of Tinderet five miles north—Westnarth-west of MakutanoMakutana (42,989?)(42/897) is a blue—black.blue-black, ﬁfine-grainedne—grained lavnlava with anorthoclaseanarthoclase and rare nephe—nepheline phenacrysts.phenocrysts. In1n thin sectionsectian the ﬁfine-grainedne-grained groundmassgraundmass is seen tota consistcansist of'Of a teltedfelted mat of'Ofanorthoclase felspars with interstitial eossyrite,cassyrite, kataphoritekatapharite and amorphousamarphaus nepheline.

The steep—sidedsteep-sided fIat~oppedfiat-topped MauntMount Blackett,Blackett. south 'Ofof the Mau Summit-LondinniSummit-Londiani road, risesrises several hundreds 'Ofof feet aboveabove the surraundingsurrounding countryside. It is composedcampased entirely 'Ofof uniform,unifarm, fine-grainedfine-grained phanalite,phonolite. and despite the finenessfineness 'Ofof grain 'Ofof the rackrock it is cansideredconsidered to behe a phanolitephonolite ping.plug. A specimen 'Ofcit” the rock,rook. 42/726,41726. shows sparseSparse euhedral phenot‘r}.stsphenocrysts of nepheline and anorthoclase set in a greenish black gtttti’ftlmllssgroundmass seen in thin section tota be crammed with minute crystals of nepheline and carrying trains of'Of flow-alignedflow-aligned felspar laths, with patches of'Of cossyrite and kataphorite. The 'Onlyoniy occurrence 'Ofof phonalitephonolite in the sauth-eastsouth-east carnercorner 'Ofof the map are:area is a bluttbluff 'Ofof lavalava amangamong the weldedhe led tuii‘stuffs of the Mau\iau Escarpment faurfour miles south-south-westsotithsouth-‘tvest 'Ofof Nessuit. The extent of the visible autcrapoutcrop is limited by heavy forest cover. The rock,rock. specimen 42/1014, cantainscontains euhedral anorthoclase phenocrysts 8mm. in length with carlsbad twinning visible Itato the naked eye. The groundmass, rich in grains 'Ofof nepheline, contains dispersed crystals 'Ofof cossyrite and kataphoriltekataphoriite and poikilitic plates of barkevikite.barketikite. A basic Clotclat cut by the thin section contains a core 'Ofof 'Olivineolivine altering mar-m3r~ ginallygin-.1 ly tota iddingsite Lindand with a rim 'Ofof harRCVikite.barkevikite.

A number 'Ofof small hills rising above the plain in the narth-eastnorth-east of the area are horsts of'Ofalderolder rock uplifted by gridiaultinggrid-faulting and partly buried by late pyroclastics. The outcropoutorop three miles southeastsauth-east 'Ofof MogotioMagotio (42/995A)(420953“) is of a granular silver grey porphyritic rackrock with rare aegirine—augiteaegirine-augiJte set in a ﬁfine~grainednevgrained matrix of trachytic Ite~turetexture of'Of anor-anar- thaclase,t'noclase, cossyritetcossyrite, kzitaphoritekatapharite and iraniron ore.are.

[11)(11) NYANDO VALLEY PHONOLITES ExpasuresExposures of'Of blue black phenoiitephonalite seen in the NyandoNyanda River bed one-and-a-halfane-and-a-half miles north-westnarth-west of'Of Kedowa immediately overlie'Overlie a finely bedded sequence of tutl‘s.tuffs, Claysclays and SUNS.sails, with a baked upper member Sent.8em. thick and terra-cotta in colour.calaur. The phano-phono— lite base dips west and lies below 6.7006,700 ft. at the road—riverroad-river bridge two-and-ahalftwo-and-a-haIf miles northwestnarth-west 'Ofof Kedowa. The blue black phonolitephanolite is overlain'Overlain by trachyte.trachyte, and in the railway cuttings one'One mile west atof Kedowa there is a thin intermediate layer of \ttewhite tut?tuff and agglomerate.agglamerate. ToTa the west of the river phanalitephonolite reaches a maximum altitude of'Of 7,100 ft.

Farther west.west, bothbath northnarth and sauthsouth of the NyandoNyanda River.River, the phonolitephanalite overlies'Overlies phono-phana- liticIitic nephelinites, a relatianshiprelationship again demonstrateddemanstrated at the boundarybaundary of a smallsmail inlier 'Ofof phonolitiephonolitic nephelinite Inin the stream bed twotwO' miles West-south-westwest-south-west of'Of Kedowa. Ex- posurespO'sures dedefiningﬁning the phonolite boundary in the railway cuttings three miles north—westnorth-west ~ of KedawaKedowa alternate between phanalitephonolite and phanolirtJicphonolitic nephelinite, and onan the hilltophilltap 1,000 feet higher the phonoiite is overlain by traehjt‘te. I 1,000 feet higher the phonalite is 'Overlain by trachyte.

South of'Of the Kisumu road,raad, phonolitephanalite 'Overliesoverlies phanaliticphonolitic nephelinites and bleached tuffs, and is itself 'Overlainoverlain tato the narthnorth 'Ofof the raadroad by trachyte which produces a crene-meme- latedlate-d scarp-line with exposuresexpasures indicating a general sautherlysoutherly dip. Evidence fromfram the roadstaneroadstone pit northnarth 'Ofof the road, 'Oneone mile east 'Ofof thethe baundary,boundary, suggests that the phano-phono» lite outcrapoutcrop consists 'Ofof at least twotwa flaws,flows, divided by a coarse agglomerateagglamerate or'Or baulder-boulder- bed horizon.

21 The phonolite of thethe Nyando Valley one mile west of Kedowa (42.3879)(42/879) is a dark blue black lava, with'With hackly fracture and abundant phenocrysts. Felspars dis- playplaya a typically plate-like form 10mm.tOmm. long, and nepheline crystals 4mm. in diameter occur singly or in clusters. No ferromagnesian crystals can be seen in hand specimen. Phonolite from the top of the hill four miles south-east of Londiani (427815)(42/815) displays a smoother fracture but contains similar phenocrysts. MicrophenocrystsMiorophenocrysts cut in thinthin section are biotite and twinned euhedraloohedral augite. The phonolites of thethe scarp north of the Kisumu road carry abundant hexagonal biotite flakes 4mmi4mm. in diameter in addition to phenocrysts of nepheline and felspar.

(12}(12) TRACHYTESTRAcHYTI!s noAND QUARTZ TRACHYTESTRACHYTI!S 0FOF LONDIANI AND KILOMBE Mount Londiani (9,872 ft.) stands on othethe waternwestern shoulder of the Rift Valley justjoustsouth of the equator. The lower slopes of thethe mountain,mountain. to the west.west, are composed of ﬂflowsows ofof trachyte lavalaw: which extend as far west as Londiani and Maji Mazuri.Mazuri, and are bounded by a prominent bounded by a prominent scarp8CaqJ feature whichwhioh can be traced almost continuously be—be- .... tween these two towns. The lava reappears two miles Westwest of Londiani capping the ;phonolite:phonolite and agglomerate hills north and south of the Kisumu road as far as the Westernwestern boundary.

To the south of Mount Londiani the trachytetraohyte outcrops do not extend beyond the line of the MoloMole River, whichwhioh catchesetches the lamJava boundary and has cutout a deep gorge in the softer pyroelasticpyroclastic rocks beneath. Lava outcrops are sporadically exposed betWeenbetween the road cut—cutting west of the Jolly Farmer Hotel north—eastnor-th-east of MoloMalo (7,900 ft.) and the valley floorﬂoor (6.500(6,500 ft.) inin billowing slopes derived from the originalol1igina[ dip of the ﬂflowsows into the valley, together with limited later fault movement.

The steep 5CaIrpscarp slopesIOpe running north-south from Kilombe Hill to the Nakut‘u—MakutanoNakuru~Makutano main road is interpreted as a lava-faced scarp. No bold fault-stepsfault—steps are seen: the original major fault line must run to the east of the foot of the present lpe.slope, passing beneath the col between Mount Londiani and Kilombe and is now covered by the mantle of later pyroclastic rocks on the Rift Valley floor.

Kilombe Hill consists of a well-preservedwell-preserVed vent,vent. with crater,crater. composed mainly of lavaslRV'as .indistinguishableindistinguishable from those of Mount Londiani. It is situated astridesstride the main Rift faulting in this area (McCall, 1964).19643. To the north some ﬂflowsows of Londiani and Kilombe trachyte entered the Eldama RatincRavine area, where they were mapped under the lakeLake Hannington phonolite group (Walsh, 1969).

TypicallyTypicaily these rocks appear on freshlyfresWy broken surfaces as granular blue grey lavas, with glassy felspar phenocrysts showing rectangular sections. Weathered surfaces usually display a typical satin-like.satin-like, silver-greysilver~grey trachytic sheen, but occasionally weathering produces a ﬁfinelynely mottled appearance with minute flecks of dark brown, khaki and green.green, giving a general “heather—mixture""heather-mixture" effect and an uggregatcaggregate colour of pale brown or brown-grey.brown—grey.

The vast majority contain pomssicpotassic felspar (5:1nidine—anorthoclase)(sanidine-anorthoclase) as occasional mac- rophenocrystsrophcnoerysts and abundant microphenocrysts,microphcnocrysts. accompanying sodicsodie ferromaonesfanferromagnesian minerals (cossyriuz,(cossyrite, ketaphoritekataphorite and sometimes aegirine.aegirine, negirine-augiteaegirine-augite or barkevikite),rvarl-{evikiteh often black iron ore, and rarely biotite,hiotite, inin a variable amount ofof‘ ﬁfine-grainedne-grained intergrowninter‘grown interstitial base,base. predominantly felsputhic,felspathic, -inin which neither quartz nor nephelinenephcline can be recognized.

In some rocksFOClLS the ﬁfine-grainedne—grained base appears to behe composed of as ousrtz—felsparquartz-felspar inter- growth,growth. and in others quartz occurs in optical continuity infillinginliilfng minute, widely spaced irregular interstices,interstices. In others ophitic plates in the groundmussgroundmass are accompanied by small clear pools of quartz. In one of the specimens collected olivine was recognized.recognized, indicating aa tendency to undersaturationunderssturation among occasional rocks of this group.group,

f"'II (a) Trachytes The only specimen of trachyte showing undersaturation in sil[casilico collected within the area is a rock carryingca'ITYingrare oliyineolivine phenocrysts.phenocrysts, from the forest boundary fifiveve miles west of Rongni.Rongai. In hand specimen (42/766)(-12 766! the rock has :1a mottled green grey base and caTriescarries abundant felspar phenocrysts. The felsparfe1spar macrophenocrysts occur singly or as clus—clusters of .pTismsprisms and commonly attain dimensions of 2.5 x Imm.1mm. They consist of simply twinned euhedral sanidinc—anorthoelasesanidine-anorthoc1ase in a flflow-alignedow—ziligned base of smaller lalths.laths. Aegirine—Aegirine- augite prisms.prisms, 0.2mm.O.2mm. in diameter.diameter, often associated with grains of black iron ore,ore. Showshow diagnostic cross sections.sections, and weak pleochroism in shades of pale ,green.green. 'l'heyThey are surrounded by a sheath of alteration to yellow-green highly birefringent amphibole, and isolated laths of the latter mineral occur in the felspathic base accompanied by a copious scatter of flakes of cossyrite and kataphorite. Olivine crystals appear in the slide as a single cluster of small fresh euhedra individually attaining diameters of 0.3mm.O.3mm. withwhth ,rMerare cracks and narrow blackened ma.:ins_maTgins. ~ The lava from the main road four miles north of ManMau Summit (423759)(42/759) has \rea-weathered to a fifine-grained,ne-grained, granular.granular, pink rock. The sanidinevanorthoclascsanidine-anorthoc1ase phenocrysts, 'partly resorbed, are acoompaniedaccompanied by several types of sodic amphibole in thethe base. Kataphorite, markedly pleochroicplooClhroic from bottle- or blue-green to khaki- or olive-green to purple-pUl'p'le- or greenish brown.brown, occurs as dispersed interstitial flflakesakes or poikilitic plates, as does cossyrite,COISsYTite,pleochroicpleocmoic from pale buff or straw-yellow to chestnut or russet-brownrusset-brown to deep chocolate brown. often nearly opaque. Most common are poorly deVelopeddeveloped prisms of bright fawn poorly-pleochroic:poorly-pleochroic barkeyikite.barkevikite. The groundmass is a fifine-grainedne—grained mosaic of felspar. The lava in the railway cutting on the northern boundary, oneande-halfone-and-a-half miles north of \‘lajiMaji Ma/uriMazuri ['42(42/916)“Ulfil appears as a sparsely porphyritic Streakystreaky grey rock.rock, and thatthat in the,the forest two miles south-Westsouth-west of Sabatia (423918)(42/918) is glistening dark grey granular and porphyritic. Both have resorbed microphenocrysts of sanidineenorthoelasesanidine-anorthoclase Whichwhich display progressive extinction across the crystals. Cossyrite,Oossyrite, kataphoritekatapborite and barkevikite are present in both. The rock from .thethe lip of a waterfall four-and-a-half miles east of Sabatia (42,9919)(42/919) is more abundantly porphyritic.porphyritic, withWith common 3mm.8mm. sanidine-anorthoclasesanidineanorthoclasc crystals accompanied by ragged microphenocrysts of aegirine-augitenegirine-augite and grains of black ironiron ore in a base of fine-grainedfine-grained felspar and dispersed interstitial fragments of optically continuous cmsyritecossyrite and kataphorite,kataphorite.

