Glacial geology of the island Stord, west Norway
ANDREW N. GENES
Genes, A. N.: Glacial geology of the island Stord, west Norway. Norsk Geologisk Tidsskrift, Vol. 58, pp. 33-49. Oslo 1978. ISSN 0029-196X.
Upland morphology suggests pre- or early-Weichselian glaciation prior to the inception of existing cirque basins formed when frrn limit was 450 msl. Directional elements indicate complete ice cover probably during the main Weichselian ice advance. Cirque parameters suggest that 250-300 m of upland has been removed through a combination of glacial and fluvial erosion. Marine shells were dated by radiocarbon at 12,860±250 yrs. BP designate an Older Dryas ice advance. lee-cap conditions sub sequent to the Older Dryas advance and a rising frrn limit during deglaciation is postulated, with the probability of nivation processes occurring during Younger Dryas time. lsostatic adjustment of 134- 138 m since Older Dryas time (12,000 yrs. BP) is calculated and a relative isostatic uplift of 12 m since Tapes transgression (6000 yrs. BP).
A. N. Genes, Department of Regional Studies, Boston State College, Boston, Massachusetts 02JJ5, USA.
The island Stord, situated some 75 km south of elevation metasediments metamorphosed during Bergen at the mouth of Hardangerfjorden (Fig. Caledonian time (Strand 1960), with elevations 1), is considered part of the strandflat region of not exceeding 60 m. Norway (GrØrilie 1940, Andersen 1965f Re cent studies on the western part of the Hardan gervidda and surrounding areas (Mangerud, The cirques 1970a, Aarseth 1971, Pollestad 1972) suggest that the island was probably immediately distal Most cirques are associated with the upland to the Younger Dryas ice margin. This study block in the north western segment of the island, was undertaken to determine whether Stord but upland outliers with associated cirque-like was glaciated during the last glacial (Younger forms occur at Midtfjell, Kinno, Mennene, and Dryas) ice advance and to determine the Store Melen (Fig. 2). Descriptions of Stord Pleistocene history of the island. cirques are given elsewhere (Genes 1974), but Brief descriptions of the Quaternary geology some important points should be noted. First, of Stord have been given by Reusch (1888), the headwall of Svartavatn East has a polished, Holtedahl, H. (1967), and Mangerud (l970a). smooth appearance with crest ridges rounded to Some surficial deposits were described during the west, indicating glacial modification. Sec bedrock mapping (Kvale 1937, Skordal 1948) ondly, most cirques have n!Jrthwesterly aspect, and the glacial map of Norway (Holtedahl, O. & and thirdly, moraines occur in several cirques. Andersen 1960) indicates some moraine deposits Fig. 2 on Stord.
Distribution Bedrock Cirque distribution with regard to quadrant The bedrock consists of crystalline and volcanic location and cirque aspect is shown in the polar rocks. in the northern part of the island and diagram (Fig. 3). The seemingly anomalous metasediments of Cambrian to Silurian age in northwest location of most of the Stord cirques the southern part. The relationship of the igne can be explained by preglacial valley incision ous rocks to the metasediments has been de along the Stord western coast resulting from the scribed by Kolderup (1931) as a tectonic line assumed Tertiary uplift (Holtedahl, H. 1967) of which separates an upland igneous massif, in the Iandblock. The origin of cirques has been places exceeding elevations of 700 m, from low explained as resulting from primarily structural
3 -N. GeologiskTidsskr. 34 A. N. Genes NORSK GEOLOOISK TIDSSKRIFT l (1978)
Fig. l. Location map of Stord and vicinity, Norway. NORSK GEOLOOISK TIDSSKRIFT l (1978) Glacial geology of Stord, Norway 35
Se ale l' 50,000 L-�-·L----L----L----L----L-----'Okm Stlecttd contour lnttrvol In meters
5 45'
Fig. 2. Stord location map showing place names. 36 A. N. Genes NORSK GEOLOOISK TIDSSKRIFr l (1978)
Elevotlons in meters
CIRQUES
l. MENN ENE QUADRANT NE SE NW SW 2. STE l NDALSVATN
NUMB ER of 3. 2 o 4 l KLOVSKARDVATN C IRQ UES 4. BOTNAVATN C IRQ UE 2 o 2 3 5. SVARTAVATN NORTH ASPE CT
6. SVARTAVA TN EA ST
7. Å SANE
Fig. 3. Polar diagram (after Temple 1965) showing cirque aspects and quadrant locations.
