HiroshimaHiroshimaGeographicalAssociation Geographical Association

Geographical Sciences vol, 43 no.1 pp, 18-36, 1988

Evolution of Coastal Laridforms inthe Western Part of Sri Lanka

JINADASA KATUPOTHA*

Key Words:evolution of coastal landforms, SriLanka, late Pleistocene, Holocene, landfOmi classMcation

Abstract

Geomorphic and geologic evidence shows four different stages {Stage I-IV} in the evolution of coastal landforrns on the west coast of Sri Lanka during the Iate Pleistocene and Holocene Epochs. The author assumes that the old

ridges in Stage I at Sembulailarna, Kiriyanl(ailiya, Pambala, Wiraliena, Uluambalarna and Kadrana areas have been fonned precedng the Holocene transgression. Low hMs and ridges in the area were coated mainly by wind blown

sand, fonowing the lower sea levels during the Late Pleistocene and Earty Holocene Epochs. Radiocarbon datings en the west and seuth coast$ reveal that the sea level remained 1rn or more above the present sea level between

6170± 70 and 535e± 80 yr B. P. During this transgression, the forTner drainage basins were submerged and headland bay beaches were ereated. Many wetlands aiid beach ridges, particularly in Stages ll, III, and IV were gradualy formed owing to rninor oscMations of sea levet after mid-Holocene. Most of these landiorTns haveaclose

relationship with main climatic zenes of the country.

1987; Katupotha, 1988) also help to deterrnine their I. Introduction evolution.

The island of Sri Lanka has a coastline over Coastal Iandform maps of the study area were 1920 km in length, exhibiting a diversity of coastal cornpiled by means of interpretation of aerial photo- landromis. The coastal lowlands with elevation graphs (1:40,OOO-Survey Department of Sri Lanka,

from mean sea level (MSL) to 30m consist of 1956) and field observations. Boring logs data of

barrier beaches, barrier islands, sand dunes, buried the text received from the department of Highways,

and emerged coral reef patches, lagoons and lakes, State Development and Censtruction Corporation,

marshes, mangrove swamps and beachrock shoals Water Resources Board and Multiplex Overseas

which are mostiy related to the Holocene Epoch. Constmction (Pte) Ltd. of Sri Lanka. Soil auger

The coral reets, beachrock and mangrove swamps observations also were carried out in selected areas.

are specinc features particularly characteristic of II. Physicalbackground tropical coasts.

This study deals with the prelirninary investiga- About nine-tenths of the island consists of crys-

tions to identify coastal Iaiidforms and their evolution talline rocks from the Precambrian age. The rest

based on weathering conditions, colour, constituents includes sedimentary formations which were depo-

of sand and height of the barrier ridges, distributionsited in the Jurassic, Miocene and Quaternary of beachrock and wetlands on the west coast of Sri periods (Coates, 1935). The quatemary deposits "Older Lanka (Fig. 1). Radiocarbon datings on the west of Sri Lanka are designated as Group" and "Younger and southwest coasts (Katupotha and Fajiwara, Group" (Cooray, 1967; 1968a). The Old- `Red

* er Group ismade up of Earth',basalferrugi- Postgraduate student, Hiroshjma University. Address nous gravel and terrace gravel deposit after from Aprl 1988: Department of Geography, University tplateau Wayland, 1919; cap after of Sri Jayewardenepura, Gangodawila, Nugegoda, Sri gravel Wadia, 1941). Lanka. Holocene deposits made up of alluvium, dune sand,

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J. KATUPOTHA:Evolution ef Coastal Laiidfdrms in the Western Part of Sri Lanka 19

se. 81' 82' 1 t

9.

EE

8.

7-

6' l

Lowlands

Uplands and highlands

AZDZIZWZ(a)Cb)(c)(d)(e)= ARID ZONE

:: DRY ZONE

= INTERMEDIATE ZONE

= WET ZONE

Mundal Lake

Chilaw Lake

Negombo Lagoon

Beira Lake

Bolgoda Lakes

Fig. 1. Location map of west coast, Sri Lanka, A, Between Udappu and the Dedum Oya outfal1 (deltaic topography); B. Between Mahawewa and Seeduwa (ridge and swale topography); C. Between Pamunugama ancl the Kelani

Ganga outfal and the adjoining area of Colombo (wetlands and bedrock related topography),

beachrock shoals and wetlands are widely distributed weathered bedrock and capped by lateritic soils are

across the entire coastal lowlands (Fig. 2). North- located close to the sea (Fig. 1).

ern and eastern coastal lowlands correlated with Marine processes, mainly waves, currents and

"intermediate", "Dry" "Arid and Zones" are widely tides, are strongly govemed by the southwest and

covered by sand dunes, sand bars and barrier nomheast monseons. The seasonal waves are im- `CWet" beaches with lagoons. Nong the southwest portant agents which control beach behaviour and and south coasts, residual hills and ridges formed of the pattem of littoral drift around the island. High

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AB III. Outline of study area h..p 4 The area betweenMundal Lake and the Kalu Ganga outfal1 extend over the about 1,675 km2 with-

in the latitude of 6040' and 8000'N and the longitude of 79095' and 80002'E. The Deduru Oya (river), the Maha Oya, the Kelani Ganga and the Kalu Ganga

are the main drainage systems of the area with

runfoff of 1,130; 1,485; 5,582; and 8,187 by mi1lion m3! per year respectively (Wijesekara and Het- tiarachchi 1983). Approxirnately 75 km2 of the tota1

area is occupied by large water bodies namely Mun-

dal Lake, Negombo Lagoon, Beira Lake and Bolgoda Lakes.