The silvery green grey lava from the hill north of the waterfaHwaterfall on the rim of KilombeKilomhe Crater <42(42/797)79.71 is indistinguishable from many of the Mount LondianiLondianf lavas,lavas. withwilth large euhedral sanidinehnorthoclasesanidine-anorthoclase macrophenocrysts,macrophenocrysts. and abundant smaller partly resorbed flow-aligned sanidine-anorthoclasesanidinehnorthoclase laths in a base characterizedcharaoterized by sodicsorlie amphibisles,amphiboles. Other tine-grainedfine-grained rocks from the Westernwestern rim.rim of Kilcmbc.Kilombe, such as 41795,42/795, include types with only sodicsorlic amphiboles and felsparfclspar prisms in a fine—grained.fine-grained, leucocra~leucocra- tic.tic, mosaic base,base. andand a streaky purple and fawn lavalnya 142.5794)(42/794) of interest because its streamedoutstreamed-out vesicles carry marginal fringes of felspar laths and ncicularacicular prisms of a yellow green,green. highly birefringent ferromagnesianferromagnesizzn mineral,mineral. probably aegirine-augite,aegirinc—augite. possibly the result of crystallization fromfrom the vapour phase.

(b)([2) QuartzQuart: trachytesrrar'ltytes (Pantelleritic(Palitelleriric trachytes)[rat/tyres) ~' The lavalam outcroppingoutcroppfng on the lowerlatter slopeSlope of Mount LondianiLoneirini three miles north-east of MauMan Summit (42/719)(425719) is sparsely norphyritic.porphyritic, granular,granular. mottled and khaki coloured. The thin sectionsection shows a texture of flow-alignedflowmligned microphenocrystsmicrophenoerysts of sanidine-anortho--senidine-nnortho- [4 l clase,close. accompanied by sodiesodic amphiboles. KataphoriteK :aphor'ite forms largerlarger crystals than in most rocks,:0 * occasionally attaining a length of OAmm.0.4mm, The rockrock also contains rare anhedralanhcdral quartzquart? crystak.crystals.

23 Lavas displayed in cuttings along the new Kericho road between Mau Summit and the LondianiLendiani turn-offturn—oil all contain small amounts of quartz ini: the matrix. They are otherwise indistinguishableindistinguishable from more normal members of the group. From a bluffbluf‘t' above the railway one mile north-westnorth—west of Mau\‘lau Summit the rock (42/714)['42‘1‘7‘l4] is a medium green grey porphyriticporph31ritic lava wirthwith small iron-stainediron—stained felspars.felspars The weathered rock has a pale powder-green mottled appearance with 5mm.5mm patchespa ches stained chocolate brown.brown Sanidine- anorthoclaseancr thocla e phenocrystsphenocr35ts occur in a trachytictrach3'tic texture of imprecisely twinned laths of felspar,lelspai'. and cossyritect1ss3. ite and kataphoriteltata phorite are accompaniedacsompanizd byI131 poorly1110113 preservedp1'ese1'31cd flakesflake: of brownbroown biotite.bi ot itc Quartz occurrs as opticallyOptically continuousconti nuous interstitial infillingsinli l1 ings in the ground-g 'ound— mass.mas In some'ome sections the sodic-amphibolessodic— amphibol es areare amplifiedampliﬁed by barkevikite,barkevikite. and poorly preservedpre.';er3ed subhedrasubh;d: a of aegirineaegiiine occasionally occur.

The most extreme member of this group,group. specimen 42/804,425804 forms a slight bluff northorth ofoi” the river,ri3'.er one-and-a-halfone-11nd a halt milesniiics north-westnorth—west of Kedowa,Kedowa where it appear:appears as a fine-grained,fine grained, glistening,glistening grey porphyriticporphyritic trachytetrach3'te with occasional vugs filledfilled with waxy zeolites. The euhedra]euhedral macrophenocrysts of sanidines1an dine 2mm. long and flflow-alignedow—aligned second- generation sanidine laths O.5mm.0. 5mm long are accompanied byb3 cossyritecoss31rite and small prisms of aegirineac girine in a fine-grained,Iine~gi :1ine1 locaJlyl11:".il1.' zeoliticJet1lit:c groundmass.grcuadmass. CrystalsCr3's 113 s of 1'111.art/.quartz, occasionally 33ithwith poikiliticpoi :‘itic’ relationrel?ton to small prisms1.1ri sins of aegirine,acgi lite. to.form111. rare poolpoolss in the groundmass,mass and display occasional crystal faces against euhedral zeolitezeolite, fifillinglling irregular vugs. The vugs,vugs of elongate sh'shape,pe have scalloped margins and attain lcnuthslengths 0of 12mm. Euhedral acicularacicu': 3r aegirine(18:.1i1 inc prisms,prisms. andandllessess common felsparfelsparllaths,atah protrude from the groundmass of the rock into the vugsvugs, which have a complete narrow lining of zeolitezeolite, probablyprobztol3 analcime,anacime. aligned'1h1'1ed perpendicularpct'pend' cular to the vug311g walls.33' LlilS The bulk of the ca3'-'it3cavity is filled by spherular aggregatesaggreg ates of clear zeolite,zeolite possibly naltrolite,natrolite with central small mosaic patches of quartz.

Further evidence of local oversaturationo3ersaturation nithwith Silicasilica is provided by the composition of the syenite boulders which strew the ﬂflanksanks of Kilombe.Kilombe In hand specimen this holo~holo- crystallinecr3stalline rock (421'(42/793)793) is apparently composed of twot33o main constituenconstituenlts,.s prismatic white31‘1111:elsp:1rfelspar crystalsc '3'stals with clearcle 111' glassyg'laSSV coresco1es attaining lengths of 8mm.\mm. and:Ed 113lessss com-com— mon ferromagnesian clusters 5-8mm.5- 8mm. in diameter.diameter In thin section the dominant felspar appears as slightlysl 1htl3'alte1cdaltered perperthiticorthoclasethitic orthociase with smaller amounts otof clear \3ell~well- twinned plagioclase within33ithin thethe oligoclase range.range The predominant dark mineral is highly pleochroic green hornblende,hornblende and subhedral quartz crystals are present attaining diameters of Imm.1mm. 2. Pleistocene (1)(ll MAHCMAFIC BASALTS,Bss'sLTs BASANITESB-SSANITES AND LIMBURGITESLIMBL'RGI'lL‘S OF011‘ LONDIANILONDHNI OlivineOl; 3ine basalt lava ﬂflowsows occur sporadically over the western third of the areaarea. Their varied3aried composition and limited outcrop at varying3ar31ing altitudes suggests they are of local origin,origin. not remnanltsremnants of extensive lava sheets, and that their extrusion occurred late in the succession.

Owing to their resistance to erosion these basalts are usually found capping hillhilI features. Basalts lie upon surfaces composed of most of the preceding members of the 'Succession,suc:ession includinginc 11d 11" eut'a.eutaxiticic welded tul‘lstuffs, blcbleached1'1:t lapiIli-tuffslapll i-tutls and sediments1'11t of Tinderet and the Mau,Man. LondianiLon iarii quartz-trachytequartz-trach3te and phonolitephonoli te lavas from Tinderet.Tinderct.

Four conical or steeply rounded hills north and north-west of Londiani,Iondiani. r’.‘rising some 800 feet fromf‘rorn the plain.plain markmarl: thelli': sites5:125, of1111‘ basalt vents. No craters are preserved,preserved. but Limutet (8,416(8.416 ft.) retains the suggestion of an arcuate form,form. and both Limutet and Les—Les- sotet (8,352{8. 3-2 ft.) show successions of basalt lava flowsﬂows alternating with lava boulders and basaltic ash dipping steeplysteepl3 and randomly on their upper slopes. In general the most basic flows are found near the:he vents,WM s, and3111 d the more extensive flowstions on the plains include1neludc the felspar-richfeISpar-rich members.

~ OutcropsOutcrop s ofol‘ olivine basaltl .151”. onc: the western:rn boundary three mile;miles south—westsouth-west L1"of KedowaKeel-0.11:1 are overlain by agglomeratesLiggloineiates dipping 2°-3°—3"' noJ'lth-east.north-east. This may represent the dip of the underlyingunderlyingbbasalt surlaL‘e.surface, and suggests the position of a source for tlieSethese lava fiowsﬂows beyond the western boundary of the map area.area

Generally these rocks are heavy.heavy, blue black mediummeLium or fine-grainedﬁne—grained lavas in which both bottle-greenbottle—green olivineoli1ine and black augite phenocrystsphenocrvsts are conspicuous, and readily distinguisheddistingu shed with the naked eye.e1‘e. The'lhe rock fracture may be'L'e smooth,«heath. or more often hackly, and chlorite- and carbonate-carbonate-filledﬁlled vugsWigs and occasional plagiodascplagioclase crystals are displayed.diSpla}'ed.

The steep hilIhill three-and-a-halfthree—and-a-half miles west of Maji Mazuri provides poor exposures of pale tuffstufts towards the base,base. while11hilc gladesglaLies on the upper slopes are paved with.1 th lL'.largerec lava boulders.boulders, An increase in profile-gradientproﬁle-gradient towards the peak suggests a lava capping, but exposure is lost on the forested summit. ln hand-specimen 421910 the lava ,:00.. ping, but exposure is lost on the forested summit. In hand-specimen 42/910 the lava isis of11.1" mediumtcdium grain,gain. with abundant felspar and occasional olivine crystals. Inla thin sectionsectit'n euhedralcubeu'ral phenocrystsphenccrysts of zoned augite and deeply cracked olivine are accom-ace-urn- panied by irregular multiple.;twinnedmultiple-twinned plagioclase laths of basic labradorite composition (An7J)'(A117). PhenocrystsPhenocrjwts attain a length of 2mm.3mm. and are contained in a 111edium«gra:nedmedium-grained groundmassofgroundmass of plagioclaseplagiocltise laths,laths. and granules ofoi pyroxene and black iron ore.

The rock (42/876)(42 “876) forming the summit of Limutet,Limutet. six miles north-westnorth—111'est of Londiani, is a heavy purple-colouredpurple~coloured finetine grained basalt with abundant 2-3mm.2—3rnm_ ferromagnesian phenocry,tsphenoer'v ts in a finelylyinely vesicu]ar\esiL‘u L'1r base.base. The thin section shows abundant close-packedclose—packed augiteLiuLvite euhedra,cubedra occurring singly or in clusters,clusters.111thwith neutral cores and pale greenL'recn outer zones. Subordinate subhedral olivine phenocrystsphenoc 11sts with11ith bold fracturesfr1etures and boundaries ae-accentuated by a bright foxy-redloxy—red margin of alteration products occur 1-1with1th micropheno-microphcnocrysts of pyroxene, plagioclase and russet-brownrusset brown pseudomorphs after oliv.ine.01livine. The groundmass,:nass. mainlyntainl) of pyroxene,p1roxene.panLLi:1seplagioclase and black iron ore,ore contains some interstitial low-birefringentlo11'~birefringent material possiblypossibl11 analcime. This rock is one of several which show an affinityafﬁnity with basanites.

The Limutetlimutet basalts overlie off-whiteoff-White eutaxitic welded crystal—tutlscrystal-tuffs and bleached lapilli-tuffslap.lli-tufls of Tinderet,Tinderet. exposed in a gully and quarries immediately to the south. Far- ther south towards the Londiani-KisumuI.t,1ndia11i»i{isun1u road basaltbaalt flowsfirms apparently overlie Londianiondani phonolitic trach1tc.trachyte. Basalt flowslion's forming lowlL111' hills north and south of the K ;sumusumu roadroad three-and-a-halfthree-and-a‘half miles west—northwestwest-north-west of Londiani (42/864)(4213641 carry large resorbed phenocrystsphenocrysts of olivine with russet alteration rims,ri:1s. neutral co]ouredcoloured augitcaugite with11ith hour- glassgjass zoning,zoning. and small vugs infilledinfilled with calcite. Microphenocrystsh'licrophenocrysts are exclusively ferromagnesian, multiple—tuinncdmultiple-twinned laths of basic labradorite occurring with a faint I flow-alignmentfio11'-alignme:1: only in the groundmass.ort‘undmass.