(Lewis 1938, McCabe 1939, Temple 1965, Flint possible exception is Svartavatn East, which is 1971) or meteorological (Battey 1960, Andrews favorably located for accumulation of drifting 1965) considerations. snow. Structural control is ev en more evi Clearly Stord cirques have originated from denced by the aspect of Botnatvatn and Klov previously developed stream eroded sites. A skardvatn where they cut along the east-west NORSK GEOLOGISK TIDSSKRIFf l (1978) Glacial geology of Stord, Norway 37 rock strike contact between granite and effusive Interpretation rocks rather than having an insolation protected northwest aspect. Characteristics of the Stord cirques clearly dem onstrate that they were formed early in, or prior to, the last glaciation. They were active Morphology prior to the main Weichselian ice advance, were subsequently overridden, and served as niva Manley (1959) gives a ratio between 2.8 and 3.1 tion hollows during the early period of deglacia to l for cirque valley length to headwall summit tion. height, whereas Andrews (1965) gives a ratio Morphologically, the readily apparent monu doser to 2.1 to l. Embleton & King (1968) ments such as Kinno, Klovskardfjellet, Store suggest that the latter ratio reflects immaturity Melen, Mennene, and Midtfjell, the total lack of of cirque development. Svartavatn East con aretes and horns, and the almost complete dis forms to cirque morphology in all respects. integration of most headwalls, characterize a However, the length to height ratio using the monumented-upland stage. With the exception 660 msl contour is 0.98 to l, considerably lower of Svartavatn East, the cirques exhibit than the values given by Manley or Andrews. characteristics of an advanced stage of modifi The cirques have been overridden, as evidenced cation in that Ahlmann's rounded-head appear by grooves, chattermarks, and faceting. There ance of old age has been destroyed by pro fore it is not possible to determine accurately longed weathering and erosion, or by glacial the original headwall elevation. overriding. By using the 700 msl or 750 msl contour Leveled places of large areal extent around rather than the existing 480 msl contour which Sæterbø, Tveitafjell, and Midtfjell are inter defines most of the present cirque headwalls, preted as 'bastions' representing old cirque the length to height ratios of other cirques at floors, in the same manner that the Brunene flat Stord are in dose agreement with that of has been developed at Svartavatn East (Fig. 2). Svartavatn East, as are the angles between tarn The Brunene flat exhibits a mammillated, de lake lips and headwall elevations. nuded surface, perhaps the 'glacial peneplain' Comparing these ratios and angles with to which Cotton (1942) alluded. Bowl-shaped length/height ratios of eight cirques at incisions, as at Fuglatjoen, Kvernatjoen, Kyr Rondane, the average length/height ratio of keveien, and Olstj., may represent almost com Rondane cirques is 1.2 to l, and the lake li p to pletely disintegrated cirques. Alternatively, headwall angle is 41.0°. Thus it seems that Stord some of these may represent plunge pools, later and Rondane cirques have smaller length/height modified by nivation processes. The origin of ratios than cirques investigated by Manley & Kartreppen at Svartavatn North and in the val Andrews. ley south from Steindalsvatn may be ascribed to Lewis (1949), using the gradient of present compressing flow at the base of small ice falls. day glaciers in Jotunheimen, calculated an 18° If the present-day relationship of precipitation average value for the slope of glacier ice con between Borgtveitdalen and the remainder of tained within valley walls extending from the Stord is assumed to have held in the past, this headwall intercept to cirque thresholds. lf the relationship could explain the intact cirque present 480 msl contour is taken as the headwall morphology at Svartavatn East, either by af elevation, an extension of this average 18° ice fording advantageous conditions for prolonged surface gradient from the cirque basin lip in cirque glacier activity or subsequent nivation tercepts an elevation above the present head processes. The rounded cirque headwall and wall in the majority of cirques at Stord (Fig. 4). crest attest to overriding by an ice sheet with The use of the 660 ms! or higher contour as subsequent nivation processes responsible for the headwall elevation at the time of cirque widening of the cirque valle y. An oceanic eli inception agrees with calculated cirque mate prevails at Stord with mild winters, cool geometry. Use of higher initial headwall eleva summers, and high precipitation, especially in tions brings the length/height ratios and cirque winter. Mean annual precipitation (1900-1949) basin lip to headwall angles into hetter agree is slightly higher in the south than in the north, ment with cirque morphology as expressed by being 1669 mm at Leirvik (30 msl) and 1490 mm Svartavatn East and Rondane cirques. at Fitjar (7 msl). Locally, precipitation can be w 00 ::.:... �li� � USEO FOR HEAOWALL ANO SNOWLINE VA LUES · C) . ·. �� ���� EXPLANATION ... ·; · :�l�-=�� l "' . ;::s ... f,o" -�..., ._.� ...... SNOWL/NE : ms/ ( UNREOUCEO) USING INTERCEPT VALUE "' (Til/ANGLES REPRESENT CI/IQVESJ ,•'"'"'� ���t:l:g!!:�� 450 "" .. ·;;··cfaL- �::������ c AVERA GE C/RQUE FLOOR ELEVATION : ms!, . t��f1 _ IDO; 1.$,18° 350 11/D�O/NT(III . ,t&...... -�,.,. fl..,tC� ....- Q .,. . . ' , . . . • , , . · i� � f off� '!-- ;�oP SNOWL/NE : ms/ (UNREOUCEO) FOR THE .- ·�sfO � -, cf >•• 415 , 11• - - � �ACTVAL HEAD• c · ;·,s �f!'!- < SU� ------f\ WALL ELEIIATION . . - .· - ,�o -_:.{� -- 480 ms/ HEAOWALL ELEVAT/ON, ANO 525 _.·j-r:.·--- TA/IN ms/ ( UNREOUCEO) FOR TH E ms/ L/P Aw•Le 700 ELEVAT/0/t -- -·· ---- · (Ill ••t•l'•) EXCEI'T HEAOWALL ELEVA T ION WHE/IE NOTED)