The present shoreline, particularly to the north

of the Kelani Ganga outfal consists of barrier

beaches, barTier islands, spits, sand dunes and

beachrock shoals. Low hills and ridges which are

coated with wind blown sand extend inland with

dfierent elevations. The wetlands such as mud

flats, mangrove swamps, marshes and so on have o been developed between the present shoreline and 'marine, Fig. 2. Sirnplified landiorrns map of Sri Lanka (After the low hills and ridges by fluvial and bioge- Swan, 1982), nic processes. The shorelme to the south of the

A: Holocene beaches and sand dune B: O;d beaches C: Lime- Kelani Ganga outfab is characterized by headland bay stone low[ying D: Fluvial, deltaic and marshy lands platforms beaches. and associated water bodies E: Undulating terrains F; Low hills, ridges and terraces G: Steep scarps, ridges and associ- The uplands, behind the coastal lowlands ex- ated plateaus H: High plateaus and plains tend above 30 m from MSL. They are also widely

covered with Iateritic soils or weathered bedrock,

waves (height 5m or over) are seen along the south- and have been subjected to fiuvial process resulting

west and south coasts during the strong southwest in the erosional land-forms. Many strearns of the

monseon season from May to September. The west and southwest part of Sri Lanka flow in a area of Hikliaduwa, on the southwest coast, is situ- northwesterly direction on the uplands and hilly

ated at a nodal point of the littoral drift. North of lands in accordance with the geological structure, Hikkaduwa the drift from the southwestern waves is But in the lowlands these strearns fiow consequently

toward the north, and south of that point it is to the in a westerly direction supplying a considerable fine material to the south (Gerritsen, 1974). The tidal range on the Sri volume of terrestrialwater and

Lanka coast is smal, ranging from the mean low lowlands. -37cm water spring (MLWS) at and MHWS at The coastal area between Udappu and the De- +40cm from the MSL (data based on Colombo duru Oya outfal is deltaic, while at the mouth the datum level 38 cm below MSL, Tide Tables, vol. 2, Maha Oya the delta is negligible in size. The lower Pacfic and indian Oceans, 1982). The phases of part of the Maha Oya and the Kalu Ganga lack deltas the tides have an obvious influence on the existence of any significance size which shows a drowned

of mud flats, tidal fiats, marshes and mangrove valey topography. A flood tidal delta is formed at

swarnps. the mouth of Negombo Lagoon in significance size,

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21 J, KATUPOTHA:Evolution of CoastalLandiormsinthe WesternPartef SriLanka

but no ebb deltas are formed in the study area. A. Between Udampu and the De(h`n` (lya o"ij1ill IV. landforms of the study area Coastal (delimc mpQg?ropluij

area comprises Coastal landforms in Sri Lanka have been This deltaictype coastal 1akes,

marshes, man- forrned by combined processes of marine, fluvial and tidalcreeks and wetlands (patchesof

biogenic, and the basement structure of the lowest grove swamps). Streams such as the Katagamuwa

and the peneplain. Three regions have been selected from Oya, the RatarnbalaOya SenegalOya flow connected the study area based on diversity of coastal land- inte a 1argetidalchannel which is to forms for detailed discussion emphasizing their ex- Mundal Lake and the Deduru Oya outfal. From

tension, underlying material and the above men- the Deduru Oya outfal to the north, a narrow recent tioned processes (Table 1 and 2). The diversity of sand barrier ridge extends about 15km leng and

ecological conditions of mangroves on the west coast about 200m wide, and is coated with low creeping

is shown in Table 3. vegetation mainly ipomea Pesccipre, SPenipc littorous A. Between Udappu and the Deduru Oya out- and Canavalia rosea. It becomes graduaky wider north from Udappu village 3). fal1 (deltaic topography) to the (Fig. Before B. Between Mahawewa and Seeduwa (ridge the formation of this barrier ridge it is assumed that mentioned rivers fiowed separately a and swale topography) the above into C. BetweenPamunugamaandtheKelaniGan- 1arge water body. Present water creeks and wet- ga outfal and the adjoining area of Colombo lancls in the area are forrned of gradual filling by (wetlands and bedrock related topography) terrestrial sedments as well as marine sediments. in the past two hundred years the sedirnentation

Table 1.Classhication of coastal landfbrms on the west coast of Sri Lanka. Extention Form Lanaform DryLand Wet1and

Marine Beach-t Intertidallevel Springtidallevel Barnerisland Barrierspit Sandbar Mudfiat Beachridge Beachreckshoal Headland Tidalflat Aeolian Dune Ceastalbe]t Duneridge Marineand Mangrovelowswarnp Mangrovehighswarnp biogenic Saltlowmarsh Salthighmarsh Marine,fluvial Mangrovelewswamp Mangrovehighswamp andbiogenic Brackishlowmarsh Brackishhighmarsh Fluvial Naturallevee Alluvialflat

Fluvial Floodplain Naturallevee Valeyplain Floodplain Upland Fluvialand Freshwatermarsh,Swamp biogenic PeneplanationResidualhillsandridges WaterbodyEstuary,lagoon,tidalcreek,stream,channel,lakeandpond