Many other limitedlimited outcrops of bebasaltsalt occurbccur around Londiani; at the Lessotet vent (I tizithreeCE J11iles"‘ilcs to :h:the north 1.1;.w111(42/880);1: at a S‘I‘l‘ll‘llsimi]ar 1e11tvent three miles t1;to the r111r1h=1enorth-no,th-ea~tﬂue-st (4219001;(42/900); and abo1eabove the 11.1tLrwaterfallall one~and«a«halfone-and-a-half miles to the southwest,south-west. Basalts at the last three localities all include plplagioclaseagioclase phenocrysts within the bytownite comnositioncomposition J range (A(An7o-An76)'“.11-An 1. and in thethellast twoL110 examples zeolitic-zeolitic-vugs1ugs Lcmland zeolites in the groundmass again suggest :1a basanibasanitictic aaffinity.ﬂini t1' CarlsbadCLirisbad and multiple t11:twinningnn ng in mmzonedd plplagio-1g: 0- clase phenocrysts from Lessotet la1alava indicate ccores.es more sodicsodi: than the outer rims. 'f"A The composition is from An66Anti (labradorite)llabradoritel to AnnAn; (bytownite).lb1‘to111'nite1.

Olivine01 vine and augitcaugite form twotau generationsoe"cratl ons of phenocrystsph cnoervsts in basaltbasal‘L specimensspecimen: JI‘RSE42/852 and 42/85342,1853 colleotedcoile cted from the lava flowsﬂows near the KerichoKeri cho road south-westsouth—west of Kedowa. CafeiteCalcite, zeolftezeolite 'and ehloritechlorite ﬁfilledl lr d 11:;1svugs oLoccurecu.” in a groundmass of plagioclase, pyroxene, blackblE‘ ck ironiron ore and possiblepo: 3 ble interstitialitn cist1ti;.lanalc1rneanalcime.

252S On the MoloMole Plateau four milesmi'es west-north-westVV'est—north—westLof KeringetKeri nege: no basalt featurefeature is exposed,e\posed but a gentlyzeiLlL.L.1meLidomed cultivated311:: 19..“ed ridge ofo.‘L‘:1L1L‘olaLe-:oioL:'chocolate-coloured 1:soiloil carries a:1 copiousLL1_1:LL1 is float of basalt boulders. The rock (42/957)14-. 9571 consistsconsis s ofol feIspar-richt‘elspar—rich oliV'in-eolivine basaltbusalt with111th feIsparfelspar andand olivine phenocrystsphenocr} ts in a base of plagioclaseplagioelase laths,laths. clusters of iron ore grains, poikilitic pyroxene,pyroxene. interstitial carbonate and amber—colouredamber-coloured mesostasis,mesostasis. with carbonate-fillednate-filled vugs.was. The sparsely vesicularVesicular porphyritic black lava from four miles south of the Lumbwa roadroad on the westernWestern boundaryboun‘ ._V' (42/829)142 829.1 was found to contain only olivine phenocrysts.phenoL‘rV‘sts. Common vugs,V‘ugs. Imm.lmm. in diameter,diameter are ininfilledfilled with zeolites.zeolites, crystacrystallizingllizing with spheruIarspherular habit,habit.t1ndand small patchesp:.tches ofoi interstitial analcime are rare.r1ro..\'eitherNeither nepheline nor plagioclaseplagioclzisc is present in the base which consists of granularLranular pyrmenepyroxene and subordinate~mbordinate black iron ore in an amber-coloured isotropicisotrOpic mesostasis.meso tasis The rock is thushus classifiedclassified as limburgite.

(2)ill MAUMAL: ASHES WITH\\ 1TH BASVLBASALTUFFTHU" ~ Much of the southern third:hi rd of the map area [sis covered by a thick mantle of soil derived from a coveringcmering of ash.:l1. These ashes,a~hes, the product of air-borne showers of pyroclasticpyroclastie matedal,material. extend far toto the south of thethe present area,area. and reach their maximum known thickness of about 800 feetfeet some 20 miles south of the southern border of the map (Williams,(Williams. Report No. 96). A Pleistocene age for these ashes seems justifiedjustiﬁed on the basis of their deep weatheringWeathering and erosion.erosi ‘n. At the base of the ashes locally are exposed lapilli-tufIslapilli-tuffs containing powdery yellowish pumiecpumice fragments set in a brownishbrLV'LV‘nish grey IlSl’lash base. The pumice fragments are some- what ﬂflattenedattened and poorly aligned. Rare fragments of phonolitic lavalaVa are also seen. Most of the area in which L‘VlauMau ashes occur is eharacter§7edcharacterized bj.‘by heavyheaV'Ry rainfall.rainfall, and the ashes hayehave undergone complete alteration to:o claysClCiVS and ciaV'eVclayey soils. Rare exposures of relatively unaltered rock show earthyearthy, poorly consolidatedconsolida.tedashes of reddish or yellowish brown colour, with finefine' Lolour-colour-bandingbanding and cross-cross-beddingbedding on all sealm.scales. Angular fl1a'gmentsfragments of obsidian up to 50mm. across are extremely common.common, and occasionally rounded calcareous concretions are seen.

(3)1‘3) TUFFS‘I‘L'Frs ANDAVD ASHESAstiEs OFor MooOTIOMtxsoito North of Kampi ya Moto\‘loto andLind east of KilombeKilom't‘e thethe greater1.12:. ter part of the countryside isis coveredcoycred by a highly varied bedded sequence of coarse-coarse-grainedgrained yellow and khakikhLLlri tuffs.tutfs Exposure isis good,good. being nearly continuous inin the beds of ,thethe Molo RiverRiVer and those rivers to the north. A specimen,51:) 'men. typical1} )ieal of much of the sequence,sedtel nee collectedeelleeted frome1m a quarry one-and-a-one»;ind-a- half miles south-eastsouth of Mogotio, has'Lia compaot,comp :t. tint-grain:fine-grained,Ll. buffbun baseease with11in occasionalwe innal rounded pellets of0. included lava, numerous small ieLspLirfe1spar CijLstL-Llscrystals and angular chins.chips of black glass. The wider-spaced larger fragmentsl‘r‘aeierits L‘Linsi».tconsist of randomly disposed uncom-uncon} pressed black and silver-grey pumice lapiIIi up to 20mm. in length. The successions11::es ' at the river crossing north of‘ Kilelwahilai‘ﬁ-a HilIHill shows about ten feet of unsortedurt' *ted khakiRh‘ tuff, with glass and pumice fragments, overlying ten feet of more com- pactpoet but'Ttit coarserL‘oL; tuff, with abundant :L’TLLse—paeclose-packed pumice lapilIi, some fresh and black andLind otherothers hollow andLLilCl yellowVe low. in colour.LL1L out OeLasiL'ingLiOccasional flattenedilat‘L: bombs two to three feet nnin diameterdiam‘br area: e included,includtti. and this:h1s bandbLiLLcl forms the lip ofLit a smallsriittil waterfall“313- l, beneath which“hich a:1 succession of bedded clays and soils represents tiethe weathered product of J layered ashes.ashes,

Good exposure of this variableVariable sequenceseouem- isi' displayed near the watermill inin. th:the MoloMole RiverRiVei threehie” miles south-westsouth-oft of Mogotio.Mogmfo The cliffs in the valley waIl are of' commonCLI'TTTZlitH'l khaki-yellowkhak yellow tufftuli‘ with\Vi:hLeL1Ltera scatter of black glassyL ._ chips and felspar crystals, and with

L] palepal; grey1; glisteningistL-ning pumice lapiIIipzli 2Omm.211111111. to 30mm.3111:1311. inin length.lentU‘ Pavements of rockock form-form-

t."'--" -- jing the11;; stream~'..-'c..:11. bed.1.-Li just above the waterfall are of harder..rdcr mid-grey tuff, with thin 7.9. .7 ‘, eutaxiticc .- ...\;L'.-.‘ compressedC."I]1p.”L‘\:-.'(f fragments lOmm. to 20mm. in length: 3' L31. and4l less.L\) common small uncompressed=..;'..:-..1.1:1c~.:. .3 pumicepunt-1c lapini. Beneath the waterfall the111.1 rockssuch are.1.? course:—coarser-grained uncompressed.'-.'-:1‘.11:'.x\:._1 khakiKhaki yellow_\.:..' tuffs. Impressive bomb-beds'1.:1".1i.1-'i1-..;~ on various scales are displayed. A rock from the streambed two-and-a-half miles west-north-west of Mogotio consistsI~I~.1\ 1)}of porphyriticFlu-"pi: pumice bombs of remarkably uniform size, about 50mm. diameter ' with111131 fresh1'1- glassy black cores weathering to lemon-yellow exteriors, closely packed in a3. dull611.1 yellow5 base with abundant flecks of black glass.

(4)'i' ASHES-\\1:F~. OFHP TINDERETI: PleistoceneP13331311." ashes,3531c. attributed3111hu-K‘E to1.1 a1 source..JLLTL. at., Tinderet,T'idi'l'g'_ mantle. I: the[i273 ridge which carri~ !the road andLind railway13:!“ .1). betweenh;- Makutano and the1113 north-westl]L‘.’i}1»‘~\E‘\[L‘i"'”corner of the area. Natural exposures ;in'."1 this111i. region are virtually non-existent, but numerousJ]L' lT‘.‘ cuttings for road and railway provide plentiful information. Even so the nature of the base of the unit, and the height at which11.11111 it overlies.11.. the111.1 earlierc.11'lic1" TinderetTfi1'dc rcl agglomerates111111-1135. and lapilli-tuffs,1.1]1i is unknown, and£111.! the111.- mappednuppcd boundary‘1).11L111.L1 must:1:'.1\-'. be.‘c aocepted.1u.'.';“.c..- as'.'1\ provisional.11".1'1 \1.1‘-1;=..

IsolatedEu 1.. .I..Z exposuresmp:- near the Sorget-Timboroa:TLRE'LMTH‘J road“- in the1h: extremeC\1"-':1..- north-west11..1 3%»w display.i. _ , completely rotted11111-3 bleached'}1 ed unC'ompressed11111‘.'111:prc\».-.1 tuffsluiTx with“3111 abundant1'11'.1111'1: 1111 20mm.2l11111.1.1:1to 30mm.313111111. pumice >\ 1 ‘l 1‘ ‘ -...: ._ fragments."i1. AlsoMx.‘ exposed.' 1‘1i.] are1"“ many' 1. well-bedded1‘ ."1 LdLl... depositsL1 withE1 sandstones,x1.11.}\-11‘;ca laminated1:11:15111cd clays, and7.1 chalky.‘hiiikj. textures.1.: a. . Occasional flakes of biotite were seen.

In1:1 recentec.111 cuttings0111‘. ' for1.1." th.’the 110111.11dnew road in11'11hthe north-west. an'- overburden'... ofi" 40' ft.1.1.1to 5051111.ft. 'of red soil covers variegated, : clays. wh;ich preserve.. a. relic tuff‘ structure . in brilliantL1"i'].|_ coloursCL” of orange, . yellow,.iiL. - redTcd and.. chocolate-brown.. . . . In the upper spiral. . . loop of the1‘1: railway,r..il‘.\;1}'. carvedc.11'1cd in111 beddedbiddcd ashes.«hex and tuffs111115 three-and-a-half1}1:'-.-.- .111.1» miles from1111111 the1h: western1125113111 boundary,i111L‘;11.i.1r}. unconformable1111-;'11111'.1:'111.1}.1‘--c cross-cutting'CI‘H‘VCUI: 111-: by'1'" differentLiiﬁcrcn: unitsLillilx in1:1 a.1 sequence5.1.1.3.. c of fine-,11111‘: medium-111.1.111131. .and coarse-grained' tuffs1'. isi1. weB"-L"i displayed.L'ixiﬁ". ' TheT11.- colourCU'i' ' range isS usually11‘..1.11'.:. from111.1111 creamc .'.11:1 to buff or khakii, with.\.I}1.x':;15.11:11|occasional yellow,ycHw-A. greengun or.11' rusty{LN} members.111..

Some81:11.1: two111.1 milesmils. farther111" east dips in:11 .1:1an 311311315119alternating serieswiicx of1.I' .i1x‘1‘..:xashes of..11' sand and clay 1, grade'\4'L vary1111:. from horizontal11.1.. 11:11:11 toIn 30°3'1" east-south-east.c..51—\.111:‘11ic.1~:. Fine-F1116: and coarse-grained yellow weathered i31'11dc11bedded ashesLJ\i—'.C\ with'- steep and' undulating dips exposed in a rail cutting four miles north-west of Makutano-‘-E.'.11.13111 (specimen1' 42/898) contain occasional much1111:}1 harderi.- and larger angular fragments-..‘.\ which prove to be of vesicular porphyritic olivine basalt.r This deposit may be related'...‘.-:d in1:1 age.1 to'11 the111. basalt115.1}: lava1.51; cones of south-west Sorlget.

