Tilting of lake shorelines in Jølstravatnet, western Norway, caused by glacioisostatic rebound
OVE NORALF RYE KLAKEGG &
Klakegg, O. Rye, N.: Tilting of lake shorelines in Jølstravatnet, western Norway, caused by glacio & isostatic rebound. Norsk Geologisk Tidsskrift, Vol. 70, pp. 47-59. Oslo 1990. ISSN 0029-196X.
Based on studies of shore levels and lake sediments, a change in outlet from the eastern to the western end of lake Jølstravatnet is demonstrated. The change was caused by the faster uplift in the eastern end. The lake history is subdivided into five phases. During phase I ( -9500 BP) lake Jølstravatnet was an ice-dammed lake. During phases Il and Ill (9500-7500BP) there was an eastern outlet and glacioisostatic tilting caused a transgression in the western part of the lake until the lake leve) had risen to the threshold in the western end. Phase IV (7500--6000 BP) is characterized by outlets at both ends of the lake. The emergence of the eastern outlet (ca. 6000 BP) marks the transition to phase V, i.e. the present situation, with an outlet over the western threshold. By making a tentative comparison between the Jølstravatnet time-gradient curve and two marine based curves from adjacent areas some trends in the glacioisostatic rebound of western Norway are deduced.
Klakegg, Geological Survey of Norway, P. Box 3006, Lade, N-7002 Trondheim, Norway. Present O. O. address: Norwegian Institute of Land Inventory, P. Box 115, N-1430 As, Norway; Rye, University O. N. of Bergen, Department of Geology, Sec. B, A/legaten 41, N-5007 Bergen, Norway.
Tilted marine shorelines is a classical field of Kræmer 1977; Holm-Larsen 1982). The divide study both in western Norway (e.g. Rekstad 1907; between the two lobes was situated at the water Kaldhol 1913; Reite 1968; Aarseth Mangerud shed at Skei at the eastern end of the lake (Klak 1974; Kaland 1984; Anundsen 1985; &Fareth 1987; egg 1981, 1987). Svendsen Mangerud 1987) and in other formerly glaciated& regions. The effect of glacio isostatic rebound on lake shorelines and location of outlet has, however, not been studied in Methods western Norway before. The lake Jølstravatnet, The development of the lake is reconstructed by situated at the watershed between Nordfjorden a synthesis of studies of the deglaciation history, and Førdefjorden (Fig. 1) , is well-suited for such former lake levels and the stratigraphy in lake a purpose because of the only 3 m difference Fuglevatnet at the eastern (Skei) threshold (Figs. in elevation of the thresholds at both ends and 2, 3). The terrace levels are plotted in an equi because the isobases cross the lake at an oblique distant shoreline diagram perpendicular to the angle (Fig. 3). During the Holocene uplift the Nor marine shore level isobases from Fareth outlet switched from the eastern to the western (1970, 1987). A Meridian clinometer, a Paulin end of the lake, causing several interesting fea aneroid barometer and maps at a scale of l : 1000 tures in the paleolake. (contour interval l m) are used to determine the The aim of this paper is thus to describe the elevation of the terraces. Holocene changes of lake Jølstravatnet and to put The sediments of lake Fuglevatnet were studied this development into the context of glacioiso to identify and date the start and the end of the static rebound. drainage across the Skei threshold. The corings The lake area was deglaciated ca. 9500 years were carried out using a 'Russian' peat corer BP, during the Preboreal time (Klakegg 1981). (Tolonen 1967) and a modified 110 mm diameter Afterthe formation of the Nor moraines (Fig. l) piston corer. The X-ray photographing of the in Younger Dryas (Mangerud et al. 1979; Fareth cores follows the methods described by Bouma 1987) the glaciers in Førdefjorden and Nordfjor (1969). Grain-size analyses were performed on den (Fig. l) retreated to Jølstravatnet (Rye 1963; sediments with loss on ignition below 10%. 48 O. Klakegg N. Rye NORSK GEOLOGISK TIDSSKRifT 70 (1990) &
N
4° 6° SOUTH NORWAY
o 10 20
C====l50 km
Fig. A map of the area showing lake Jølstravatnet situated just inside the Nor moraines (Younger Dryas). loset: key map of southernl. Norway.