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Table2. Detailsof geomorphic units on the westcoast of Sri Laidca, Geornorphicunit Form Process Agent Underlmgmaterial Age Beach BelowspringtidalMarine Waveandcurrent Unconsolidatedquartz Holecene level sand'shelldebriesr Banierisland Variable,aboveMarine Waveand Finetocoarsesand'shell'Holocene springtidal current shelldebries level Sandbar BelowspringtidalMarine WaveandcurrentUnconsolidatedquartz Holocene level sand;shelldebries Beachrock lntertidallevelMarine CementationFinetocoarsesandwithHolocene sheUdebries,Heavy minerals Beachridge Variable,aboveMarine Waveand Finetocoarsesand Holocene springtidal curTent level Sanddune Variable,belowAeolian Wind Quartzosesand Holocene 10m Duneridge Variable,betowAeolian Wind CompactaeolianmaterialHolocene 10m Titalcreek Alltidallevel Marine TidalcurrentMudandsilt Holocene Tidalflat intertidalIevelMarine TidalcurrentMudandsilt Holocene LagoonalbeachBelowspringtidalMarine TidalcurrentFinesand,mudandsiltHolocene level MangroveswarnpBelowspringtidal MarineibiogenicTidalcurrent, level streamwa-Mud,siltandloarn;peatHolocene ter,'organlsm

Mudflat BelowspringtidalMarine tidalcurrentMudandsilt Holocene level Saltmarsh IntertidallevelMarinefoiogenicTidalcurrenti'Mudandsilt;peat Holocene orgamsm Brackishwater belowspringtidalMarine,fluvial/ Tidalcurrent, Hoiocene marsh level biogenic streamwa-Mudandsilt;peat ter,org. Freshwater Abovespringtidal FluviaVbiogenicStrearnwaterf Mudandsilt;peat Holocene marsh level org, Smmp AbovespringtidalFluvialfbiogenicStrearnwaterl Mudandsilt;peat Holocene level org. Bogflat AbovespringtidalFluvial StrearnwaterMudandsilt;peat Holocene level Mttvialflat AbovespringtidalFluvial StreamwaterClay,siltandsand Holocene level Pointbar Abovespringtidal FIuvial StreamwaterClay,silt,sandandgravelHolocene level Naturallevee Abovespringtidal Fluvial StreamwaterClay,siltandsand Holocene level Floodplain AbovespringtidalFluvial StreamwaterClay,siltandsand Holocene revel Valeyp]ain Abovespringtidal Flttvial StreamwaterClay,silt,sandandgravelPleistocene? Ievel Headland Variable Eustatism? SubmergenceSandstone,Crystalline ? oremerg- rocks ence ResidualhillsandAbovespringtidalPeneplanation'nses WeatheringLateriteandlateriticsoil ? level

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J. KATUPOTHA: Evolution of Coastal Landiorrns in the Western Part of Sri Lanka 23

Table 3. Ecological conditions of mangrove species on the west coast of Sri Lanka.

Treesize(height Species inmetre)FrequencyoftidalinandationNormalsaiinitySoil Extent dayslmon,

Rhizophoramucronatarnedium(8-15)IC191.2-3.0deepsoftmud edgeoftheestuaries,lagoons andtidalislets 'estuarles Rhizophoraapiculatamedium(6-10)IC191.2-3.0veryfinesand 'estuarles BruquieracylindricasmalYbush(3-5)<10 1.a2.5stffelay Bruquieragymnorhizamedium(6-IO)<10 LO-2,5wellaeratedveryedgeoftheestuaries,lagoens finesand andtidalislets ' Ceriopstagal medium(6-10)2C301,2-6,Ovariable lanclwardedgeof]agoons

Avicemiamarina smallfbush(3-5)2C30 1,2-9,Ofinnmud tidalislets Avicenniaoficinaissmaf(3-5) 10-20 1.2-3.5variable landwardedgeoflageons Aegicerascomiculatumsmal(3-5) <15 O.572,Oloatn riverbanksfarfromsea Xyloearpusgranatummedium(8-15)15-301.0n2.5sandymud nearriverbanks LumnitzeraracemosasmalVbush <30 1.0-9.0loainandclay riverbanksnotfarfromsea Excoecariaagalochasmab(3-5) <15 LO-2.5clay landwardimgesoflagoonsor'estuanes 'estuanes Sonneratiacasiolaristal(15) 2C30 1,O-3.0deepmud,coraletc, Heritieralittoraismedium(6-10)<10 1,O-2.0sandyloam landwardfrirtge,nearriver banks Nipafruticans smal-medium(5)20-30LO variable irmerestuariescreeksJ Hibiscustiliaceus small(2-3) <10 2.0 sandyloamandclay]andwardfringe,nearriver banks Scaevolasericea smab(2-3) <15 1,O sandyloam Zandwardfringe,nearriver banks Clerodendroninermesmall(1-2) <10 LO sandyloamandc}aylandwardfringeofestuaries Morindacitrifolia medium(6-10)<10 1,O sandyloamandclayestuaries,riverbanks Pandanustectoriussmal(2-5) <10 2.0 yariable landwardfringe,riverbanks AcanthusMcifoliussmal(1.0-1.5)IC20 LO variable landwardfringe,nearestuaries

(Katupotha, 1982; Madduma Bandara, 1982; Swan,1982; Tnasley and Fritsch,1905 and field survey).

rate has been raised up due to the clearing of extended further east than at present, and this infers forested land in Intemiediate and Wet Zones for that it resulted from a transgression during the

plantation agriculture by Europeans instead of gra- mid-Holocene.

dual filling by natural processes and human activity The area to the north of Udappu village is mainly chena (shifting) cultivation prior to the mid- occupied by extensive sand dunes, The coastal eighteenth century. Similar physiographic changes dunes occur 300km along the coast in Sri Lanka

prevaded at the Chesapeake Bay, on the western mostly in the Dry Zone in accordance with a long dry shore of Maryland (Froomer 1980). season and adequate sand supply (Swan, 1979). The lagoonal deposits in this area consist of silt, Dtmes on the wet tropical coasts are generally consi- dark clay and sandy clay with intercalations of sand dered to be poorly developed because of prevailing (Fig. 4). They are very soft when wet and ex- environmentat conditions Genins, 1964; Pye, 1981; tremely hard when dry. Sand and sandy clay de- 1983a and 1983b). Sirnilar conditions appear on the posits mixed with oyster shells have been reported west coast of Sri Lanka (Fig. 3). The sand dunes

several metres down from the surface, about 2km to of this area are mainly transverse dunes in type

the east frorn the present Dedum Oya outfall extendmg 5-10m or more in height. Parabolic