InIn a.1 dcupdeep madstone2.191.131:- quarry1'1111'11'1'} 11.111half a1; mile1113].: southwith of.15 Makutano a 5"50 foot1.11.1.1 sequenceswam“ is1': exposedc\p.1x-:.1 in111 whichic‘1'1 well-bedded1.1.11 11.11c claysch}; and1;:1d ashes9511c» are confined to the lowest few feet .and the remainder:'c1:1.1.1 composedg. 171:1.‘w1'd of coarse-grained tuff with only rudimentary banding. The hilltop.111 one1111-: miler1 south-south-east\-.1=.1'E1—a.1111]1— of MakutaJno. l'.1k'.1|.:111..1 outcrops flaggy bedded ashes which dip121;: south-west“11113141t at'.1‘: someM1111: 12l: o.. InI.“ hand specimen 42/904 the rock, of sandstone grade, is

compact,.':111-'1..;'1. well'1 :ll cemented.1-111.nicd and1:111 khaki-coloured,k1:- .. - with the bedding 1'14 locally accentuated by layers33¢ s of.1:'...11111Icanodules up to11:1 4mm.4211111. in.11 diameter.di.1m1:1'21.' TheseThem are1 composed6011.111}.1 of.1: an:.:1 outer.1111cr rim1'111 of.11' light:brnxxn.brown, e~tremelyc.1111. meiv fine-grainedi1 material, often..1!12:1 includingincludzng?a.1 corecar-.1 of rock:.1:k matrix,11111131; but1111‘. in3118.1:others11c . completelyc..1n1pi.::‘j-.' obliterating11bit- the core withirh homogeneous fine-grained cement. In1:1

thin section little' IQ can- be. recognizedu.) :1in the fine-:grained'IC- base of clay minerals, but num-'1'1': eroussmall pyroxene.111a crystalscast-1k and'.1n-d occasionalOCCLSI'DHHI rock1.. fragments with large..1 olivine pheno-‘ ..r-u— crysts suggest it is of basaltic. 1C composition.L'C-i ‘._I“'."1I1. 01“..

The outcrop of.711." bedded'11:.iucd ashesink-Cw on1111 the:h western flank of Mount Londiani is exposed in Ian erosion' scour5.1-1.1 nearnu": a:1 farm111111: trackI ' two-and-a-half miles west of the peak. The beds (are finely laminated ashes of sand and clay grade, with rare 20mm. layers of fine tuff. They exhibit a general shallow dip to the north-east.

"'. (5)151 KILELWAKziVV'A HILL PHONOLITESPUDNOUTES The zeolite-richzeolfte rich lavalt'a fl.owsﬂaws of KilelwaKllcd Hill,Hill. four miles west of111 Mogotio,Mogoiio. are unlike any phonolitesphonolitcs seen elsewhere in the :1area,ea. in teXJturetexture or in composition. The low hill risesrisea above the plain composed«1111;103:2111of MogotioMugotio ttitis.;'111dtuffs, and probably111'0h1'1blV' marks the source of111' these lavas.lavas. The riverfl‘q bed,bed. deeply incisedted in tuffsmil 5 and agglomeaJgglomerates,Tdtéi separates the twotwo outcrops of lava.lava 111In hand specimenSpecimen 421“)2342/923 the 1511/11lava is :1a hacklyhacl~ly grey black sparsely porphyritiepoDphyritic rock1"Ock with a:1 waxy texture. I'llin thin secticnsection occasionalvectuitmrzl fresht'rcgh simply twinned euhcdraleuhedral sanidinesanidine- ,anorthoclasemorthoclnse phenocrystsphentx'ryxt.» up to 5111111.5mm. in 1127tlenigth are accompanied by 1":rarere subhzdralsubhedral partly resorbed micrOphenucstsmicrophenocrysts of111 aegirincdugiteaegirine-augite and 1n1loimeanalaime pseud01111.11'phspseudomorphs after nepheline. DispersedDi>persed ﬂflake'Sake: .ofof cossyrite,Couyriic. prismsprism: and granules of aegirine.augite,aegirine—augite. and grail18grains of black iron ore :1CCC1111ps1r1yaccompany minute randomly oriented felspar laths inin 11a low—low- birefringent,birefringent. locally isotropicixtitrcpic base,21111.1. whichVV'hieh is ldrgelylargely composed01111131114111 of analame.analcime.

The'l he rock of113‘ the:heinorthern1C1‘1hern outcropnutcrnp containscunninx rarer.1:'C rounded phenocrystsphen 1C:V‘;15 of sanidine-sanidinc— anorthoclase,;r10rth0cl;.15c deeply colouredC11 11111e6elongatel1111gz11e prismsprism» of aegirine.augiteaegirine—1111:1211: 51ndand microphenocrystsmicmphcnocrysts of nepheline. The groundmassgroundmasx has a:1 boldyhol dV striated texturetexture withVVith elongate lensesicnscw composed mainly111:1i11ly of analoime,:1n11lc1111e subordinate unaltered nepheline,nepheline. crude"'Udc spherularSpl‘ICTUlLLT aggregatesavgregntes of11f zeolite,zeolite. and1:111 raretire central1:11trzzl calciteCQIl‘Slic cores..‘11re\_ The'lhe darker .surrounding<111'r0111 ' patches" 1e> contain(11.11113 bl.1Ckblack ironimn ore,are. cossyrite,91111:. kataphoritekatztphoritc and111d aegirine:1 “Prime 11ror aegirine-augite,LiL git. neatiggzte. andand. .a11 scattersome.“ of.11 ran-ran— domly disposeddimmed miorolathsmicrolnths of0:” felsparfelspar is commonC111111110n to both lightlight and dark streaks.lCLilCh.

. 1m(6) TRACHYTESTmritV'rrs' 111:OF THE1111 NORTH-EASTN1T)l{TH-?.\\"I TrachyteTi'dt‘h'tte lavasl.1\' .. exposed111' nixed byhV faulting111:1 '1 11.111near the eastern3.1111.1111 boundaryhaunting, north-eastnorth-e19 1:111and ~11111h7south-

east13.1.51 of1.11 KampiK:1111pi ya1 1 MotaWatt) are con&idered130115.111. 1111 to behe youngerV1.111neetht1:1than thethe phonolites1- 1h1’1n1'1lites and11nd trachytestruchyteg ofol' the extreme north-east.i111rth—ez1st. An example taken from aI; highly pumiceompumiceous lava1111.1 from thethe minor warpscarp two milesmil~ north-eastr'..':tl1~e.:st of11f KampiK:111111i ya Moto,\lwtrv. 42(1000,42 1131111 cont3JinsC13"i1lll< .small~m£1ll patches11.111115; 11fof compactCumpfit‘t blue and11nd blue green basehmc surrounded\tirrotmtlcd byh}' a:1 hrghlyhighly vesicularVC.\.C'.1l;«.r swidedswirled texture,texture. with pore spacesipaces stained rusty11151): brown.hrmx'n. includedIncluded fragments11191113113 attain:11: .11 lengths of1.1? 211mm.20mm. and rarererc felspnrfelspar phenocrystsphcnurrysu reachtench 3mm.3111111, x\ 311x11.3mm. Inl:1 thin section51“;t veryV'erj. fine-grainedﬁne-grinned patches11:1tel1e~ can.11 behe {lig—dis- tinguishedEinguishcd from glassy material111:1terictl which rsis typicallytypically devitrifiedtVitriliCd intoin‘qo spherular aggregates of111' felspar:‘ekpnr lathslathx and:1111l 17:1l::\flakes of11f sodicmoiic amphibole.'1111ph3l11’1le. OccasiunallyOccasionally ferromagnesianTerran :gne .111 and felspaJffelspar prisms1e111-; protrude111'111'1111: intoirtto, the vesiclesVe< Tl l:beyondVund the margins111.111.1111; 115of the 3.11.131glass. 'IheThe highly pum-51:11} iceousigenilx and11nd contaminatedC:111131111'1.1ted lavalgw flowflmV exposede\;11'1%etl at:11 the top111p Cfof the fault111111 scarp53.1111 on13:1 the cuterneastern boundaryhounddrv south-east\Ullhll east of KampiKrir11pi ya1‘11 MotaMoth isi3 tentatively correlatedCtiri'elnted with the rock:'.1;'l< imtjust described.db.-. izjcd.

(7) EUTAXITICEUTAXlTlC WELDED\VELDED TUFFSTITFR OF0F THE111E NORTH-EASTN1 111111—5551 Hillill featuresfe.1t:1:‘c< standing\i.tﬂLllﬂLi above:1':.'.:1Vt: the levellcl of1"? the:h: plainpinir‘. in the north-eastmirth-cast includein:..1de'i’l1.11sethose one.311: milemil.- south511:11'1 of.11 McCallsWet ill; SidingsAiling and the fault[11.11. scarpwarn south-east>1)1:‘.h~e:1.~‘. of01f Kampil\':111.111._V.-..‘l1.1111.ya Moto. These featuresl :1turo are'.-.:e nearlyDearly buriedhurled by MenengaiMencrzgii pumice.pumiCe. The fault1".1ult scarp52111111 isis capped6.111111311 by a.1 laval.1V.1 flow171111 30311 feetto thick,1h. .11.. highlyEifghly stJreaky\t:‘e'.1l\_V' and.11111 vesicularVesicular in texture:ex'ttir: and11:11] contaminatedC1.1111._r11111:1te;l byhV much includedin lutietl debris..1. hit?

The pyroclastic:3} 1111.11.10...“ material:11;1'C:’fal forming1.11:1:111; most:111M of11;‘ the1h: scarp«0.111) is extremelyexircnnm} ill-sortedill-<11rted and:Lnd coarse-nurse- .grained andLlCl isi3 characterizedcl1:1“;1:‘.e1i3ed byh}. un1111C1111.11;‘eascdcompressed pumice bombsi:1.1111h\' of porphyritic110:;1l1jhitfc black glassglam up1111111to a:1 foottact in{:1 diameterdit1111etcr and.1:1ti compact.1111:p..t1. angular.1.1.111l.11 fragmentslL‘lTiETIIS of11f glass124.151 and1 :1.liiVI‘tlava blocks.hlucks

Often coarse' Re and.11111 medium111etijizn1 grainedneg1 fragmentsit'ngnmnis are.11: closelyclosclV packedp....\etl and littlel.ttlc interstitial .Ul material is.\ seen; sometimes blackrune}; glassclass as11x bombs and:111dl.11_1.111'1er11.ifragments constitutesC011

~- 1 East af1111‘ Kampi ya_\';1 Mata31111111 capiauscopious flaatﬂoat af1'-:' ,giant151.1111 pumice111111110: bambs1111;? associatedC1L‘ 11.1131". with“h thisLhIs farmatianformation litters the1hc cauntryside.12111111 n‘szde. Aggiomcrs‘esAgglamerates and11nd 1311111111111:eutaxirtic 111115tuffs in this lacalityl111.=..i1y dis—dis- playa):1131' '11 wide1.11111: calourcolour range,' " with\xizh «15:11deep grey1': , and brown predaminating,predominanng. but11111 always displayiagdisplaying: the11113 characteristic.htu': abundance of black171.1111 glass.1.11.»;

3. QuaternaryQuatemary (1)(‘11 BLACKASHES OF1,11: RaNGAIRONGAI PLAINANDNJORO TheseThose distinctin‘:distinctive black ashes with glassy bombsb'Ombs are11m 11widespread1116;111:3111 in the Menengai\Icncngui 1111::1area, 111111and 1111:are attribtzttdattJributed toto. :1a source :1:at that11.1: 11111::11volcano. Tl1c11‘Their 'Outcr'Op0111111111 extendsext ‘s1111 from12111 the plain casteast 111'of Njoro,N1111‘1,1_ l,‘1‘:11‘;11hbeneath thetho laterI11" ‘ silver-greysih 1‘1' y pumice11111111112 beds,ocds andan is seen<1‘1‘n again315111." in:‘1 thetho bedsbc-Lls 'Ofof thethe MoloMala, R'OngaiRongui and BissoiBiss'OiRivers. The[‘11: black ash11sh 'OverliesDVEI‘il‘ CDUfOiT‘LlILlI‘1_'canfarmably the yellow—yellow- bedded11311111 113111511111Mogotio tuffs,111111;. the1l11‘ boundary1‘1111111'11161' lying (1111‘1'o‘x'131111telyapproximately 1111111,;along Thethe line ofaf the:11: KampiKr1111pi ya5'11 Motn-EldumuM'Ota-Eldama RL1V'111'c‘Ravine r'Oad.111.111. ArtAt the s'Outherns11111l161'11 extremity of its 'Outcropoutcrop the black .1;hash 'Ofof the R'OngaiRow 1 Plain overliesaverlies the 'Older11111131 b11111»;black ash 'Ofof the ElhwgonElburgan area,urea \1‘1‘with71 which it sharesshare; many characteristics.111.1:1'1‘1‘1‘ris11cs. The b'Oundaryboundary between11131111111 the two.‘1‘11‘11 is ill—deiII-defined.ﬁned.