750,1.2,3 1° - " 750, 1.4,35° . � 700,1.0,28 ...... 2.2,24 • !0• . . . 70 O, �760,1 .4 . 0 0, 2.5,21 ?0011.1:31• . - · · · 66 . . - :::-··: . � 810,1.1,300 470 <"'::::..-_-_-:·. . � 7 ./'' •oo,2.1,2o• ooe- :� ·€""'f'. JO 720 - 690 msl 660 .9 600 .8 � ::s; 540 liJ 7 -..J 480 � .6 - 450 msl Cl) 420 . 5 � 360 C) � 300 11 � 240 - 240 msl Cl) .3 Q: 180 � .2 liJ � 120 .l -- 60 msl 60 11/H .2 .3 .4 .5 .6 .7 .8 . 9 JO o/A Fig. 5. Hypsographic curve for Stord. 450 ms! elevation corresponds approximately to maximum elevation of upland outliers and cirque headwalls. 240 ms! elevation corresponds to supposed 'bastions' of earlier cirque floors. Area determined by polar planimeter and plotted according to Strahler (1952: 1117-1 142). higher, as at Borgtveit (350 msl), midway be rected to 300°. The se occurrences indicate tween Leirvik and Fitjar, with a mean annua! complete overtopping by thin, topographically precipitation of 2833 mm (Hafsten 1965). controlled ice, probably not associated with the Glacial debris in the form of either subdued main ice direction when ice moved independent moraines, widespread glacial drift, or con ly of topography. centrated block deposits conforming to cirque U sing only the previously described cirques valley mouths, suggests cirque activity after (Fig. 3) the average cirque floor is calculated to ice-sheet cover. At Midthamarsåto a�d be 320 ms!. The value for the snow line based Kalderassen, the highest elevations at Stord, on the 480 msl headwall elevation is 380 msl there occur abundant chattermarks, quartzite (unreduced). and nordmarkite erratics, in addition to faceted Triangles (Fig. 4) representing inost cirques surfaces and several large glacial grooves di- and cirque-like forms on Stord show midpoint 40 A. N. Genes NORSK GEOLOGISK TIDSSKRIFf l (1978) elevations between tam lips and intercepts of Firn limit the average 18° glacier ice-surface gradient. Calculation of snowline (Embleton & King Østrem & Liestøl (1964) calculated a present 1968, Flint 1971) suggests that the average mid day firn limit of 1400 msl for the east side of the point of these intercepts should approxi Folgefonna ice cap. Firn limits of 1300 msl and mate snowline at time of cirque formation. 1200 msl were calculated for the Melderskin and Unreduced snowline values for the 480 ms! Ulvenos areas respectively, as western exten and 700 msl headwall elevations are 415 msl and sions of the ice cap. Pytte & Østrem (1965) also 525 msl respectively. These values, calculated calculated firn-limit values on Folgefonna of by using the average cirque floor value (350 1410 msl on the west and 1350 msl on the east msl) and the applicable headwall elevations, sides for 1964. Values of 1370 msl on the west suggest the possible range of snowline eleva and 1390 msl on the east sides were calculated tion. The 450 msl snowline elevation value (un for 1968 (Pytte 1969). The intervening years had reduced) is obtained through the use of the similar but fluctuating relationships. average midpoint and the 18° glacier ice-surface The glaciation limit at the central-southern intercept. The 450 msl snowline value is in ac part of Folgefonna was determined to be 1500 cordance with the pronounced areal surface in msl (Østrem & Liestøl 1964:325). Pollestad Fig. 5. The previously mentioned bastions are (1972) determined the glaciation limit to be 1420 at lower elevations than the existing cirque msl on Melderskin with both the firn limit and floors. glaciation limit depressing to the west, but with It is hypothesized that the se cirques, of which the glaciation limit approximately 100 m above only the bastions remain, were initiated when the firn limit. the snowline was considerably below 450 msl. Similar relationships of firn and glaciation After cirque coalescence and the development limits were determined from Ålfotbreen to of the mammillated bastion surface, snowline Gråsubreen through Nigardsbr.;:en. The differ rose initiating the 450 msl snowline phase of ence between this firn limit and glaciation limit cirque forniation, resulting in the present cirque was 70-100 m within 50-70 km of the coast configuration at Stord. (Østrem & Liestøl 1964). The existence of a plateau ice cap is post Pollestad (1972) extrapolated a probable ulated for the final phases of glacial activity, glaciation limit of 13�1350 msl for Ulvenos. where cirques became heads of glacial valleys He calculated a glaciation limit of 900-950 msl containing