Organic matter was removed with 10% H202 curve) can be constructed (Fig. 10), giving age solution. The coarse fraction (>63 ,um) was wet estimates of the different lake phases. sieved and the fine fraction analysed by the hydrometer method. The minerogenic sediments are classified according to Wentworth (1922). Where loss on ignition exceeds 10%, the sediment The present lake is named gyttja. Loss on ignition was performed Lake Jølstravatnet (207 m a.s.l., Fig. 2) is a typical at a temperature of 550°C (Sønstegaard fjord lake (Strøm 1935) situated well above the & Mangerud 1977). The results are shown as percent marine limit. The present outlet Iies at Vassenden ash content. Pollen preparation and identification at the western end of the lake (Fig. 3). The 80 m are mainly according to Fægri Iversen (1975). broad outlet river (the Jølstra) passes across a & By combining the radiocarbon datings from bedrock threshold which has a veneer of boulders. lake Fuglevatnet with synchronous lake leve! The level of the threshold is 205.9 m a.s.l., being gradients of lake Jølstravatnet, an approximate the lowest recorded water leve! in the lake. time-shore leve! gradient curve (time-gradient Lake leve! ftuctuations have been recorded NORSK GEOLOGISK TIDSSKRIFT 70 (1990) Ti/ting of lake Jølstravatnet, W. Norway 49
E w
Fig. 2. Lake Jølstravatnet seen towards south, with present (Vassenden) and former (Skei) outlets. (Photo: Normann.)
daily since 1903 by the Norwegian Water A threshold at the northeastern end of the lake Resources and Energy Administration (NVE). (at Skei) is only ca. 3m higher than the present Before regulation of the water level in 1951 the outlet threshold. Due to road constructions it is highest flood peaks reached 2.1-2.5 m above the impossible to determine the exact threshold level, threshold level. The mean water level lies 1.1 m but allowing for minor errors the level of the Skei above this level. threshold can be estimated to Iie at 209 m a.s.l.
l l l l l l l l l l l l l l 217 l IH.HYKL.EBUST l "' l •226 / Yl M'l'lUBUST •zzl'8 /
N.ÅS�2� l/ l l l l l l 'lOm 100m l l
Fig. 3. Terrace levels (m.a.s.l) along Jølstravatnet. For Vassenden and Skei the elevations of the thresholds are given. Isobases for the Nor leve! (Y ounger Dryas marine shore leve!) are from Fareth (1987). 50 O. Klakegg & N. Rye NORSK GEOLOGISK TIDSSKRIFT70 (1990)
Table l. Raised terraces at Jølstravatnet.
Locality Description ma.s.l. Interpretation
Hammar Small delta plane dipping 5°-7" towards the lake 230 Lateral Strand Remnant of a small fan delta 217 Lateral Tåhaug Shoreline, ca. 50 m long 214 Lake phase I Sanddal (l) Fan delta with surface dipping � towards the edge 218 Lake phase l, early? (2) Shoreline and small delta 214 Lake phase l Nedre Åsen (l) Fan delta 228 Lateral (2) Small delta 221 Lake phase I Ytre Myklebust (l) Small delta and shoreline 219 Lake phase I (2) Fan delta eroded in front 222-226 Lake phase l, early? Indre Myklebust (l) Shoreline 219 Lake phase I (2) Small fan delta and discontinuous shoreline 217 Lake phase I Dvergsdal Fan delta, eroded 235 Lateral Sandvikja Small delta 208.5 Lake phase Il Breelvskaret Fan with frontal delta sedimentation 213 Lake phase Il Sygnesand Eastem part of raised delta 215.5 Lake phase Il Vik ja Small delta 209 Lake phase Il Årdal Raised delta, foreset dipping towards the lake 218 Lake phase I Ålhus Raised delta plain with low dip towards the lake 212 Lake phase I
We assume that the two thresholds were sta mark older Jateral lakes to the glacier and are not bilized by ftoodsduring the deglaciation and that used for the history of Jølstravatnet. The levet of no postglacial erosion has Jowered them sig the ice-dammed lake is constructed on the basis nificantly. of terraces at the mouth tributary valleys which we assume were deglaciated nearly at the same time as the lake itself. Such a deglaciation pattem seems to be the case in Ålhus and Årdal, and the The Jølstravatnet glaciallakes (lake terraces at these sites are interpreted to represent phases I and Il) Jevels during lake phase I. Corresponding terraces The Jølstravatnet glacial lake formed when gla in Sanddal, Tåhaug and Myklebust (Table l) sup eiers in Førdefjorden (Fig. l) retreated to Jøl port this interpretation. From these terraces the stravatnet white a glacier from the Kjøsnesfjorden shoreline gradient of the ice-dammed lake can be down towards the lake