The black ash ofaf thothe RongaiRangai Plain has 1111113a fine-grained‘ 1111211 porousparaus base. itsIts friable1' 1:1ble texture :111dand susceptibility tato 11111511111erosi'On preventsgvmcnts it 1111‘111111‘3farming lllfifl‘it‘dmarked fegiures:features: its most commoncamm'On c‘xpoiurcsexpasures are:11: in1:1 the1h.- 11flaors121‘; 1:11af 1111'":river beds, 1‘ 1; uhwhere it’ linx'ai'hbinvariably has11‘s :1a <1:111.11_saaty waterlogged, 11111161111111121‘.appearance. 'lhcThe base11.1;1‘ colour.calaur, m'Ostnio~t 1:1'11111'1'1onl;camm'Only black.black, \111‘11";varies 11::1lllacally toto. 11.1rkdark purple.urplc andand purple [greygrey with rusty partings.;1_11':111gs;\A Vvarierty11131;. 'Of1111' includedIncludsd closcipmckcd.clase-packed, Iightlight coloured,col'Oured,

grit-sized" pellets' of lava are '11'1'1accompanied11111111511 by 1111111111angular 11111:chips 111'of 11.1.1111black glass‘1: lass 11111and plate-plate-1 1 like felspar crystals, which occasianally111 attain111 1,a llengthh (11‘of Smm. A high1 1.... 1 11*.111.1rli:1'1proportion 11157af ,the:11: included fragments11.1gn1o111s 11?“:are af1,15 uncampressed11111p1‘cs~;1l pumice,13111111112. usually11:: 7 blackblack, inI11 Colo-131'calour (indand .oo.arsely11:;TsciV \Cjiulkm,vesicular, but (1‘1“1‘Lnlxl711accasionally, '.\1thwith 911411111striated 11‘1111111.‘texture .;1‘:11and .‘ia graygrey (11‘ar (11,111streaky 3111‘}:grey and green colaur. Same angular fragmentsgments of 11113113111:trachytic l:1\'11lava carry' 1y 3111111f1‘lsp1'11'3mm. felspar phena-phe 110- crysts. The base shows little camp action and 'On breaking, fracturesz ' ‘ttires :1\'o11_lavoid the 111111111311included f;ragments,.1,;g1‘.11‘1‘11>. 1:111115'giving broken1,1:‘1'1Ixcn faces a11 pebbledqhlet'l :1pp611;‘.:1:1c1‘appearance {1:111and concealing11111111111111; the1h;- pumiceous3111111111111 n.11urcnature of'Ofthe inclusions.

Highly \dri.'1blcvariable in proportion, bu:but 1111111-quite commonoammon generally,zensrg‘dlv. :11:are lalrgeglassylargo 1‘11»; bombs.bombs, ’l'l‘uscThese 111‘1‘111'accur 1‘1in all1:1l sizes\l/C§ fram111,11 two.121.1,: inoht‘;inches to.11'," two.1.1111 {061feet in1:1 11111110‘613111'1diameter, and (11111112111131although oeca- SIi'OnalIy{1111:1111 1‘11u11equidimensional‘L-nsioniil :11:are usually1101111111. 1‘11‘1~:1‘:1"{1:considerably 11111111311.flattened. They present11"c;{'11t h1ihlyhighly lTl’é‘gUlJfirregular LLIHUCDOlLlam'Oeboid sham;shapes 111-111with rough mpyropy s11r111c1‘s'.surfaces. ThoThe homhsbombs are composed of'Of porphyr'itlcparphyritic blackHugs; glass withh 511111111‘1s11111‘Smm. felspar 11prisms.isn‘s. Thelhc '1csICl1svesicles, 1‘aften1:17 s:\1’1‘1‘;llseveral cm?“centimetres1111:1113 long.I'Ong, vary widely in111 size.size, shape and(111d distribution within the bombs. A strestreaky(Ll-i}. 5structure1111311111: is1; frequently1161111511111 displayed,1151111111511. withxxilh contoftcdcontorted lenses ofaf fresh dense L..11cl1black glu,§s1_13£‘€l’ld€dglass suspended withinuithin '11a coarsplycoarsely 11‘511‘11l111'vesicular swirled«1111311 matrux.111111111. AlteratianAllei‘nti‘on l‘C‘SllilSresults 511in tracts111111; of1,11 green1:1"c and:11111 greengoon grey colouroolaur throughautthroughnu? theth1p11mi1'cpumice.

OftenDitch the red1111 sailsoil for1111 two.11111 'Or111‘ three feet averlying1,1\é'l'l}'lllf._f black 11shas'hanon the RongaiRongzii ,PlziinPlain carries1‘.11'.'11" a ooncentration1‘01‘1131‘111r:1tion 'Ofof thesethose bombs,bombs. which‘1‘. 111m occur11:1‘111' in such numbers locallylo" that111111 theyIhcv areC116 callectedcollectml and11:11 used11,;211 in511 building7111151 1:; dry15.11 stane walls.‘ No\o bedding1101111111; was11,1; recognized1:311: 1.1.11 in1::any:m 'Ofof thesethose deposits,dcpos‘its. but 'Occasionallyocczis o111'1lly small-scales1111'1ll-s11:_1‘ calumnarcolumnar j'Ointingjointing was1:1,; 'Observedobseiwe‘l :12in maremore homagene'Oushomogeneous exposures.exposures, 11:;as in ,thethe MalaMolo R111".rRiver one milef‘l'EiiC S'Outh-westsouth-west 1,1,1'Of Rongai.Rongai, SculpturingSciilpttiring 11:”af theseW15: s'Oftsoil astashes bf»by river1‘1‘11'1' Actionaction risis common.common, miand >I"cll‘l.\streams 11.:1‘noften mn3'1 :7 in111 thethis battoms1101111111.; af11f narraw1111111111 garges.gorgos.

RemovalRc‘mi1v11l, 1:11of soil to 1111111111“-reinforce :11:the railway: way embankment11:1‘c17111knicn1 has11:1; resultedrc;1:lictl in111 {1:111:50extensive cx-exposures[insures adjacent to.to the railway13.:l1.1;1y 1110tW'O miles east-south-eastc:1s‘t-s0utl1—e:.st 'Ofof Rongui.Rongai. A furtherfarther mile111111“. downJohn 1111‘the line from“"1111 R111Rongai :1a pit1‘1 13.31151";exposes thrcethree feet of 1‘C1lred soil 1111512"overlying_ close- packed,pad-1111 fine—1111311311fine-grained khaki 11111311311coloured tuff1.1111 11111171;carrying striated>[l‘111t’d g1“grey pumice:1111‘131‘1‘ frag-1‘.€"tsfragments :1:11;land felspar':1:1_1 crystalscry 1; Smm.5111111. long. Underlying thisthus is waterlogged\‘-':1te1‘l11gged blackblag}; ashuh with langelarge bombs of11‘” porphyritic glass.

. ---- A similar seqULnLcsequence isl5 d15pl3;.c5idisplayed inl1"; :1a pitpll near the main roadread {ourfour milesmllc5 south—cw.south-eas: L1:'Of Rong,ai.’ .SomeSome four feet1:33. Oi:of rered 501'}soil overlie fLDUTfour feetfeL—t Ofof yellow and ochreeusochreous ﬁfine-graanedne—g ncd :tuffs, Lindand dust:dusty purplewurpl: black frlahlcfriable ash ‘L1f1l1with abundant bembsbumb5 over one foet{0111’ in «1'.dia- 'meter. 511111131Similar black L15l1ash 15:15was 0.151311115eenceuntered hmcalhbeneath {ourfour feet of redred soil.soil and an equal thicknessnc5s of pale uncensolidated,LlflccrlsL‘vlldatL—Ll. uncempressed,111 comm: 553d hurlhmlullhorizontally\' banded ill‘w‘er-L’rcj.silver-grey Munc—Mene- ngaingLLl pumice twomo milesmllcs nLlr‘Lh-noril1-uc5lnorth-nerth-west of 351L110.Njoro. Thehe ash15h therelhcre is of dark brown c0101colour with small$111.11 angular lava3.15: fragments113311121115 and11nd large bumbsbombs hin a friable ba5cbase L‘ontuimngcentaining 1‘felspar"elven crystals and pumice lapllll.lapilli.

(2)l2) PYROCLASTICSPYROCIASTICS ANDANT) SEDIMENTSSLDIMEN’I'S OFr’Lr RONGAI1111351351 PLAIN AND5.513 MADMALL SLOPE RecentRectal dcposldepositsﬁ con the plain near{1631' NjomNjoro are represented only11a by well-bedded,1Lxcll—hedcled. glisten-g155tcn— ing, greygrcy unoons'Olidated[111L11115L1lid:.: :Ll pumice.'1111111Lc ElsewhereEl5c11 lL1 [c hewever,llmwxcr. particularlypurtlculmf} to:0 the ne[lthUt‘l'lh and nor,th-north— west,we5t. theIhc Pleistecenel’lL‘15lLL‘LLnL‘ te111 RecentRL‘ LL 1’ successien[ILL‘C55 L::1 isl5 expandedcnimalml with theH1: inclLHlnr‘.inclusien 'Ofof yellow sediments"11-3115 and white'5l1lteblL-mcl1elibleached tuffsu1l5 atLt thell‘li‘ base,hi. '6. andml lateralml variati'On1.111.111 isl5 'Observedobwrxcd fr'Oml‘tom J:1elativelyL’l‘. freshfro}. [ELLEN-11115;medium-grained glistening.LW: :1: pumicep11111l3L onL". the plainvldln in5.1 fhtthe east,1-2151. tato impressivel:1:pr:55i\c accumulations.LLL.131[1U 33111115 ofL119 much11111 l1 finer-grainedlmc" :lrlLd claysLlay and ochreaus11-;l1tL-L1u5 sandy5L1miy soils5L1ll5 producedpi’L‘LILxL‘d by‘m weathering'~\cL’11l1L‘r111gL1l1111L*:'.15l1L’1[1‘Ll1-of finer ash en the scarpLLLirp 5309551111l1cslopes to the west.1.1951.

SedimentsSLL’liH‘L-nn :11:are pearlypuvrl} displayed[l55pl.’1§.‘cLl inl? sectiens5LL‘IlL‘n5 :11at bendsbel1d5 1:1in the1l1L MeleMole Rl‘v'L’l‘River fromTrvm 1550twe miles northnerth 10to :110two miles :50L11l15wes:south-west 'Ofof Rongal.Rengai. InT11 river[her sectiens5601mm the[he 5cdln1ent5sediments censist003535101"of medium-[’11L-L’l1'um- and:111L’l fine-grained,llnagl‘glnéd. yeHow)cllma’ and bull.buff, bandedl1:~.r1Ll:Ll tuffaceous[UNI-@0115 L’L;depositsL’151l5 shawing5l1L11111L’r CLcoarse’[[5L- and [1:165grzllncdfine-grained bedding and unconformableunconl'omL’Lblc crL’1555L'u‘Ltingcrass-cutting 'Ofof €JLL‘llLrl1earlier by3 l:[lLrlater beds,bed5. 1’15as in 5pcclmenspecimen 4242/762Th: L‘mmfrom ane-and-a-halfL1[1e~:.[1d—lh;;lf [11llc5miles .50uisouth-westﬁ-uu51 of Rang-ml.Rengai. 'l'he5cThese sedlmemssediments gradejzmdc upward intoH1111 layeredl;~.}crcd clily5.clays, iren-richMon—rich sol§5soils L111Lland ochrcomochreaus sandstanes.5:111L’l5tL’1nc5.