(Cooray, 1967) which indicates that a large lagoon dunes and a series of dune ridges are strongly

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24 imgeFL\43-1,1988

-gr

EXPLANATeRYNaTES

Extent Pro[tss::2( Gtomerphit unir

avmu Fore tiuneSand

dure end dune ridge

BeFFier beech or b?:igRa

Old berrier ridge

:e::::s EIEIE]EptBeechrock {ebove MwwS)

Bee[hFo[k (Itt)

Tidel fletLagoonel

beech Eww-eeI,,uad flatMengrove

sHentp

MershSuempNatura1

ievee -･･.mpRllllllilMtwuaDigpm

Peint berBog

1es flatAbendened

chanre]

Alluviei flet

velley plain or Mge,g

Residuel htlls end rtdges x wa

ts River or streem

II Leke er pond

Fig, 3,Sketch map showing landfbrms around Mundal Lake. Bore holes and cross sections are referred to in Table 1. (1) Stage I, From Late Pleistocene to Early Ho!ocene; (2) Stage II. Between mid-Holocene and first phase of the Late Holocene C6600-3700 yr B.P,); (3) Stage IIL Late Holocene (Around 3700 yr B.P.); (4) Stage IV. Modern. Similar stages are shown in Flgs. 5, 7, 9 and 11,

oriented seuthwest to northeast in direction owing Jayawardana, 1979). Dug well observations at to the prevailing wind. Quartz is the main consti- Kiriyankalliya reveal that the ridge is covered by fine tuent of the dune sand, and is mixed with other to medium brownish black to light gray sand (Figs 3

heavy minerals of imenite, garnet, hypersthene, and 4). However, no sheil or shell fragments were

sillirnanite, rudie, zircon, monazite, spinel and so on. found. Soil auger observations on the eastern bank

A narrow elongated ridge rises 2-4 m, to the of Mundal Lake revealed that the ridge consisted of east of Mundal Lake (Fig. 3). ln many places of bluish gray clay and coarse sand with well rounded this ridge, and the low hills and ridges of the adjoin- granules. The weathered bedrock in the area is ing areas are covered with Red Earth formations predominately granitic gneiss rock and its level is rwayland, 1919; Cooray, 1968a; Dahanayake and approximately 10 m below surface ievel (Fig. 4).

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JKATUPOTHA Evolution of Coastal Lan（廷o s m the Western Part of Sn L ka

・ ⊥ ：轜 − 擁 i：：iiii窟il瓢 1篇 ≦ 響 飜飜 蕪 ・ 1響：1∴ ・ ・ 一 一 ｛；：ll；1』 贈kmノ iiiiiiii − w ＝Water body ” l：：1：：ll、e 」 」 〔 ・ ∴ 一9 ）

嵋 ，、 ， 心 奪

くa ） 麟幽 欝∴ iFlne to じ oarse1sand （h 〕

謂 CL δy

Weatheped bedrock ，。 ） 麒 黜 糊 Flg 4 Cross sectlons and the stratlgraphy of lowlands between Mundal Lake and the Deduru Oya 二 outfaj1．

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Z6 fugeflF\43-1,1988 f II M `b

Rs. ltr AMBAKANDA-ILAthx ilRANAWI'L'At' r-

K

ct)g

- km -SSi

Fig. 5. Sketch map showing landiorms between Mahawewa and Katuneriya. Bore holes and cross sections are referred to in Fig. 6.

ridges extend parakel to the coast as we- as in a B. Between Mlxhawewa and Seeduwa (n'cige and northwest to southeast direction, varying in height swale topclgrapiut) (Figs.5 and 7). Abeut 1.5km to the east from The whole coast in this area is made up of a Marawila Town, the soil layers of these old banier

berm and a series of inlandbarrier ridges. These ridges (about 5-10 m in height) are composed of fine

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J.KATUPOTHA: Evolution of Coastal Laridf6rms in the Westem Part of Sri Lanka 27

,,E+80j

-10 = -I fd Foredune

d = bune fd dmgw ob ch st w Nlr = Marsh

mg = Mangrove swernp K77.=V. L r :t River

ob = OLd ridge (beach ?)

w = Vater body

ch = [hanneL

b ob st = Stream ch 1 = Bore hoLe XN)K= weathered bedpock M lfsxxxxxXN,silN ×××××

m+5

"} if,'s･'/t,f,I:'･e.;.i;/.:tt:.i't:tttJ/.;t-t･L,CLayFine o sand ;I... Fine to [oarse -5 ÷ :.･;-: send with ctay : :･ =･-:-i--t-t;--it-:i-:':'-::::"::;･:.--t----:':':::':':'::1,si:f.:.ri-tt+:i::::'::'":':':':::t".･Fine sand with ctay Medium te cearse -10 sand Nith cLa>t Mediurn sand [earse sand with -IS gravet Coapse sand

Gravet with cLey

-20 '.e ?' Weathered bedrock

?