ice streams of all types which over with firn limit at approximately 800 msl for the flowed cols and eroded other cirque-generated plateau area west of Ulvenos and correlated forms such as the aretes and horns. these limits with the Fjordbrestade of Y ounger The Store Tjørnadalen area is drift covered Dryas time. Pollestad thus assumes a fim- and and has associated kettled lakes and large glaciation-limit depression of 400± 50 m for patches of till on bedrock. In many places no Younger Dryas time in that region. bedrock protrudes and moraine ponding is evi A climatic snowline depression of 450-600 m dent. This entire area and most other parts of at Lysefjorden in Ryfylke (Andersen 1968) and the uplands have characteristic 'dead-ice' ap a 600 m depression of the firn limit on pearance suggestive of snowline elevation Sunnmøre (Reite 1966) suggest the magnitude of above the istand preventing nourishment of the firn limit depression in southwestern Norway ice cap. The lower ends of the valleys Ieading during Younger Dryas time. from the cirques have shattered valley walls, The average westerly depression gradient of and weathering and erosion are evident. How the glaciation limit calculated by Pollestad & ever, the relative freshness and smoothness of Pytte is 2.6 m/km. the upper parts of the valley walls and the ex Extrapolation of the glaciation limit from Fol istence of protalus ramparts near most hollows gefonna to Stord using this average 2.6 m/km attest to nivation processes during the last depression gradient yields a present-day phases of glacial activity when the snowline be theoretical glaciation limit of 1350 msl and a firn came elevated above Stord. limit of approximately 1250 msl. By using the average firn limit of 1310 msl on the east side of Folgefonna in conjunction with the previously mentioned 2.6 m/km glaciation Iimit-depression NORSK GEOLOGISK TIDSSKRIFT l (1978) Glacial geology of Stord, Norway 41 gradient, a present-day firn limit of 1140 msl Table l. Pollen and spore count, S14-8, Beivik. over Stord is obtained. An average of the two firn limits thus obtained yields a present-day Deter- Question- Total mined able theoretical firn limit over Stord of 1200 msl at Midthamarsåto. A firn limit of 800±50 msl is Bet ula 10 2 12 obtained for Younger Dryas time over Stord. Salix 8 2 JO As will be shown later, the calculated isosta Pinus 6.5 6.5 tic adjustment of Stord suggests that the firn Other AP 3 3 6 limit could not readily intercept the highest Artemesia 7- 7 Gramineae 5 6 parts of the island, Midthamarsåto, Kaldersas Koenigia islandica l l sen, and Kattnakken during Younger Dryas Other NAP 7 3 lO time, all lying within a reduced range of 660-700 msl during Older Dryas time. Total pollen 47.5 11 58.5 Hystrix 261 Spores 18 Stratigraphy The pollen count performed by cand. real. Lotte Seising, The late Quaternary environment of Stord was and the identification of Koenigia islandica by cand. real. DagfinnMoe, both of Universitetet i Bergen. not favorable for the preservation of an exten sive Pleistocene stratigraphic record. The oldest Pleistocene sediments on Stord are dark gre y, silty-sand to sandy-granule tills which overlie thick and containing numerous pebbles, com bedrock. The angularity of clasts within this prises the upper unit of the Stord section. This unit increases northwestward from Fugleviki to ela y, overlying only bedrock, occurs intermit- Breivik. At Breivik, marine shell fragments are . tently as patches along the southeastern coast. concentrated in the lower 0.8 m of this till. At Borgtveit whole shells in fine condition are Shells from this zone have been identified as randomly distributed throughout a 1-m section. Chlamys islandicus, Mytilus edulis, and Maco Shells identified as Mya truncata, Hyatella arc ma calcaria, and were dated by radiocarbon at tica and Chlamys islandicus were collected 12,860 ± 250 yrs. BP (DF 445 T-1171, from approximately 0.3 m above the base of the Trondheim Radiological Laboratory ref. no.). section and dated by radiocarbon at 10,600 ± The presence of Hystrex and Foraminifera in 140 yrs. BP (DF 546 T-1370, Trondheim the pollen analysis spectrum (Table l) of the Radiological Laboratory ref. no.). fossiliferous till indicates that the sediment was initially deposited in a marine environment. The pollen in the sediment was probably redepos Ice-recessional topography ited, and is suggestive of vegetation occurring during either Oldest Dryas or Bølling time. Constructional and depositional features are in Till fabric analyses of this unit from widely close accordance with iceflow indicators and separated localities yield mean azimuths which afford good evidence of deglaciation chronology range from 272°-345°. Sediment parameters and (Fig. 6). directional elements of tills are consistent with The topography north of Årskog is predomi the view that all the tills on Stord were depos nantly erosional except for isolated patches of ited by the same ice advance. subdued ground moraine on the northeastern Above the till and bedrock is a 0-17 m thick segment of Tveit. Stoss-and-lee forms at zone of well-sorted and