SedimentsKcLllt11er115 freml'rnm ay; similar5l[1:1'l:[1‘ part[111! 'OfL15 1h?the successien5L1L'Lc551L"11 are:1:‘c::l5L1alse expesedL\11L 5LLl1in theIll; road[‘LuLl an[in the[l1- northrtLJrzh bankbunk 'OfL1l' ‘Llicthe Mala’ RiverR' ' faur[Luv miles[111l 5:111:Ll1—1VL5[sauth-west 1'Of Rangai,R1 [1: _ where,1,5.I'1Li1‘1s. withwill: lilL-cllcdbleached ,tuffs,111?.5. they[hL’j 1'Overlie..L‘ t!rachyte1;LLl1_\[L- lava and[dd underlieL111Llcrl1c .echreausL1Ll1reL’1115lrLLlLlLbeddedl soilsN‘llN and ashes,L5l1c5 Th»;-The sediments5Lxli[:1L‘11l5 :11at thisthl5 lacalitylL’1L“. .13 form1111111 a,4 varied,“\L‘L‘L—lC'd. well-bedded\1cll-ll3c’3LlLLl 5L’L’111LnL'L-sequence Lit-1111'dipping same5L’1111L 10°lll 6.15%east- nerth-east.[11.1ftl1-m51. TheTh: 5:1L’c355lm1.successian, Ll'Ofwhich\xhlL’h 501116same ll)10 feetlc‘L.are L’ll5lL1XLLldisplayed in111 b‘LmSL5banks at1? 1h:the raadside,r1~gul5lc. (has a basal.LLl member[remix-1’ twa feetl’cL“ thickilt1L‘l5 'Ofof alternating:lltL‘I‘nLlllrv thin'Ll1l11 units1111115 'Of01 laminatedllmlnxlcd clayL141 and sandstene.:1L15113[1c. ThisH115 isl5 .1\-’'Overlain by. same13 faur[\111: fcctfeet ofOf cea

Thellic‘ l‘L‘LIL‘Vl'lateral L‘L}11'»L1lL[’L‘equivalent L‘I‘'Of tlli’the bedded—dLlr:Ll 5nil5sails :m-cland pumice of 1h:the plain[9.1111 15is t‘the weathered, “azhcr ed preduct 'Ofml ’Ll‘Lcthe finer-grainedma} 111.1[L1material. :l flfrom LE1:the 5.1111:same pumice 5l‘.n'»1:r5shewers which 11:;5was Utrans-[15— perted[7113‘ 21111113.fa

30u

1""---0 j

dations11mm fromfrom beds. of fresh glistening grey pumice through yellow rotted pumice beds to10 soilsmils and claysmay» can be demonstrated in roadside expasures two ta three miles sauth- west\R'L‘r: ofIIi' Njoro,:\_. and in a river guJIy three miles west-narth-west of Kampi ya Moto.

TheseThL’SC beds53L“ have}‘-_:=,'I" also praved particularly prone ta severe erosian. Scauring has fre- quenJtly remaved up to 100 feet of bedded sails and clays, lacally producing vast amphitheatres with nearly flat flaors and vertical walls. Differential ewsion 'Of beds 'Of slightly differing campaction has produced steeply terraced walls and pillars, and weirdlyII-..IJII weal}:sculpturedIurcc‘ farms. Elsewhere severe badland tapagraphy and deep ravines have develaped. These distinctive land farms -occur at intervals around the faat 'Ofthe S{:arp I from the flanks of Kilombe to Egertan College, three miles sauth of Njoro. J Spectacular examples of erosian are alsa seen in the river gully three miles west- I n'Orth-west of Kampi ya M-oto, beneath and:Ii to the1-1“ eastas: ofin the railway five to six miles north-west of Rongai, and an the interfluveser'ﬂ nee fourII-L1r milesmi.cg south-wests-I.L: of R'Ongai.

I ~ I'):(3) \TI;\"I..\(]\1MENENGAI PUMICEPw‘Ilﬁ'E-L Much 'Of that part of the Rongai PlainPlat: nearestn: Menengai,Xi :III': from. north of Nj'Oro toLI:- south of Kampi ya Mota, carries‘; ar: mantlemanilc 'Of0!‘ uncompressedLmIsI n‘rsscd humicpumice.c.0w-ngOwing IL"to isits completely unconsolidatedJ] dIliL‘d .1112]and uncementedLIﬂCL‘ mcmcd natureIIII‘ILIr'- iIit farms:\. s no[In features476% I'm-dand m:natural‘ exp'O- .‘I.-.]l’::-\Jsures are virtually nLIIl-L’Xnan-existent."It. T_II.:iITracing r [Euthe 0.2-1'OutcropCF-C'Y‘D of'Of theII:- formation‘1;- -iI': isi; not difficult h'Owever, as due to limited \L‘ffsail (we:'Overburden”PM: d-I:I‘_- r'Oad."I‘-;';Li IIIIIJand railr tracks expose pumice in shallaw cuttings and drainage gullies. Elsewhere empty silage pits proved a valuable source of infarmation.

The pumice ~s glistening pale green grey 'Or silver grey ,in calour and occurs in roughly equidimensionaJ Iapilli. In ovaid fragments a distinct striatian and elangatian of vesicles parallel to the long axis occurs. The lapilli vary in size from one ta ten milli- metres in diameter, with the coarser fragments occU!fring in beds on the plain towards the eastern margin. The deposits are horizontally banded in units of six inches ta twa feet, and within units fragments are remarkably uniform in size. This reflects the made of deposition from pumice showers within which a high degree 'Of sarting occurred. The deposits are remarkably homageneous also, usually containing pumice lapiIli 'Only, but sometimes including a small proportion of minute chips 'Ofblack glass. Successians 'Ofmore weathered beds shaw layers 'Offresh pumice aHernating with layers 'Ofyellow 'Or chocolate col'Omed soils, rich in pumice lapilli. Exposures 'Of bedded pumice in raadside banks, shallow rail cuttings and gullies are extensive on the east 'Of._ the plain,I particulalfly in the disused rail alignment from six miles south 'Of Kampi ya_\_'1 Moto“mo ta the eastern baundary, and in the Kampi ya Mato- Nakuru road from three miles sauth-west of Kampi ya Mato.

Gaad expasures 'Of bedded uncansoIidated pumice are shawn in the law scarp four miles narth-east 'Of Njora, attributed ta erosian 'Ofdeposits burying a hidden fault scarp. The layers show a persistent easterly dip 'Of some 10°. Towards the top 'Of the succes- sionL"'.'.' aL'. mare:I'EC‘I'C campact,I20:"? [).'-.' fine-grained greenish tuff harizan with rare pumice lapHLioccurs. TheI-“;I; thintn sectian«cation (42/1010)€42 shows pellets of included lava, occasianal felspar crystals, and vesicles 'Of pumice infilled with laths 'Of vapolJlf-phase mdnerals, recagnized as fel- , spar and aegirine-augite.

(4) SEDIMENTS 'OF RIFT VALLEY FLOOR AND KIL'OMBE

Capping a low hill eight miles south 'OfI“ Njara_-\'iI' '.L.Lare bedded depasits_-'Jl ~1I< irL'iLZLiingincluding <'sedimentary horuzons,hm hmﬁ, whichuhich are well exposed inI". theY}‘..’ motorF7“- “TO-I" track skirting the summit. The featureure isJ's interpretedinterprcted as a tiltedIlcd hOTSt1'0. SI developed\IIImcI-j as a:I result:‘cault 'Of grid-faulting. The de- positsL5 consistI-onsisi mainly of khaki yellawyawn-I beddedh: dcd tuffs‘I .1": with" . h rottedroIIc’ yell'Ow pumice fragments. averaging:IIcrIIging 5mm. acrossacross: butbul whichM" 'OccasianallyI'3cc:1.~',i0n..I _\ reach 30mm.3Omm. and rare chips of black

31 1‘. glass. The outcrops weather into.3110 flaggy1].; -, pavementspmcmurm of:31" pock-markedpock— ILLr'xu-Li slabs5121b) withwill“: :1a per-pt"— sistent dip of 8°: ’ to{0 10° to the east.crest. PoorPum— andamd scattered«L 'cd exposurescxr-o.su:-.s onor: thet5: sIopingsinging pl'ain:11 to' the eastcast and north-eastnor:h—c;1si;‘-Uinipoint to‘ similar rock types in that locality. The sedimentssc which formed in the1113 old01d sailor—lakecrater-lake of ,thethe KilonKilombe vent aJttained a mid-(flex;thickness a:of overx - 400‘ : feet..L‘LL Om":Only iiiat the 151?Lip n:of th-the waterfall, where the drainage streams Ofof the crater funnel into a deep gorge, are medmedium-grained.5" ‘ tuffs- 11k seen,seen. the succession aboutabove providingLT ' 11;: limited1:311:14?' exposuresapt-Lu '- mainly113* of finelyii: laminated' ' clay-x.clays, \xiihwith \occasional wellucli cementedgcrrcnlcd bands.mm»; Gently terracedtor banks suggest a cyclic11c reputitionrepetitlion duriduring. deposition, with some two1‘10- toIc. three132:: feetL‘s-2 of finelyﬁnd; laminated grey andi white3 daysclays on.overlain. by up to six inches of fine-graineddined [111?tuff with:‘1 ILa com“compact and firmly cemented:m-cntcd one inchInch cucap-- ping. The clays are preferentially eroded and occur in diminutive-. cilifcliff features1:11:13? while the harder bands provide cobbled pavements, perfectly horizontal and often over ten feet wide. Exposures .11at the111:: foot50.1: of01 theIhc waterfall1523' are also of sediments, being finely banded pale brown gritswits (1ndand clays,QELQA. d1\}"il_.‘displaying infilled mud-cracksmud-g I'LiJ'ii andand cross-bedding.L': (5){5| SEDIMENTS\LDHJLNIS OFin MAUN11: SUMMITSl'M‘ffi A3; largeLlrgc area311:1 of05‘ poorpour exposureexp-51:3: in thethc regionE‘s-'flsn of[‘1‘ Mauﬁrm SummitSunnnf‘. is causedCiill'w’d by‘r-f. a:1 mantlermmtiu ofm“ fine-grained1%: @1336 pyroclastic11):, .JaIE: debrisd-Lif":~ nowm'x‘. weatheredthat: tot- a sequencesegue of bedded‘m‘d' ' clays.

Before!3135-“6 theI‘m? Molo\ir-Ir- RiverRim-r111:cut backEck fromfr'nm [htthe eastago: andan] the11‘: KedowaE\;-.- RiverXWL'" from.‘u'r"- the west'1‘,l the levellcwl area.Jrca wasv; 5 probably moreIT :c extensive,extensivc‘ and servedsced asin :1a depositorydepodtury not onh‘only forfu r direct ash-fall material511:1):-but also;.\:3 forIn.“-debris washed down from the higher ground to north,1 anduh south. PoolsAmi: formedi'n‘mcd locally,fog. and"mi muchr':11i;1‘.- ofu? the1|“: material is consequently waterlain.Ian. Expo-Exp-J» sures are.,.'u poorpour buthi; bedded1-» clays and,1 soils with iron-clay horizons can be seen near! the{in the road-riverma: crossing:1 three-and-a-half milesm‘lv north-north-west of Molo and in'A theihg road1-16 quarryLuvs‘y and51;} exposures; down towards the111: river, one-and-a-halfL miles north of Molo.“.1130. In1.“. a=. dam6.1.“. in the1.x forest:.-I:\‘: two-and-a-halfL‘.‘.L--1-'\i»_.»h,..{ milesmifu north-north-east.‘.("'lhii‘nv'lhnLd'. of Molo‘J-J-i-c beddedr-dcﬁ depositsqty-(nib ofn: sandstone grade are exposed lin the spillway.' ExposuresFumxdru‘ of.H' apparently.g‘pll‘rnl‘y waterlainxxulcrlin‘: materialr) are di,splayed in a road cutting two miles northnth of£11.. Mau\., Summit, where'.'-\}‘ak:u 505'? feetfcgl of gently undulating bedded ashes and tuffs are displayed.

VI-STRUCfUREVI~91"RU (TURF: The western wall of the Rift Valley\:-..}c. dividesdividb the MoloMule. area.11).: into two.,,,7.uu ‘unequal1:12:14.”, 1 units;11:11:51' . the larger, the highlands of the rift shoulder to the west,A L ~.L_ and11:” plains of the rift floor to the east. Unfortunately little direct evidence can be obtained regarding the style and age of faulting as the greater part of the pyroclastic deposits post-dates the major periods of rift formation, and considerable pyrodastic activity continued until relatively recent times. Even on the eastern margin, at which longitude Recent grid-faulting is well- displayed to the north of the Equator, a deep mantle of unconsolidated bedded pumice from Menengali obscures most of the evidence of tectonic activity. As the Molo area includes the western wall of the Kenya Rift Valley at the latitude corresponding to the transverse Kavirondo Rift Valley one might anticipate information on the mode of rift formation from the site of their junction. The evidence suggests however, that the compJex tectonic pattern at depth has produced an area of c~~stal weakness particularly amenable to exploitation during periods of volcanic activity, and no evidence of a junction between the north-south and east-west rift structures is seen. The area includes the major fissure volcanoes of Mount Londiani and tbe Mau (the latter extending out of the area to tbe south), the central type volcanoes of Kilombe and Tinderet (the latter centred just west of the north-west comer of the map), plugs marking ancient volcanic vents at Mount Blackett and also in the Nyando VaHey, the late-stage basalt vents of Lessotet, Limutet and others on the plain north of Londiani,

~. ~::::!!I ~_c,'

and zinc,the MenengaiMenanga‘. C'ald-Caldera at the eastern913'ern margin,margin. apartapar: from others011—9'. which may not have been recognized or may be buried beneath la'1er‘ deposits.""roms. The accumulated deposit!! ifrom‘.:'n thesethe»: sources\"..‘-!J.'C.'_"*~ resultr- in this area being the highest onLh". the western shoulder of the rift, and;; Q quitequilc possibly_m‘->>.n£_\' carrying the greatest depth of volcanic material on the vaIley floor.