Fig. 6. Cross sections and the stratigraphy of barrier ridges between Mahawewa and Katuneriya.

sand, gravel sand and ciay beds. Dug well observa- south, a narrow unstable recent spit extends about 1

tions at Wirahena and Koliniadiya near Katuneriya km long, and further south the beach ridge becomes

show that these ridges are composed of reddish- fairly stable and wide. Dug well observation at

brown to bright-brown or yellowish-brown fine to Ettukala about 600m eastward from the present medium sand (Fig, 6). shore shows that the surface layers of this ridge are The outfal1 of the Maha Oya has been defiected composed of yellowish brown fine to medium sand by sand spits which intermittentiy changed. This (Fig. 6). unstable condition is due to the influence ef flood From Palangature to Seeduwa weli-developed

water as well as changes of wave direction during barrier ridges are separated by swales (Fig. 7, ridge

the southwest monsoon. The soil gayers of the and mmel after Cooray, 1967). A long swale

northern spit which is presently stable show thin which is extended between Palangature and Seedu-

layers of fine to medium sand and coarse sand with wa, has demarcated low ridges and high ridges in the

heavy minerals. From the Maha Oya outfal1 to the eastern part of Negombo Lagoon, and their heights

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28 tampRV#43-1,1988

'. IV

p:7sc oQ

ULUAMBALAMA

s R

IIIglgiNEGOMBO

AGOON

Fig. 7. Sketch map showing landiorms between the Maha Oya outfal1 and Seeduwa.Bore holes and cross sections are

referred to in Fig. 8.

vary from 1 m to 3 m and 5 m or more, respectively. from the Deduru Oya outfal to the Maha Oya outfall

At Dalupotha, Uluambalama and Kadirana areas the intemittently, and from Negombo to the Kelani

high ridges are made up of dark brown to yellow Ganga outfal1 continuously. Sllch cemented beach sand extending up to 4m deep from the surface sand (beachrock) widely occurs along the retreating (Fig. 8). The layers beiow this sand consist of light coasts (Russel, 1959 and 1962). Beachrock on the

gray or grayish white coarse sand and pebbles with west coast is rich in ilmenite and other heavy minir-

clay. als mainly magnetite, spinel, rutitle, Zircon, garnet Well developed beachrock shoals in the inter and sillirnanite (Cooray, 1978b). tidal zone (ITZ) and slightly above MHWS extend

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J．KATUPOTHA ： Evolution of Coastal Lan（iforrns in the Western Part of Sri Lanka 29

・ b ・b ・ b ・ b 駕

” 7 b ＝Bar 厂 ieris しand bob ob sbf br 臨 ： 堕 …△ ’＿ へ 頴 ’ 州 R ｝ ・ 亨 旦 “ ］・ ；・ ・ Ha ・ ・ “ ｝iil；i，i ob ＝ 0 しd ridge （be 己 ch ？）

bf ＝ Bog fL ヨt

「 ＝River

s ＝Swamp 卜 B・ ・ e ・… ・ ・ “ 臘 h・ ・ ed 圃 …

7 0 2 」＿■■騨國■； km （o ） m 1： i6 窟 澱 Fi． ， t。 ． ，dium se ． d の∵．∴r．．似r．，’’∴r

＋ 4

tt−tt−ロtt’t 「・：・ ． 2 ．’賠 （m ） つ Fine sand

、， 門edium to coarse sand Ol ．一．＿ L −2 M ， d ・・ m ・an ・ … h ・ L・ ・ ？

Weathered bedro ［ k 〔q ）

、 1

（P ）

0

−5

−fO

−15

−20

Fig．8． Cross sections and the stratigrraphy of the area between the Maha Oya out f訓1 a皿d Seeduwa．

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30 thXN･\43-1,1988

ture of this area (Fig. 9), and consists of peat de- C. BetweenPalatrgatureandtheKelaniGairgaout- posit which covers aii area of 21 km2. This deposit and the cuijoining area Cotombo fatl of (toetiandshas an average thickness of 3.7m and maxirrium and bed,ocle related topagtmphy) thickness of 5m (Dissanayake 1984). Three hori-

Muturajawela marshy land is a significant fea- zons have been identified based on the stratigraphy

4.･1･t ''=-,

-'.-Q't'''

PAMUN vt

o

Fig. 9. Sketch map showing landiorrns around Muturajawela area.Bore holes are referred to in Fig. 10.

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J. KATUPOTHA:Evolution ef Coastal Laridfbrrns in the Western Part of Sri Lanka 31

A c e

Peat + pLant Leaves and roats

m-IS

Peet + tree trunks

-4e ----!--!

Thin tLay LayerCLayey peat ctay

Fig. 10. The stratigraphy of the peat deposit at Muturajawela (Dissanayake,1984).

of the deposits (Fig. 10). The top horizon has been V. Evolutionefcoastallandforms formed by an accumulation of plant debris in a

lagoonal environment. The middle horizon was Four Stages (Stage I-IV) in the evolution of formed by a sudden input of terrestrial sediments coastal iandforms on the west coast, in late Pleis-

and debris of higher plants into the marshy Iagoon. tocene and Holocene Epochs can be identfied based This horizon indicates a sudden environrnental on geomorphic and geologic evidence. Wayland