interbedded sand and Kalvanes, sparseness of drift, and the domi gravel with minor amounts of silt. At Breivik, nance of westerly striae suggest that the north Fugleviki and Vik, where the sand and gravel em segment of Stord was essentially ice free overlie till, the contact is sharp. The areal dis during the last phase of Older Dryas glaciation. tribution of these deposits is patchy and in In marked contrast, the Fitjar area is termittent throughout the island, occurring characterized by hummocky moraine topogra primarily at elevations from 44-55 msl. The phy extending from Breivik to Vest-Bostad and sediments at Vatna and Vik are deltaic in origin. south to Vik. A large end moraine approximate A massive blue to bluish-grey clay, 0.5 m ly 1500 m wide, herein named the Fitjar 42 A. N. Genes NORSK GEOLOGISK TIDSSKRIFf l (1978) l Sea le l' 50,000 4 e lun S.lected contow- lntwvol in meters 59" 55' rzJ GJ � m - rmm MARINE SEDINENTS D SHOREBAR [El DELTAS 0 El FOSSIL LOCALITIES 8 TERRACES 00 ERRATJCS s MORAINES [i] "DEAD ICE" TOP OGRAPHY [j] BOULDER CONCENTRATI ON B ESKER (ARR OWHEADS SHOW CURRENT DIRECTIONS) 59" 45' la DRAINAGE CHANNELS ASSOCIATED WITH DEGLACIATION B STREAMLINED HILLS AND RIDGES (LINE ORIENTED TO LONG AXIS) 05"15' Fig. 6. Surficial geology of Stord. NORSK GEOLOGISK TIDSSKRIFT l (1978) Glacial geology of Stord, Norway 43 Moraine, extends from the Midtfjell western moraine. The submarine ridge extends as an slope west-southwesterly to Tangen near the arcuate form from Fitjarviki to KolØyhamn. northern end of Storavatnet. The Fitjar Moraine Sediment accumulation is also evident at en ds at V oil, at a ridge interpreted as a shore Færøysund on the northeastern coast. bar. The surface of low relief on which Fitjar is Most islands comprising the archipelago on located is topographically expressed by fluvially the northwestern coast are barren of deposits. incised and terraced flatland. Those islands, or parts of islands, correspond Hummocky moraine surface characterizes the ing with the trend of the submarine ridge are Stord '_Vestern coast to Almås in the south, either drift covered or have large boulder ac although bedrock control is obvious in many cumulations along inlets or coasts. instances. Boulder accumulations are as Widespread, thick submarine sediment oc sociated with the Fitjar Moraine and are prom curs in Leirvik. These deposits are interpreted inent in nearly every bay and coastal segment as interbedded silts and clays deposited during along the western coast. the Younger Dryas ice advance which reached · Gravel pits occur along the Storavatnet shore Huglo and surrounding areas. and extensive delta deposits at Vik and Nymark A linear, symmetrical ridge, 14 m high, is Church both have northwesterly dipping foreset superposed on the marine sedimen( at Fit beds. Associated with the Fitjar Moraine com jarviki. This ridge, interpreted as an esker, ex plex are patches of stratified glaciofluvial de tends from the approximate midpoint of Fonno posits, as at Nymark, Bostad, Dyvik, Kvernaty, to the Fitjarvik coast at Urda. and the valley leading from Nymark Church to Large drainage channels radially distributed Storevatnet. around Stord imply greater erosional capability A delta at Vatna with southeasterly dipping by early postglacial streams than by the present foreset beds and poorly stratified deposits dip streams occupying these valleys. ping northeasterly along the Borgtveit coast are the only glaciofluvial deposits along the east southeastern parts of Stord. Terraces at Varden Late Pleistocene shorelines and Raunholm in the northeastern section are comprised of sandy to gravelly sediment. Large Evidence of former sea-leve! stands on Stord boulders are common along the Stord eastern include the shorebar at Voll, the terraces at coast, particularly in the Fugleviki area. The Varden, Årskog and Fitjar, and the Vik and boulder pattern suggests an ice-rafted deposit. Vatna deltas (Fig. 7). This is interpreted as deposition by 'Ra' ice Glacial-isostatic uplift along the southwestern during the Y o unger Dryas ice advance to coast of Norway is considered to have been Huglo. discontinuous (Marthinussen 1960, Hoppe et al. Echo soundings interpreted from echograms 1968, Aarseth 1971, Pollestad 1972) because of utilizing a Simrad Scientific Counter EK, the effect of high elevations and deeply incised Simonsen Radio A/S, Oslo, Norway, revealed fiords upon calving and ice recession. The dis extensive submarine deposits around the Stord continuity of uplift is attributed to differential coast, which, with deposits on some islands off release from ice load (Holtedahl, O. 1960) or to the northwestern coast, are considered to be discontinuity of isostatic recovery itself (Fægri associated with the Fitjar Moraine. 