Breimsvatnet still existed estimated to be 1.2-l.Om/km (Fig. 4). (lake phase l, Fig. 4, Klakegg 1981). Terraces at Some terraces (e.g. at Sanddal, N. Åsen and the mouth of tributary valleys are evidence of this Myklebust) which are situated a few metres above lake phase (Table l and Fig. 3). An equidistant this shore levet are also assumed to represent this shoreline diagram perpendicular to the Yo unger lake phase, as lateral ice-damrning at such Jow Dryas marine shore levet isobases (Fareth 1987) levels can be excluded. Because of the short time has been constructed on the basis of these terraces available it seems unlikely that the gradient of the (Fig. 4). Two glacial lake phases, an ice-dammed ice-dammed lake has changed beyond the inter phase (lake phase I) and a frontal glacial lake val estimated above. The higher levet may be (lake phase Il), can be traced. The burst of the explained by a somewhat higher threshold-Jevel ice-dammed lake caused the transition from lake at Vassenden during early parts of this lake phase. phase I to lake phase Il. The burst (9500 BP) ( -9500 BP) Lake phase I Extrapolated eastwards to Skei the lake:<:phase I Some high-lying terraces (e.g. at Hammar, shorelines point far above the threshold here. The Strand, Nedre Åsen and Dvergsdal) probably retreat of the ice lo be therefore caused a burst of NORSK GEOLOGISK TIDSSKRIFT 70 (1990) Ti/ting of lake Jølstravatnet, W. Norway 51
=:n ::l� <"" 100 -,::l 20km ..... o ION a. o -::l 3 10 - 224 " 1,2 3 !Jl m/km :"0 ...... 222 ...... Iåfiiriii' 1,0 m/km -c> :tl!rro,cl!s N 220 a- o Preb. 218
216
Bo. 214
---
202
LAKE PHASE I ( w
Fig. 4. The Jower part shows paleogeographic maps of the lake phases (I-V). On the first two the shaded areas are glaciers with arrows showing ice movement. On all maps the large arrow shows which outlet (Vassenden or Skei) was in use. The upper (teft) is an equidistant shoreline diagram for Jølstravatnet, where the terraces and thresholds are projected into the projection plane shown in Fig. 3. The vertical zones that the shorelines passed through during the different lake phases (paleogeographic maps) are marked with different hatching. (Note that the radiocarbon datings (to the right) and chronozones are turned upside-down in order to match with the diagram.)
the ice-dammed lake. The lake level dropped by Skei. The shoreline gradient representing this some 12m as the threshold at Skei became the event is estimated to be l. Om/km. controlling leve!. This marks the transition to lake Glacioftuvial and glaciolacustrine sediments phase Il and the establishment of the outlet at along the drainage route northwards from Skei 52 O. Klakegg & N. Rye NORSK GEOLOGISK TIDSSKRIFT 70 (1990)
m.a.s.U 220
10 ' � 215 l o l l
� 5 � 210 � § !l!o -Cl 1::1 si -5 200 -10 3 B.P. 195 10 years o LAKE PHASE l Fig. 5. Calculated shore displacement curves from different parts of Jølstravatnet. (The curves indicate the displacement of the mean water leve) of the lake.) The lake phases indicated in the lower part correspond to Fig. 4. to the lake Breimsvatnet (Figs. 6, 7) indicate a considerable sediment input from Jølstravatnet area during the deglaciation. In the former lake Skredevatnet (Fig. 6) grave! beds below couplets of sand and silt indicate the subglacial/proglacial origin of these sediments. A subglacial drainage is also supported by the gravel bed F in the Fuglevatnet (Fig. 8, Table 2). On the other hand, the kettle-hole !akes and the glaciolacustrine sediments (Fig. 7) are of pro glacial origin. We assume that the gravel in Skredevatnet is the result of a subglacial burst of the Jølstravatnet ice-dammed lake. The great discbarges may have dissected greater parts of the damming glacier and mark the transition from subglacial to proglacial deposition among rafted icebergs which occupied the kettle-hole !akes. At the southern end of Breimsvatnet (Fig. 7) Rye (1963) described two main terrace levels. The upper terrace at 91 m a.s.l. is a small sandur delta (marine limit). The lower terrace at 81 m a.s.l. probably covered most of the valley floor over a length of more than l km. This terrace may represent a link between the lake levels of Fig. 6. The Skei valley. TheIevels of Fuglevatnet and Skrede Jølstravatnet and the marine levels, as high sedi- vatnet before the artificial lowerings are stippled. NORSK GEOLOGISK TIDSSKRIFf 70 (1990) Ti/ting of lake