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° .