1943). Sediment is lodged along bedrock slopes and Andersen (1960) has assigned an Older on the inner portion of the bay, approximately Dryas/Bølling age to an approximate 46-msl 75-80 m below sea level at Fitjarviki. The strandline at Lillesand, southern Norway. This morphology of the submarine sediment is ex strandline corresponds closely with sea leve! pressed as a wide subdued ridge. Smoothness of delimited by the Fitjar Moraine, the shorebar at the ridge surface and depth of echo penetration Voll, and the surface of low relief distal to the of the deposit suggest loosely consolidated, moraine. fine-grained to gravelly sediment. Marine sedi Assuming contemporaneity of formation, ment of similar characteristics occurs along the marine limits based on barometric measure coast at Fonnosen, Kobeviki and HellandsjØen ments of 44 m at Voll (Fig. 7, no. 4), 48 m at at the Ålforo coast. Locally, as at the TelØy Varden (Fig. 7, no. 3) and 52 mat Vatna (Fig. 7, area, the irregular surface suggests a bouldery no. l) suggest an 0.83 m/km isostatic uplift gra- 44 A. N. Genes NORSK GEOLOGISK TIDSSKRIFf l (1978) SEA LEVEL INDICATORS l. VATN A DELTA a. SURFACE b. TOPSET- FORES ET CONTACT 2. BO RGTVEIT FAN 3. VARDEN TERRACES 4. FITJ AR SHOREBAR �. Fl T JAR TERRACE 6. VIK DELTA a. APEX OF DEpOSIT b. LOWER SURFACE 7. ÅRSKOG TERRACE ISOBASE CONSTRUCTION A 8 ISOBASES REFLECTING ISOSTATIC UPLIFT SINCE OLDER Sa 60 . .2 DRYAS TIMES �50 la0 GR ADIENT EQUALS .... 3�a . 40 4 •&b A � .83 m/ km .5 � 30 <:> ., � 20 ., Q: • 3b � 10 • 7 � • 3c o 6 o 2 4 8 10 12 14 16 HORIZONrAL SCALE IN KILOMErERS Fig. 7. Reconstructed isobases on Stord showing uplift since Older Dryas time. NORSK GEOLOGISK TIDSSKRIFT l (1978) Glacial geology of Stord, Norway 45 dient. Isobases constructed using these meas uplift of 12 m also is calculated since Tapes urements trend N. 6° E. Using the Stord to transgression, based on Milliman & Emery's pograpliic map elevation for Vatna, 48 msl, curve. isobases trend N. 9" W. The 0.83 m/km isostatic gradient compares with RØnnevik's (1971) 0.77 m/km isostatic uplift gradient in Haugesund for Discussion isostatic compensation since Older Dryas time. It is possible that isobase construction could The presented observations do not support the have more northwesterly orientation than N. 6° concept that , Stord was glaciated during E. and would include or be near all plotted Younger Dryas time. Also Younger Dryas ice points except fan apices and lower elevation would have had to override the Huglo terraces. The Vatna delta surface encompasses Halsnøy-Sandvoll island chain at the mouth of a zone rather than a point; therefore, a value Hardangerfjorden and project an ice tongue at between the calculated isobases is probably least 1.5 km across the body of water, presently more correct. Isobases were constructed based 320 m deep, separating Huglo from Stord, an on topographic map elevations because of the irnprobable model of ice advance. It is more uncertainty of barometer readings. probable that ice flow was inhibited by to Tapes transgression on Haugesund has been pography in agreement with H. Holtedahl's demonstrated by Rønnevik (1971), who corre (1967) ice dam effect of the Huglo-HalsnØy lated an east-west 10-14 msl terrace level of 0.2 Sandvoll island chain. m/km gradient with Fægri's (1943) highest Tapes The 800± 50 msl firn limit value calculated for limit on Bømlo. The Årskog terrace and the Stord for Yo unger Dryas time combined with middle terrace at Varden correspond with the the isostatically reduced 665-700 msl upland Tapes transgression limit on Bømlo calculated elevations argue against the probability of ac by Fægri. tive cirque glaciation on Stord during this time. The gradient of the assumed Tapes shorelines However, the existence of nivation hollows on between Årskog and Varden is 0.6 m/km. Fægri Stord during Younger Dryas time is probable. (1943) used Tapes measurements at Etne and The interpretation of late Pleistocene history Halsnøy, inland from Stord, to correlate Tapes on Stord is based on the relationship between transgression on BØmlo to those areas and the structure and form of glacial features on found that a 0.6 m/km gradient corresponded Stord with those of surrounding Sunnhordaland. well at each locality, although Fægri thought The westerly striae on the Stord uplands in free the gradient slightly high. Extrapolation of position and the oldest westerly striae on the Tapes shorelines from Stord to Etne and northwestern coast associated with the erosion Halsnøy using the 0.6 m/km gradient correlates al features at Kalvanes indicate a westerly ice within l m at Etne and 2 m at Halsnøy with movement and correlate with that movement Tapes shorelines in these areas. Extrapolation to attributed to the maximum extent of Weich Bømlo is within 3m ofFægri's lOm Tapes line. selian glaciation (BØe 1949, Holtedahl, H. 1967, Complexity and inaccuracy of Holocene sea Mangerud 1970a, Aarseth 1971, RØnnevik 1971, leve! limits (Curray & Shepard 1972, Bloom Pollestad 1972, Holtedahl, H. 1975). 