Jølstravatnet, W. Norway 53 + + + + + 1km + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + � Til l, mostly discontinuous or thin cover l l Mass movement deposits D Glaciolacustrine deposits � Glaciofluvial deposits t:?:·::.:;·J Fluvial deposits � Peat It" ·.Jtl � Bedrock © Kettle- hole Å\ Fan x91 H11rine limit (ma.s.I.J Fig. 7. A simplified Quaternary geological map of the superficialdeposits in the Skei valley between Jølstra vatnet and Breimsvatnet (Fig. 1). mentation rates are necessary to build such a l) because the corresponding levels in the western terrace during the fast Preboreal emergence. part Iie beneath the present lake leve! (Fig. 4). The raised glaciofiuvial delta at Sygnesand (Fig. 3) represents the end of this lake phase as it (9500-9200 BP) Lake phase Il coalesces with the deglaciation of the lake. The During lake phase Il Jølstravatnet was a frontal gradient of the corresponding shoreline is esti glacial lake with a retreating ice-lobe in Kjøs mated to be 0.8 m/km, which indicates a change nesfjorden and a lake leve! controlled by the in gradient from l. Om/km to 0.8 m/km during eastern threshold at Skei (Fig. 4). The start of lake phase IL this lake phase is defined by the burst of the ice dammed lake and the end by the deglaciation of The Jølstravatnet postglaciallakes the lake. During this lake p hase the glacioisostatic tilting of the land caused a regression to the east (l ake phases Ill, IV and V) of the outlet (Kjøsnesfjorden) and a transgression The postglacial history of Jølstravatnet is divided to the west of the outlet (Figs. 4, 5). Raised into three lake phases (Fig. 4) based on the outlet terraces from lake phase Il are thus found only migration fromSkei (lake phase Ill), via a water in the eastern part of the lake (Fig. 3 and Table shed phase (lake phase IV) to the present outlet 54 O. Klakegg & N. Rye NORSK GEOLOGISK TIDSSKRIFT 70 (1990) Table 2. Sumrnary description and interpretation of the Fuglevatnet beds, based on 16 cores from the lake (Klakegg 1981). Bed Description Interpretation A Bed A is a brown, coarse detritus gyttja with relative constant Bed A represents the present situation with thickness in the centre of the lake (270-350 cm), increasing Fuglevatnet isolated from Jølstravatnet. to more than 500 cm towards land. Most of the bed is homo Only a couple of small brooks enter the lake. geneous, but a few sand and silt layers are observed (Fig. 8). The water transport throughout the lake is The layers cannot be traced from core to core and therefore have very low, resulting in high organic sedimen to be of local origin. The loss on ignition varies mainly between tation rates (Fig. 8). 30 and 50%. Particles of sand and fine grave! are identified on X· ray photos all through the bed. B Bed B is a 10-30 cm thick sequence of laminated gyttja. The X-ray Bed B is correlated with the later part of photo (Fig. 9) reflects the variations in organic/minerogenic lake phase IV during which Fuglevatnet was content showing laminae of 2-10 mm thickness. The laminae are isolated from Jølstravatnet. Reduced drainage irregular and unpaired. The loss of ignition varies from lamina to across the Skei threshold resulted in an upward lamina, but shows a general increase from about 10% at the bottom increase of organic sedimentation rates. The to about 25% at the top. The upper limit of the lamination defines irregular lamination is caused by water leve! the A/B boundary. A t•c date across this boundary gives an age of ftuctuationsin Jølstravatnet. The upper bound 5970±70 BP (T-3599A). The pollen assemblages at the lower (P2) ary of this bed defines the end of lake phase IV. and upper (P3) boundaries (Klakegg 1981) are dominated by Alnus and Betula. Corylus and Sorbus are also important elements of the vegetation at both boundaries. Ulmus has risen from insigni ficant at the lower boundary to 6% at the upper. c Bed C is a 15-100 cm thick layered sequence of gyttja silt. Loss Bed C is deposited during lake phase Ill and on ignition values of 5-10% dominate. An intemal, 1-2 cm thick parts of IV. The low mean influx of organic layer of graded sandy silt (Cl) is distributed throughout the lake. matter below the A/B boundary (Fig. 8) The dark colour is due to the presence of biotite. indicates drainage across the Skei threshold which prevented organic sedimentation in Fuglevatnet. D Bed D is a 2-13 cm thick bed of laminated