`Red change. Marine gastropods and pelecypods occur- (1919) reported that the wide occurrence of

ring as fossils of the bottom horizon indicates that Earth' is connected to the aeolian process, and there were palaeo sea level oscMations (Dis- desert like conditions occurred in much of the low- sanayake, 1984). The extensive lowlands on the lands during the Pleistocene Epoch. If one hundred lower part of the Kelani Ganga, especcialy at Mat- fathom and ten fathom isobars are accepted as sub- takkuliya, kelaniya, Peliyagoda, Urugodawatte, Mah- merged shorelmes mentioned by Daraniyagala

gawatte, Maradana and Beira Lake areas have been (1958), it is inferred that there were lower sea

filled with various types of sediments. In the area levels in Late Pleistocene and Early Holocene

between Colombo and Mount Lavinia, the residual Epochs respectively. Following such dry clirnatic

hilis and ridges are close to the sea (Fig. 11). In conditions the aeolian sand piled up as paralel bar- the area frem the Kelani Ganga outfal1 to Colombo rier ridges, and coated low hills and ridges in the

and to the south of Colombo, the bedrock outcrops lowlands. The author infers that the formation of

are intemittently seen in between offshore and the the narrow elongated ridge to the east of Mundal present shore. To the south of Colombo, along the Lake has been formed preceding the Holocene southwest and south coasts predominate headland transgression. Similar ridges are seen in the areas bay beaches clearly show a drowned river yaley between Mahawewa and Katuneriya, between topography. Palangature and Seeduwa and the area to the south

of Colombo. The author considers that such ridges

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32 tempN#43-1, 1988

v"T

x as

s w #

COLOMBO

(v)

.Cx).

NARAHENPITA

BAMBALAPIT.lilieli-i

xDEH1WALA

MOUNT it)L

- km

.2ssf. ,/ . Ipt .4

Fig, 11, Sketch map showing landforrns around Colombo.Bore holes and cross sections are referred to in Fig. 12,

belong to Stage I. Based on the underlying mate- to attain a given state is 1ikely to be greater in the rials of the ridges (gray clay and coarse sand with arid tropics (Gardner and Pye, 1981). The climatic well rounded granules) it is inferred that fluviatile conditions in the Intermediate Zone have faveured and aeolian conditions occurred during their deposi- formation of reddish brown to bright brown or yel- tion. Iowish brown colour of the barrier ridges between The rate of dune reddening in humid tropical Mahawewa and Seeduwa. The author supposes climate gives some clues to examine their evolution. that these ridges were formed since mid-Holocene, Climatic conditions are favourable for such reddening and belong to Stage II and Stage III. The bright in both arid and wet tropics (Noris, 1969; Gardner yellewish coleur and unconsolidated sand of the and Pye, !981), but the length of the time required berms and sand spits indicate that they are very

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J. KATUPOTHA: Evolution of Coastal Landiorms in the Western Part of Sri Lanka 33 bw1 b mf bf × Y,p,,:,,lllilN)<"twi$ilisiglSSIIiliNXssilit b,.Break-ater Y tw tw i ww ' ' ss" i ' tw -t 'sss i ･ iE /illlllilllliiils. tlllil,i ,,k l t .lllSIIt lllllilx ktiti･,i - llikm bedrock llillll,;.Weathered il..1 W= g!. lllllilllllct) +s (u) i,

o -s E:IIs (r) liiil/)11i/l 1-!-l -tD Eilii

-IS

FiLLed soiL -20 Send -ith eLay

Coarse send, greveL -25llllllll vith cLay {w) Eme sand (v) (x} PeatPeat

-ith cLey

STLtMediurn

te coarrse sand

Coanse sand "ith cLay

EiliiilFine to medium sand with ctey Fine ta rnedium sand

Medium sand

Peat with medium sand

Weethered bedrock

Fig. 12. Cross sections and the stratigraphy around Colombo.

recent in formation. has been gradualy fi11ed with terrestrial se(lments During the high sea level in mid-Holocene the as we" as marine sediments foming marshes and

Muturajawela swamp was a part of the Negombo mangrove swamps following the lowering of sea lagoon. Since then the southern part of the lagoon level. Wadia (1941) reported that the spM level of

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34 te-F}\43-1,1988

1

m 31,

o SiLt with marine sheLLs

ofSiLtliving species

-ith peat. but no 4N 'ttti`sNN mar ine sheLLs -2 NNt

Btack cLay fuLL of 5xNx

marisandyne sheLLs -4 xxxNx BLue and [Lay w]th

partLybLsckcarbomized weod --6

IA -hite sand N x N Lateritic cap of gneissic N -8 weathered bedrock km

Fig. 14. Depth sequences of sandstone reefs Fig. 13, A stratigraphic section in Beira Lake, generalized (beachrock), Celombo (Swan, 1982), Colombo

Beira Lake in Colombo is about 2 m above present the Holocene transgression rapidly occurred up to MSL, and that its present bottom formed of fossil 6170 ± 70-5350 ± 80yr B.P. (Katupotha and Fu- marine shells which extend from 3 m to 6 m depth jiwara, 1987), and it is inferred that minor oscMa- from the surface level (Fig. 13). The hills and tions were continued later. Fine to medium sand in

ridges at the Fort and Kotahena were offshore island the upper layers of barrier ridges are assumed to be

during the Holocene high sea level. Such evidences wind blown in origin. (3) Stage III: Late Holecene

and the formation of beachrock also reveals that (around 3700yr B.B.); it is assumed that the

retreating condition had begun since Late Subborial beachrock of slightly above the MHWS level and ITZ

(Katupotha, 1988), although it continued at a low on the west coast are in this stage. (4) Stage IV:

rate. Weil-marked beachrock of different levels Modern (recent beaches, sand spits and so on). also show palaeo sea levels on the western continen- Diversities of coastal landforms in these stages tal shelf of Sri Lanka (Fig. 14). have been formed by the combined processes of The stages which are identified by the author marine, fluvial and biogenic, and the basement struc- are summarized as follows: (1) Stage I: From ture of the lowest peneplain. Most of these land-

Late Pleistecene to Early Holocene; low hills and forrns are in close relationship with main climatic

ridges in the area were coated mainly by wind blown zones of the country.