1972) prohibit an absolute isostatic compensa lee movement on Stord subsequent to the tion value. However the Fitjar Moraine delimits Weichselian maximum ice advance is suggested sea level at time of deposition and yields a by predominant northwesterly striae on the relative isostatic-eustatic rise of 44 m since Old northwestern coast, and southwestern striae on er Dryas time (12,000 yrs. BP). Using Curray's the southern coast. The striae pattern in con 40 msl rise value for the past 9000 years (in junction with whalebacks at Årby and Vest Curray & Shepard 1972:16), an isostatic com Bostad conform to coastal configuration and re pensation of 81-S5 m is calculated for Stord flect topographic control of the ice advance. (A and a possible 14 m isostatic compensation complete description of the striation localities is through the last 6000 years. Applying the 44 m given in Genes 1974.) isostatic-eustatic rise of Stord to Milliman and Till fabrics with westerly component at Emery's curve (Milliman & Emery 1968), a rel Fugleviki and Sandvikvåg and till fabrics with ative isostatic uplift of 134-138 m since Older northwesterly component along the western Dryas time (12,000 yrs. BP) is calculated. An coast of Stord parallel striae directions. Clast 46 A. N. Genes NORSK GEOLOGISK TIDSSKRIFT l (1978) inclusions with higher percentages of subround Glacial drift in Svartavatn East, extending characteristics at eastern coast localities than into Borgtveitdalen, suggests cirque activity -along the western coast are interpreted as dis postdating general deglaciation during Older placed beach deposits which have undergone Dryas time. Glacial striae at the tarn lip, imply little transport corresponding to Bergesen's ing ice flow down valle y, support this view. (1970) allochthonousmoraine classification.Tills The areal distribution of 'dead ice' deposits in on the western coast of Stord have higher upland areas is suggestive of a combination of a percentages of angular and subangular clasts, rising firn limit and general deglaciation. and increase in subangular characteristics to The difference in elevation of firn limits dur ward Fitjar. The difference in roundness ing Older and Y ounger Dryas times must have characteristics is attributed to longer distance been minimal. Andersen (1%5) subdivided the transport than those tills farther south, whereas Trømso-Lyngen substage in the north into two the occurrence of inherited forms in all tills glacial phases corresponding to Older and attest to short-transport, autochthonous con Younger Dryas times, with an intervening phase ditions. of less glacial activity. He was unable to de The dose correspondence of till fabric and termine any difference in firn limit elevation for glacial striae azimuths, alignment of erosional the two time periods. The Younger Dryas firn features, and pattern of deglaciation sediments limit value calculated for Stord is probably support the view that a single ice advance, the too high to account for cirque activity in Older Dryas, occurred subsequent to the main. Svartavatn East, whereas a slightly lower firn Weichselian ice advance. limit during Older Dryas time would account for Submarine sediments interpreted as till at Fit cirque activity. This view is consistent with the jarviki and the associated scattered till deposits interpretation of a rising firn limit and normal on surrounding is lands are considered part of an deglaciation to explain the general deglaciation end-moraine complex, the outer series of of Stord during Older Dryas time. moraines designating the assumed extent of Stord was completely covered with ice during advance. maximum extension of the main Weichselian ice The fossiliferous clay till at Breivik contain advance, approximately 20,000 yrs. BP, as in ing broken shells dated to 12,860±250 (f-1171) ferred from faceting, grooves, and erratics on places the ice advance as having occurred dur Midthamarsåto. The predominance of westerly ing the Older Dryas Stadial. striae along the northwestern coast of Stord, the The pollen analyses (Table l) of the fossil whaleback field at Kalvanes, and the 'P' forms bearing till suggest an open vegetation cover on the northern coast of Tveit are also consid predominantly of herb community. 63 percent ered to reflect this ice movement. of the AP consists of Salix and Betula with the Studies of sediment characteristics between rest of the pollen probably indicative of long outer and inner Norwegian shelf areas (H. distance transport. Hoitedahl 1%5), occurrences of striae and Chanda's (l%5) pollen diagram of Brøndmyra erratics in the Møre-Romsdal area (H. at Jæren has Gramineae, Cyperaceae, Artemi Holtedahl 1%0), and seismic profiling of as sia, and Rumex species extending from Oldest sumed terminal moraines on the Norwegian Dryas to Preboreal time. shelf (H. Holtedahl & Sellevoll 1971) suggest The high percentage of Salix in the Breivik that the last ice sheet extended to the edge of spectrum is abnormal and is associated with an the continental shelf and support the view that oceanic climate (L. Seising, pers. comm. 1973). Stord was completely ice covered during the The presence of Koenigia islandica indicates main Weichselian ice advance. heiiophilous, non-littoral, bare-soil, arctic-alpine Anomalous striae with north-south compo conditions (Hulten 1950, Lid 1963, Danielsen nent at Kalvanes and in the Leirvik area suggest 1969, Florin 1969). The Artemisia finds, also an ice movement which possibly predates the heliophilous, support this conclusion. main Weichselian westerly ice