Some 15 to 20 miles north of the Equator, which defines thelh: northernr20: boundary of the MoloMmln area,a- the fault zone responsible for the major‘.' down{iG‘A'nUtrs-C.throw, and hence considered to define the western margin of the Rift, extends north-south between longitudes 35°36' and 35°40' east. The steep eastern aspect of Mount Londiani, although a lava-faced fault scarp ,I rather than C.a fault-line, indicates the position of a major fault buried beneath rift floor I depositsdepmizs toin the[he immediate east. This feature also dies abruptly to the south where, beforehct'ni': the latitudeJordan: a-of the main road, post-faulting pyroclastic and lava deposits a're I bankedbnnked againstIN? the slope.alnpc. To the west of Njoro and Elburgon the easy gradient of .{ theL11: western“3% wall of the Rift is interpreted as a gently terraced slope, with the total downthrow(jaw shared between several faults, and carrying a thick mantle.:.-cL"-'of later' pyro-m - dasticelastic deposits. This region demonstrates an offset to the east in'.1'| the {HumanposiJtion OI‘of Lh-‘cthe weal-c211western Rift' wall, with minor fault features assuming a north-north-westerlymicrl‘. trend.' At-\i the”Eh: southernsc- ‘ boundary the two most distinct fault \Scarp slopes pass out of the area betweenbaht-can 1011git'udes35°54' and 35°57' cast . In attempting to date the rift faulting in this area one is hampered by lack of direct evidence. However, geologi,sts working in better exposed country to north,r:.'1 and south have reached. cu conclusions which, although showing a range of_ differentcram inter-inlet— pretations,'_ proxzprovideﬂc a fair consensus of opinion in favour of the infliaatim:initiation of riftr 1-.\_. formation in KERR"Miocene times, probably sited along a topographical low (Pulfrey, 1960,.V.\ . |_ p. 12: McCall et ai, 1967, p. 196). Similarly there is general agreement that te-;".nnictectonic actiVEI}acmvity was renewedﬂux-«'25: in .‘Jidd’cMiddle or“' Upper Pleistocene times with development of a patternKerr. of grid-faults and 111-;the produc-produc— tion of many minor hornts and graben on the rift floor. Faulting has- "recurred(i '01".on :1a small scale, mainly as renewed movement along pre-existing lines of weakness, sporadically to the present day. Thus the major north-south fault, \‘uwhoseh as receded lava-faced 1,. scarp forms the eastern aspect of Mount Londiani, should probably be referred to the original riLt faulting of Miocene age. The complex of smaIl faults downthrowing either east or west on the scarp slope near the main road in the centre of the area, together with the pronounced graben features which carry the Njoro River, and the Njoro-Mau Narok roads in the south- east, should be referred to the later stage of grid~faulting, in Middle OTUppeT P\eis- tocene times. Aho to be tentative1y included in this group are the faults upthrowing the lava horsts of the Rift Valley floor in the extreme north-east. rn this context it is significant that of the] ,400 feet drop in the base of the lava flow, between the road cutting at the Jolly Farmer Hotel north-east of Molo town- .ship and the exposures near the Molo River bridge at the foot of the scarp slope, only some 500 feet is apparently the 'result of the accumulated down throw of all faults, the remainder representing the original substantial dip of the lava flow.

The. \'a.\l\t~ O~ the. ~()Thg'3.i"P\'3.in,tnose etched by the Molo, Njoro and Rongai rivers, and that forming the scarp on the eastern boundary north-east of Njoro, are probably of even later movement Faults of the latter two age groups are largely implied from evidence on aerial photographs. In the graben in the south-east the features are most pronounced, particularly the west-facing walls, but exposure is sporadic in the stream- beds and almost non-exisknt in the \Scarps and on the surrounding hill tops.

33 ~ 1

VB-ECONOMICVila—ECONOMIC GEOLOGY

1. Building Materials Building StoneSrorze The main geol'Ogical12321111: 212211 1'292111‘22resource of'Of the 3.723area ~sE< building1311111111113. stone,5111112. which is 2:11.11quarried£22 fromfrom the1112 M'Ogoti'O.‘v‘Iogotio tuffs,111113.. the1112 ElburgDn131011151111 black11111.21. ash,:1s:1. and{111d various13.111111; local1112.11 V1211—well-cemented«3211 211121;? 1111711211;hDrizons in bleached1112512t 111113tuffs and 2131:11111‘1‘1212‘12a.agglDmerates, E1111but 31:11:13:mainly fromfrDm eutaxitic1211.111): welded tul‘fx.tuffs. 2‘1A <1s1g12single stDne210112 quarry is Opemwd'Operated 'Onon the:52 outcropDutcrap 'Ofof the qu.Lr:z-tr:1ch)'12quartz-trachyte three-and-a-half1hrcc—md—2—hcz‘1f 1111125:miles west-sDuth-westwest-sauth-ucst 'Ofof Londiani. TheT112 KedawaK111111115. £10112stane is1.; 'Of111 high313111 {111.1111}:qualilty, and.11111 is1< much 1:111:11]efavDureidowingowing 10to' 1112;110111111113'the praximity 'Ofof its DUtOrop1111127051 to'113 the1112 131111.railway,V and:‘md the1112 lack1:121. 11:”'Of $11,221.10:superiar stDne512112 nearer tato KerichDK21121‘0 and Kisumu.K:

2. Water BoreholeBorehok' Data A,\ great::r2 :1: many111.1111. ban;~hales171012110123 have11112 been112211 driHed11.7111211 far111." water1.1111121 in111 the1112 211111115221cultivated part "Ofthe area,(112:1. particularlyp.11111.11.11.17i for1111 farms1:1:‘111‘5 nearF12 ElburgDn,‘. i.in the1112 vicinity11211113 of Mau“an Sum:Summit, 9.17111and (1‘1an the11.2 paDrly11111111111.31:watered2;" Rongai1121:1111 ‘ Plain.1 Search far" water“3121‘ has11.1; met1112‘. with111111 varying\kj‘lﬂn’ 2 success,“121155. failures1.1 11‘W‘h21beingﬂ 1 11“.:71particulary11:111.. cammDn ~n the regiDn. "Of(1{Kim‘Kampi1.11 :17:ya 310111MatD, 11112where2 the1111211211211water- table(111161115111;lies at a cDnsiderable21.311511121111312 depthdep ‘13 (McCall,512C111 1957B,11: mpp. 39)..191. A31 51111111121131summary of111‘ borehole111.112.}11112 inf'OrmatiDn1111‘11‘111311011 extracted fromfram the1112 record:recDrds 'Ofof the \1,Water(1111.1; Dev-1 21 elopment91—11211: Department,1. 231111112 Nairobi,Nairo'm follaws.113111.1s

r--- -~ """""'- -.- ;;;

BoreholeB-Whnl’c RecordsRccnnh

‘ ' \ No.“\ [ “C, 1' V ‘ k “A a; \ Locality (feet) (feet) (feet) ga ons I Depth I Water struck I Rest level I per(Ylldhour) C61 .. MauV Summit...... 352«f _ 219,238,325ﬁ‘ ‘2 285ﬂ I 300 C62 .. S.E. of Rongai .. .. 466 Dry

C63 .. S.W. of Rongai " .. 350 343 176 I 250 C67 .. Mau Summit...... 300“\ ? ? 60 C68 .. Mau Summit...... 375 365 ? 60 C69 .. Mau Summit...... 402 390 302 500 C108 .. Turi ...... 130 75 I 65 500 C131 .. Turi .. .. 150 120i48 43 1,500 C132 .. Turi ...... 150 138-140 52 200

CI47 .. W. of Rongai...... 318 160,233 I 69 155 CI48 .. W. of Rongai...... 254 ? 88 90 C196 .. E. of‘ Rongai1“ ...... 400 380 205l 1,000 I — C202:l .. S.J, of'\“Njoro ...... 450 435 124 2,000 ...... 460 150 C205 S. of Njoro I 380 340 C208:» .. W.{v of_ Njoro~1. .. .. 655* - - Dry I : ' C211, .. Njoro..\J .. .. 444 305’ I 295 400 C214 .. N.E. of Njoro .. .. 400 210 180 650 C220,‘ .. f3, qr .. .. 500" 470 110 S. of Njoro fl , 1,800 C223W .. S.s"of Njoro .. 613 350,500,5703 175 1,440 C241 .. .. 400 70 240 Molo township , 140,270 C258A .. W.ofNjoro..‘K .. .. 348 300-348 182 2,300 C272 .. Keringet .. 235 215 42 500 C273 .. Keringet ...... 416 370 280 2,040 C277 .. N. of Keringet .. .. 428{ 380x 300 2,080 C288 .. N.E. of Elburgon .'. .. 600 350 269 70 C293 .. E. of Rongai‘v ‘ .. 4157 390 375\ 900 C322 .. S. of Kilombeh .. .. 600 - Dry .. C327 .. S. of Njoro .. .. I 449 ; 1,7; a ’ 116 2,400, 130,350,-449 3751 C329 .. Molo township“W ‘ .. .. 318 270-290‘7‘ ‘ 1 241‘ 2,200L I C330 .. W. of Molo ..> .. .. 300“H 240:fix 515! 620‘ 7AM :1‘} ’1 C332 .. Keringet ...... 5045H4 470 I 230 500 C333 .. Keringet ...... 2722A: 170,235A‘ ,“ 80‘1 700’ .1 C334 .. KeringetK217 ...... 4063 390‘ 4 200H") 2,550; ”.1 C344 .. N.\ of»‘ Keringet.L'I‘i. .. .. 453 435 270‘ Q 2,100:1 W C349 .. W.\\', ofMI‘ FiveE“ ’ MileMM; PostPm; .. 425 390L 90wt; 6707 ? I :‘w W C351 .. FiveHm: MileBIND PostHM .. .. I 600 ' I 120 203 C375 .. _\.h,x,~fE‘.51r:mj1 .. 130M I ?1 N.E. of EIburgon 200 I 2,700 C376 .. ElburgonEH‘ Argon .. .. 300 230-265“ w ”A 1203111 2,500 C379 .. N.V- of”5 ElburgonIZH‘L 1'1““? " :: I 245‘ 220:ZH Artesian.\1\1C\l‘ 2,280 C422 .. Molo\Ioir» townshipmm :15:t .. .., 260 150-260‘5“; Tm 13030 2,900 C423 .. N.E..\,'fi. of01’ ElburgonE innrgun .., ..,. 7007H1} 640-680mm 7 M1 210:10 1,700 C476 .. N.E.\‘F. ofmi‘ Molo..\ltrflflw .. .. 33013w) 211-2352'] :3; 140HE] 720 C485 .. Njoro..\jom n ", , .. ..‘ _ 394W—‘r 275Z 7 210:20 3,000 C501 .. S. of KilombeKil-bafllfic ", ..‘ 3503S1.) 265‘55 221“1 650 :52‘ M). 450 '77 C502 .. N.E.NE. ofm‘ Molo..Main. .., ..M 5005m 253,290,450 177 \ 1,040 -500 C503[)3 .. NHE of \[L‘iOH ..,, H 520:20 ~ -i 20“‘1 C516(‘516 .A.. E. of MoloHMO .. .. M 630 110‘ 3 .U‘ (315 3383““ 900".‘Ud C541(7 41 ... . \ N.E.S. of0f ofKilombeKiiflMolo..mhc .. ::. , \ 40040V) \410,280,300,317$1.300.550, 615\317 223“3 \ 620(CW 332, 365 C5577 .. 3w. m'\l;C;1H< Sidings H 560 -L I - 7 DryDr} 6 . _ I Esageri , _ A . . . 400 -_ -~ Dr} C576 .. EsageriS.W.of McCalls.. Sidings.. 400 I I I Dry :: I

IiIr =

BoreholeBnrohoic Records-(Contd.)Records—I: C Emu-‘51“. 1

I Yield Depth Water struck“..ix' RestRoe: levelEcxc. Locality (feet) (feet) (feet)[‘1‘c r (gallons I per hour) - S. of McCalls Sidings 270 - - Dry S. of McCalls Sidings .. I 820 - - Dry E. of Elburgon .. .. 300 200 200 1,342 S.W. of Kampi ya Moto .. 406 345,397 337 1,100 N. of McCalls Sidings .. 550 - - Dry E. of Eburgon .. .. 750 - - Dry E. of Eburgon .. 490 425-490 249 1,200 N. of Elburgon .. 670 455 450 25 E. of Rongai .. .. 518 480--500 430 1,300 Kampi ya Moto .. :: I 602 - - Dry Esageri ...... 464 195,380,455 155 1,200 .. .. 438 170-410 140 300 .. .. 600 316 412 I S.S.E.ofofNjoroTeret .." 445 N. of Elburgon .. .. 972 824 550 1,000...

S. of Rongai " .. " 551 92-195 82 1,300... J

S.E. of Rongai .. .. 360 264 212 2,000I

S.E. of Rongai .. .. 599 527 510 1,300._.