sand, and old barrier ridges were formed. (2) VI. Conclusions Stage II: Between the mid-Holocene and the first

phase of the Late Holocene (6600-37eO yr B.P.); The weathering conditions, colour, constituents

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J. KATUPOTHA:Evolution of Coastal LandfOrms in the Western Part of Sri Lanka 35

of sand and ef the height the barrierriclgesin the Ceylon, SPolia Zevtanfaa, vol. 31, pp. 148-176. coastal lowalands on the western part of Sri Lanka Cooray, P. G. (1968a): The geomorphology of part of the northwestern coasta1 of Ceylon. Zeitsch,ilt show four different stages of coastal landiorms dur- plain YTr Geomorphotcrgis, Supplement Band, vol. 7, pp. 95-113, ing late Pleistocene and Holocene Epochs. Based Cooray, P. G. (1968b): A note on the occurrence of on this evidence the author assumes that the old beachrock along the west coast of Ceylon. Journal of at ridgesinStageI Sembukulama,Kiriyankalliya, Sedimentory Petrolqgy, vol. 38, pp. 650-654, Pambala, Wirahena, Uluambalarna and Kadirana Dahanayake, K. and Jayawardana, S. K, (1979): Study of

Red and Brown Earth deposits of northwest Sri Lanka. areas have been formed preceding the Holocene `Red Joumat Geolagfaal Society of Indin, vol, 20, pp. 433- transgression. The Earth' forTnation to the 440.Deraniyagala, east of these ridges have been deposited by wind P, E. P, (1958): The Pteistocene of Gaylon. action following lower sea levels during the Late Geylon National Museum, Colombo, pp. 5-15. PIeistocene and Early Holocene Epoches. Dissanayake, C. B. (1984): Geochemistry of the Muthur- Although, radio-carbon ages from buried coral de- ajawela peat deposit of Sri Lanka. Fuei, voL 63, pp. 1494 -1503. posits at Akurala and emerged coral reef patches at Froemer, N. (1980): Morphologic changes in seme Che- Aurala and Akurala Telwatte reveal that the sea sapeake Bay tidal marshes resulting from the accelerated level remained 1 m or more higher than present sea soil erosion, Zeitschdi flZr Geomotpholagie, N.E, Sup- level between 6170± 70 and 5350 ± 80yr B.P, plement Band, vol. 34, pp. 242-254. During the above mentioned period the forrner Gardner,R.A.M and Pye,K. (1981): Nature, origin and significance of red coastal and drainage basins were submerged and headland bay palaeoenvironmental desert dune sand. Ptzrg?/ess in Phrsihal Geqgropiry, vol. 5, beaches were created. Many wetlands and beach pp. 514-534, ridges, particularly in Stages II, III, and IV were Gerritsen, F. (1974): Coastat Etrgineen'frg in Sri Lanlea. formed owing to minor oscruations of sea graduaily Report on U.N Misson, p. 64. level since mid-Holocene. The author supposes Jennings,J, N. (1967): The question of coastal dunes in

tropical humid clirnate, ZeitschtVt that the sea level did not lower below the present fiirGeo)notPhotagie, voL 8, 150-154. ievel between mid-Holocene (Main Atlantic) and pp. Katupotha,J. (1982): CoasinlStrtrctures andLand Clse in Late Holecene (Early Subatlantic). the Southwest Coastal Zone of Sri Lanka, pp. 4-5, (unpublished report). Katupotha,J. (1988): Hiroshirna University radiocarbon Acknoledgements Dates-1, west ancl south coast in Sri Lanka Radiocar- This study was carried out during the author's Docto- bon, vol. 30, in press, rate programrne at the Department of Geography, Hiroshi- Katupotha,J. and Fuiiwara,K. (1988): Holocene sea ma University, which was sponsored by the Ministry of ievel change on the south west and south coasts of Sri Education, Science and Culture, Japan. The author is Lanka. Palaeageagr Pataeoecol. Pataeocli?nat,, (sub- indebted to his supervisor, Professor Kenzo Fajiwara for rnitted). his advice. Thanks are extended to Professor Macus Madduma Bandara, C. M, (1982): A Suroay ofthe Coastat Kaiunanayake, Department of Geography, University of Zone ofSn' Lanka, vol. 1, pp, 40-51 and 120-135. Sri jayewardenepura, Sri Lanka for his great assistance Noris, M. R. (1969): Dune reddening and time, loutual during the field work, and to Dr. Takashi Nakata, Depart- ofSedimentory Petiviqgy, vol. 39 pp. 7-11. ment of Geography, Hiroshima University, for reading the Pye, K. (1981): Rate of dune reddening in humid tropical manuscript and prQviding critical comments. Thanks are environment. IVdinre, vol. 290, pp, 582-584. also due to Mr, Joe Hicks and Ms. Akiko Ono, Hiroshirna Pye, K. (1983a): Dune formation on the humid tropical University, for their help on this manuscript. sector on the north Queensland, Austraia. Eartlt SuJY2ice -381. Process and Lantijlotvns, vol. 8, pp. 371

Pye, K, (1983b): Coastal dunes, PtqgvTess in Plrysicat References Geag,mpio', voL 7, pp. 531-557. Coates, j. S. (1935): The geology of Ceylon. Caylonleur- Russel, R. J. (1959): Caribbean beachrock observations. nal ofSctence (B), voL 19, pp. 102-115. Zeitschtpt.fiirGeometPholqgi'e, vol. 3, pp. 227-236. Cooray, P. G. (1976): An introduction to the geoiogy of Russel, R.J. (1962): Origin of beachrock. Zeitschfiejiir