movement. Although the pollen spectrum results cannot Striae at Rekstern with north-south component be interpreted with certainty, an arctic or tund subsequently crossed by striae with westerly ra pioneer phase of vegetation is suggested, component and the questionable striae with with vegetation placed probably in either zone north-south component at Fonno also could la (Oldest Dryas) or zone lb (BØlling) time. represent this movement. These observations NORSK GEOLOGISK TIDSSKRIFf l (1978) Glacial geology of Stord, Norway 47 are in accordance with Undås (1949), who sug Conclusion gested an old, north-directed ice movement along the southwestern coast of Norway, but Stord retains morphological evidence of at !east Andersen (1964) has disputed this ice move five phases of late Pleistocene history: ment. Evidence of the main Weichselian ice Subsequent to the main Weichselian ice advance when ice was not influenced by to advance the ice retreated during the Bølling In pographic control manifested in the Tveit area terstadial to the head of Hardangerfjorden where all roches moutonees trend 270°; the (Mangerud 1970a). The Fitjar substage ensued well-preserved 'P' forms north of Sandvikvåg during the Older Dryas ice advance which was and along the Fugleviki coast; bedrock knobs at responsible for the deposition of the Fitjar Olstjøni overridden by westward-flowing ice; the Moraine. roche moutonee field at Kalvanes, and the lnvestigations of fossiliferous tills and subtill overrun cirque headwall at Svartavatn East. sediments around the Bergen area indicate that The Fitjar Moraine complex in connection they have originated from widely different times with dissected till deposits at Fugleviki, (Mangerud 1970a, 1970b, Mangerud & Skreden Kalvanes, and Vik associated with the Older 1971) and that marine sediments have been Dryas ice advance. overridden or displaced by ice (Mangerud lee recession following Older Dryas glacia 1970b). tion manifested in stratified outwash deposits at Mangerud (l970a), Aarseth (1971) Aarseth & Dafjorden, Nymark and Vik, the marine-out Mangerud (1974) and Pollestad (1972), mapping wash plain at Fitjar, kettle !akes, and outwash 'Ra' end moraines deposited during the deposits along both coasts. Younger Dryas Stadial, show that these The final phase, difficult to distinguish from moraines around the Stord region extend from general ice-recession characteristics of Older Herdla north of Bergen, follow mainland coastal Dryas glaciation, concerns evidence for the ex configuration generally around Bjørnafjorden, istence of a plateau ice cap. Radially distributed and continue along the eastern coasts of Tysnes drainage channels, moraines in proximity to and Huglo and the northern part of Halsnøy. cirque mouths, protalus ramparts as at Tveita The glacial striations on Stord formed subse fjellet and Sæterbo, glaciated valleys with rock quent to the main Weichselian ice advance are steps as at Steingilsheddi, the delta at Vatna, assumed to have been formed prior to the marine outwash at Jektivik and Borgtveit, and Y ounger Dryas ice advance, as no evidence is the 'dead-ice' topography on upland areas are available to indicate that the inland ice reached all suggestive of ice-cap conditions on Stord Stord during the Y ounger Dry as Stadial. following general deglaciation of Older Dryas Sediments at Blomvåg, dated by radiocarbon time. at 12,700 ± 350 yrs. BP and 12,200 ± 350 yrs. The 240 m.a.s.l. and 450 m.a.s.l. tablelands BP, have been corre1ated with fossiliferous correlate with snow1ines that fluctuated ela y, dated by radiocarbon at 12,470 ± 150 yrs. throughout Weichselian and probably Pre BP (Mangerud 1970a), at Sandviken near weichselian time. Bergen. Mangerud described the movement of Evidence of ice flow and ice recess ion on and the glacier front as having retreated landward around Stord, in conjunction with radiocarbon past Blomvåg prior to 12,700 yrs. BP (BØiling) dates and interpretation of submarine deposits, with subsequent advance after 12,200 yrs. BP. indicate that the Fitjar moraine is of Older during the Older Dryas Stadial. Dryas age. Marine limits, correlations of sur The Fitjar Moraine is the first demonstrated rounding area deposits, and estimated depres morphological evidence of Older Dryas inland sion of the firn limit and glaciation limit all ice advance in the Hordaland district, if the support this interpretation and also indicate that Breivik deposit is properly correlated with the the Y ounger Dryas advance did not reach Blomvåg and Sandviken deposits. Stord. Acknowledgements. - A special debt of gratitude is owed to Professor Hans Holtedahl and Dr. Jan Mangerud, University of Bergen, for helpful discussions both in and out of the field. The author gratefully acknowledges the cooperation and en couragement of Professor E. H. Muller, Syracuse University, 48 A. N. Genes NORSK GEOLOGISK TIDSSKRIFT l (1978) Syracuse, New York. Professor William Newman, North Fægri, K. 1943: Studies on the Pleistocene of west Norway, eastern University, Boston, Massachusetts read the manu BØmlo. Bergens Mus. 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