S.E. of Rongai .. .. 250 130-204 109 1,300-_ S. of Njoro ...... 700 423,512-700 387 800 Mau Summit Station .. 550 550 66 1,200 N. of Elburgon .. .. 351 188,295 130 1,800 S.E. of Rongai .. " 495 481 475 600 S. of Kilombe .. .. 397 120,280 135 900 Elburgon ...... 390 198 69 3,150 Elburgon .. .. 600 201 113 2,720 S. of Kampi ya Moto 610 - - Dry f“- I N.E. of Rongai .. :: I 442..‘43 400-425.::_1:';. .515 392w: 1,800 r~ N.W. of Njoro .. " I 80050;] - - Dry 'l.\ rm Njoro ...... 410 355-390 263 2,820 (a. Molo...... 453 ? ? 2,500 C{1 S. of Kilombe .. :: i 310 90,255,280 62 I 2,100

C.1“: Mau Summit...... 420 95,160,320 75 412 Cr‘x_ N. of Mol0 ...... I 400 100,250,360 23 900 Cr‘: 395-445, 360 2,200

470-485 m C :1. 360,615,630 295 950 C("-I \:. 140,250,350 80 450

4.. CIf». 120,390 70 2,100

{a C I" 640 575 1,000

rx C I'. 520,570 520 1,760 Cf'. 150,225,450 110 400 I CF- 52, 166 20 2,400 Cf“ 120, 180 38 2,000 Cr. 38 2,400

p. 41, 180 I C 68, I?,. 330 I . 45- . 2,100

C("x

CI491.-. .. Njoro .. . . 532 352, 496 280 2,300

F“ CI504.. Turi.. .. 535 I 483 300 80

I CI585r--. .. S. of Njoro .. 510 : 305 285 300 CI638.ml .. i Mau Summit. . 505 470 435 1,030 CI641F‘- .. i N. of Elburgon .. I 600 445 250 1,350 ﬂ 717 167,421,680 120 1,000 CI647 .. I S.W. of Rongai i

I:"'"t ... Boreholeo Records-(Recc»rd~'—“ ’3Con7 (d.)

Depth IWater struck I Rest level Y1~ld No. I Locality ( (feet) (feet) (feet) I ga ons I I per hour) I C175~1 N. of Rongai 361 275,335-350' 241 3,000 C1809 .. I W. of McCalls Sidings 608 Dry CI867 .. ' N.E. of Molo.. 633 570-590 518 1,000 C1869 .. S. of Rongai .. 410 320-410 210 750 502 300 C1873 .. N.E. of Molo.. 737 I 680 C1888 .. N.W. of Londiani 374 330 160 3,500

C2099 .. E. of 1v10]0 . . 565 1 260 274 420 C2152 .. S.E. of Kilombe 660 I Dry C2157 .. Mau S'lmmit .. 770 I 642 572 2,200 C2181 .. Mau Summit. . 750 730 I 250 1,200

C2226 .. Mau Summit. . 760 I 645 600 240 C2276 .. I S. of Njoro .. 450 '228,355,4341 108 2,000 C228 I .. E. of Molo 435 300-400 1 250 960 C2286 .. N.E. of Molo. . 650 620 390 2,000 C2323 .. Mau S~'mmit . . 375 I 270,330 I 180 1,500 C2346 .. Rongai township "I 202 I 70, 140, 180 46 1,000 C2366 .. Mau Sl'mmit . . 416 350-380 I 300 3,000 C2428 .. Mau Summit. . 676 I 150,420,620, 132 1,200 C2448 .. E. of Teret 949 630,800,920 515 1,600 500 C2505 .. Molo township 600 1100,400-450 80 C2532 .. N. of Menengai Ha]t 930 I 600, 785 598 1,400 C2553 .. Molo township 550 220, 420 1 220 1,500 C2563 .. S. of Rongai .. 284 160 67 3,000 C2564 .. N. of Elburgon 718 346,520,630 352 3,000 C2579 .. Keringet . . 542 160,420,485, I 50 3,000 C2640 .. N. of Keringet 752 1 200'644'730 534 1,300 C2641 .. S. of Kerisoi .. 615 450,505,610 318 1,300? C2665 .. Kedowa 632 - Dry C2678 .. Kedowa 450 385,425 1 140 550 C2680 .. Molo. . ..I 280 260 160 300 C2692 " W. of Londiani ..I 410 208-390 153 2,000 C2693 .. N.W. of Londiani ..I 450 360 200 1,200 C2700 " N. of Kerisoi 560 I 260-520 159 520 C2726 .. Kedowa . . 404 120,312,404 80 220 500 C2795 .. S. of Njoro 230 180 1 25

1 C2808 .. N. of Londiani : ~ I 277 270 257 600 C2809 .. N. of Londiani 240 40 ? 360 C2810 .. N. of Londiani 140 50 ? 260

C2831 .. N. of Londiani 520 I 500 52 650 C2832 .. Mau Summit. . 176 160 90 240 C2853 .. S.W. of Rongai 582 540 330 650 C2879 .. N.W. of Molo 100 Dry C2880 .. N.W. of Molo 240 I Dry C2882 .. Molo township 750 270-750 320 2,200 592 33 1,000 C2894 .. S. of Njoro .. 64, 293, 553 I C2938 " N.W. of Londiani 178 178 90 250 C2973 .. Molo township 739 720 162 700 C2985 .. N.W. of Londiani 476 220-445 256 2,000 C2995 .. Five Mile Post 400 ? ? 100 C3005 .. Elburgon township 407 ? ? 4,000 C3072 .. Mount Blackett 608 60, '340 I 24 3,600

480-580 I C3076 .. I Kedowa 600 240,477,5781 83 3,000 :"n H '.\'.\\'_‘_f\1u~1;§' 465 440 I 126 2,500

C3150 .. I N.W. of Mol0 I 37

. VIII-REFERENCES\ iiI—RICE’EREVCECS

r; " ‘. .4 m AL Baker,m. B.{'3 H.,' 1958.-"GeologyH W ( LI‘ _ of7 the:‘L Magadi,‘w KL. Area".\ L. Rep..‘w. Geol. Surv. Kenya 42. Binge,' F.‘ W., 1962.-"Geologyh 1 -L c. ofL the‘1": KerichoKM. Area".KILL Rep.f Geol.. Surv.s , Kenya[\Lu “50. . . u Bullard,t E. C,.. 1936.-"Gravityv x MeasurementsH. w: in EastL M Africa".X L Phil Trans.. ~ Roy.\ Soc. 235, _.‘<_<:445-531. V. Gregory, J. W., 1894.-"Contributions to the Physical Geography of British East i Africa". Geog. Journ..4 4,‘ _289-315;. ‘ _408-424;7 505-514.I . . -. . .— «- "ﬁ’ 1920.-"The_ i 1:“ African‘ L H RiJitKM Valleys".‘x \ Geog. Journ.'1/(‘3 56,{R 13-47;.74 327-328.m. r.‘_.\

1921.-"TheL‘ 7 ”'i :10 Rift9‘ Valleysq \ and‘I‘d GeologyL mm. ofW East} m Africa". SeeleyKroc; Service,. London.

.. r ‘ — :V w L r‘ Jennings,[L‘ D.'1 J., 1964.-"GeologyW ; v of theW Kapsabet-PlateauL‘ \ .1" MIL 1 Area",L Rep. Geol Surv. Kenya 63. Kent, P. E., 1944.-"The. 7 » Age and Tectonic7 .» Relationshipsf of East African“.1‘ L' Volcanic Rocks".“w ~ Geol. Mag.u. 81,8. 15-27.if Leakey, M. D.‘ and1 L. S.\ B. Leakey,. 1950.-"Excavationsr\L ...m.~ ~t the Njoro4 River\ . Cave".. O.u.P., Oxford. V x’ Maufe, H. B., 1908.-"Report Relating to the Geology of the East African Protec- torate". Colon. Rep. Misc. Ser. 45. Cd.. 3828. McCall,. G. J. R., 1957A.-"The‘ ‘4 ‘ Menengai Caldera. Kenya1L. Colony". XX Int. Ceol. Congr., Section 1, Vol. 1, 55-69. - 1957B.-"Geology and Groundwater:" Conditions(H'»I‘:.J"1MW< in the Nakuru Area". Tech. Rep. 3, Hydraulic Bran~h, M.O.W., Nairobi."aim. - 1962.-"Froth-flow Lavas Resembling Ignimbrites in the East African Rift Valleys". Nature, 194, 343-344. 1963.-"Classification"(’{;;a§...iy:urw ofn“ Calderas:t...ﬂd:'..~. 'Krakatoan'‘KI‘nkam'm' and”mi 'Glencoe'‘Iyicnmc‘ Types".T;‘-'~J Nature, 197, 136-138.

1964.-"Kilombe”£1.01”. Caldera,1. 1L“.. Kenya."KL. ” Proc.[Dhu, Geol.(if“‘. Assoc.“"“V 75,75 563-572.:1".:’<fi:r 1967.-"Geology2967. “Ecology ofn? them: Nakuru-Thomson'sNukwm Ihonxon's Falls-LakeF.1HVT1JLL- HanningtonHLLHIlinglflTl Area",\rrr." Rep.R.4» Geol.(.‘m’. Surv.mm, KenyaK. 4» 78.7x. McCall,“\YCCILH. G.(r, J. H.,H.. Baker,Baker. B. H. and:‘Rd .I.J. Walsh,\\ 15“.. 1967.-"Late1%? 779'l 1L1: Tertiary‘I'L‘riim'y dand Quaicz'mi‘Quaternaryfi' SedimentsScdinwcms of the KenyaRwy; RiftR... Valley".\';L1Ie}'”. Inin "Background"Backgmund toLu EvolutionF“ 411111a inin Africa"..\fr.g.1" Ed.Eli. W.\‘». W.\\. BishopBiahop and.r‘Li D. Clark,Hark. 191-220.1413311. UniversityL’nixcrsflf.’ of“I ('21.;Chicago..!L’) Press.P.‘ C». Neilson,\L‘iixun. A. ‘1“T., 1921.-"Igneous1921. "lgncnm RocksRod-u from{rum British EusLEast Africa"..\1‘:‘ic.1". Appendix~\ppcndix IV to "l'he"His Rift Valleys 12ndand Geologyfleeing} of EastEnd Africa",kr‘rILu”. by .l.J. W. ('Et'cgor}.Gregory. Prior,. 'im‘, G.(j. T.,1'. 1903-"ContributionsNu} ”(f «.‘r11:'ji.‘L.:imn~. toIn the:11; PetrologyPqinfiogy u?of BritishB31? \h EastEm: Africa".\l‘rjgzi“. Miner.'xfbwr. Mag.film. 13,13 a 228-263...._..’“‘\—263. Pulfrey,PUWCY- W.,Wu 1960.-"GeologyWW» “(53010:43' andmd MineralMiriam] ResourcesResourcm of0f Kenya".Kenw". Bull.HIM. Geol.(".‘w'. Surv.Mm: KenyaNew)»: 2. - 7 1960.-"ShapeWM). 4’Shu‘p: ofuI' theWe Sub-Miocene\LI}‘—.\Ii(!fi.‘[li‘ ErosionErmiun BevelBw-l.c. inin Kenya".Reflux“. Buns".Bull. Own”.Geol. Surv. Kenya 3. Ross,‘ . , C. S. and:md R. L.1., Smith,Smith. 1961.-"Ash-Flow196L~ ".»\.sh-Flow Tuffs:Tufi‘s: theirtheir Origin,Origin. GeologicGeologk‘ RelationsRcluiinns

and Identification".ldcntiﬁc.xtim”. ProfeS'SionalPr‘nfcx‘xinna] PaperPapcr Nw.No. 366.366, (jamGeol SU"\‘,.Surv., U.S.A.USA.

J D) 38"

1'--= Shackleton, R. M., 1955.-"Pleistocene Movements in the Gregory Rift Valley". Geol. Rdsch. 43, 257-263.

Smith, R. L., 1960.-"Zones and Zonal Variations ill Weldedu.“ Ash Flows". Profes- sional Paper No. 354F, GeoI. Surv., U.S.A. Survey of Kenya, 1959.-"AtIas of Kenya"..\3‘“ﬂ < Thompson, A. O. and R. G. Dodson,,_ 1963.-"Geology of the Naivasha Area". Rep. Geol. Sllrv. Kenya 55. Walker, E. E., 1903.-"Reports on'- the1‘ Geology of the East\, Africa Protectorate". Colon. Rep. Misc. Ser. 11, Cd. 1769. Walsh, J., 1969.-"Geology of the Eldama Ravine-Kabarnet Area".‘ Rep. Geol. SUTV. Kenya 83.

'r Williams, L. .A. J.-"Geology of the MauM .1 Area". Rep. Geol. SllTV. Kenya 96 (awaiting publication). Willis, Bailey, 1936.-"East African Plruteaus and Rift Valleys". Carnegie Institute, Washington.

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