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地 理 科学 vol．43 no ．1 pp．18− 36， 1988 一 Geegraphical Sciences（C陀魏 κ 融 の − GeomOtPho’αg飽， vo1 ．6， pp．1 17． Tansley， A ， G ． and Frits（血， F． E ．（1905）： The fiora of S ，B ．（1967）： Characteristics。 f coastal sand and their Ceylon littoral， The New phytelogiSt， vo1．4， no ，1， pp．1 − − depositional environments 血 sou 止 west Ceylon．ノ翩 α l 17and vol ．4， PP．27 55．

Tropthat Geograf》 voL 24 ．30− 42． Of 尠， ， pp Wadia，D．N．（1941）： The geology of Colombo and its 甲 − Swan， B ．（1979）： Sand dme in the h id tropics： Sd envh ons ， SPoliz ZeyianiCa， vo1 ．23， pp。10 18． − Lanlca， ZeitSchnnプ諺プ Geotnotpho’ogie ， vol ．23， PP．152 Wayland， E．エ （1919）： An outline of the stone age of 〜 171。 Cey｝on ． SPolia Z¢ ytaniCa， voL 11， pp．85 125， Swan， B ． （1982）： The Cotxslal Geotnoゆhology （ゾ 5万 （Recelved： 15 Nov ．1987）

L 砌 々a ， 、肋 introdtcctory s 麗 roay ， Univ． of New England ： ， （Accepted 21 Dec，1987） Am 亘dle， New South Wales， p ，10 and 130．

ス リ ラ ン カ 西 部 に お け る 海 岸地 形 の 地 形 発 達 史

JINADASA KATUPOTHA

ー ー ス ン カ キ ワ ド 海岸地 形 発達史 ， リ ラ ，後期更 新世 ， 完 新世 ，地形分 類

バ ー い ス リ ラ ン カ 島西岸 の 海岸低地 は，主 に完新世に 形成 され た砂州 浜， リ ア 島， 砂丘 ，沈水ある は ー ー ー ラ グ ン や 地 マ ン グ ロ ブ 沼 沢 地 ビ チ ロ ッ ク よ り成 る。 そ の 中 で も 珊 や 隆起 堋瑚 礁 ， 湿 ， ， ， 瑚礁 ー ー ロ マ ロ づ っ い ビ チ ッ ク ， ン グ ブ沼沢 地 は ，特 に，熱 帯 海岸 を特徴 け る地 形 と な て る 。 ー ー の バ ア ッ ジ の ビ チ ロ ッ ク や の 1 お よ び 気候 条件 ， 砂 色調 や 組成 ， リ リ 高 度 ， 低 湿地 分類 （図 ＞ ， 14C ン ゴ ど の and 西 海岸 な ら び に南西 海岸 で 行 っ たサ 化石 ， 貝化 石 な 年代測 定結 果 （Katupotha Fujiwara，

は の つ の に 1987； Katupotha，1983）か ら， 西 海岸 にお け る地形発 達過程 ， 以 下 よ うに ， 4 時期 分 け られ る

こ とが 明 らか とな っ た 。 〜 （1） Stage I． 後期 更新世 完新 世前期 ： 本地 域 にみ られ る低 い 丘 や リ ッ ジ （5 m か ら 10m あ る い は ー 上 の は 主 に に わ れ い バ リ ア リ ッ ジが 成 され た。 そ れ以 高 度） ， 風成砂 覆 ，古 形 一 − （2） Stage II． 完 新 世 中 期 完 新 世 後 期 （6600 3700 y． B． P＞： 6170± 70か ら5350± 80 y． B． P．

て は に た て の （Katupothaand Fujiwara， 1987）に か け ， 後 氷期 海 進 急 速 進行 し 。 そ し ， そ 後 も小 さ な振 幅

っ こ で 。 わ に お い て は よ 1m は が あ た とが 推 測 きる す な ち， 完新 世 中期 ，海水準 現 海水準 りも も しく そ

バ ー れ 以 上 高 く，当時，旧 流 域 は沈 水 して headlandbaybeaches を形成 した と思 わ れ る 。 リ ア リ ッ ジ は − 〜 3 5m の 高度 を有 し， リ ッ ジ の 上 層 部は主 に風 起原 と思 わ れ る細 中砂 に 覆わ れ て い た。 （3） Stage m ， 完新 世後期 （約3700年前 頃）： 西 海岸 に お い て 平均 高潮 位 面 と潮間帯 よ りわ ずか に高

い ー ロ こ 位 置 に存 在す る ビ チ ッ ク は ， の 時期 に 形成 さ れ た と考 え られ る 。 研 究地 域 で は ，完新 世 中期以

降 の 海水準 の 小振 幅 に よ っ て 低 湿地 や 浜 堤 な どが 形成 さ れ た こ とが 推測 され る 。 しか しな が ら，完 新世

中期 か ら後期 に か け て ， 海水準 は現 海水準 よ りも低 くは な らなか っ た 。

： の ど の が わ れ た （4） StageIV． 現在 現成 海浜 ， 砂嘴 な 形 成 行 。

々 の い び い た 各 の 時期 にお ける 海 岸地 形 多様性 は，海，河川 ，生 物 と っ た営力 が結 付 形 成過程 や ， 準

い こ の の は に 平原 最下 部 の 基 底構造 に 関連 し て る 。 そ して ， れ ら 地 形 大 半 ， 国 内に お け る主 要 な気候 帯

対比 で きる 。 ・ （広 島大 院，1988年 4 月 よ り Department of Geography， University of Sri Jayewardenepura， Gangodawilla， Nugegoda， Sh Lanka）

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