clayey silt. The loss Bed D is the result of sediment input from the on ignition increases from l to 5% from bottom to top. The pollen Jølstravatnet frontal glacial lake (lake phase assemblage at the lower boundary indicates an open fores! vegeta Il). Upward increase of organic content indi tion of Betula, Salix and Juniperus. Corylus is also represented. cates vegetational immigration, which is sup ported by the pollen assemblage resembling the late Preboreal vegetation found in the lowland of western Norway (Kaland & Krzywinski 1978). E Bed E is a 5-25 cm thick well-sorted silty sand with no organic Bed E is probably the result of the first sedi content. The upper boundary is sharp. ment input in the lake and may be correlated with the start of drainage across the Skei threshold (the start of lake phase Il). F Bed F is at minimum 60 cm thick poorly sorted sandy grave! with Bed F is deposited during the deglaciation and no organic content. The bed may be correlated with the grave! is probably the result of subglacial drainage bed which surrounds this kettle-hole lake. prior to the kettle-hole formation This drainage may be connected with the burst of the Jølstravatnet ice-dammed lake. at Vassenden (lake phase V). Stratigraphical evi dence for this development is found in the sedi Lake phase Ill (9200-7500 BP) ments of lake Fuglevatnet. This lake is situated The transition from lake phase Il to lake phase at the Skei threshold in the deepest part of the Ill is definedby the ice retreat fromthe lake area. valley ftoorand therefore received all the drainage There was no change in the outlet which was crossing the Skei threshold (Fig. 6). In this way established at the Skei threshold at the beginning the postglacial lakes of Jølstravatnet are closely of lake phase Il. The lake levels during lake related to the stratigraphy of Fuglevatnet (Table phase Ill are therefore determined by the Skei 2, Fig. 8). threshold. NORSK GEOLOGISK TIDSSKRIFT 70 (1990) Ti/ting of lake Jølstravatnet, W. Norway 55 LAKE 14c PH ASE DATINGS GRAIN SIZE B CLAY § SILT D. SAND GRAVEL A 900 B V. 5970±70 (T-3599) N 0,4 m 9260±100 (T-3598) LEGEND c.§ GRADED SANDY SILT A � COARSE DETRI TUS GYTTJA o1-�-1�-- CLAYEY SILT B� E lZ:) LAMINATED GYTTJA · SILTY SAND GYTTJA SILT FEi:d- SANDY GRAVEL (� ·•· ·• - Fig. 8. The stratigraphy in Fuglevatnet represented by core XV. The glacioisostatic tilting described under higher than that above the A/B boundary (Fig. phase Il continued, and with the outlet at Skei 8). threshold (Fig. 4) the regression in the Kjøs nesfjord area and a transgression in the western part of the lake continued (Fig. 5). The shore Lake phase W (7500-6000 BP) leve! gradients changed from about 0.8 m/km in Lake phase IV is the watershed phase with drain the early part of the phase to 0.27 m/km at the age across both thresholds. The water leve! in the transition to lake phase IV. At the latter gradient Vassenden area continues to rise from the first there was a minor but continuous flow across the overflowat a gradient of 0.27 m/km until all water Vassenden threshold at mean lake leve!. flows in the deep river (shoreline gradient of In Fuglevatnet most of bed C (Tab le 2 and Fig. 0.20 m/km). During this phase the drainage 8) was deposited during this lake phase. The low gradually shifted from Skei to the Vassenden mean influx of organic matter below the A/B threshold, and a regression of the whole lake boundary indicates drainage across the Skei started (Fig. 5). threshold which prevented organic sedimentation In lake Fuglevatnet reduced drainage across in Fuglevatnet. Therefore no Preboreal rise in the Skei threshold resulted in higher organic sedi organic content is registered in this lake. The low mentation rates. Laminated bed (B) is interpreted organic content (10%) cannot be explained by to reflect irregular changes in the outlet river at high inorganic influx as this value is only slightly Skei during the period the outlet switched from 56 O. Klakegg & N. Rye NORSK GEOLOGISK TIDSSKRIFT 70 (1990) Skei to Vassenden. The irregular (unpaired) !ami nation (Fig. 9) excluded any form of cyclic change in sediment input, e.g. seasonal changes as described by Renberg (1976, 1978). Lake phase IV ends when the highest floods are no longer traceable in the sediments of lake Fuglevatnet. This event is radiocarbon-dated to 6000 BP. BED A Lake phase V (6000 BP until the present) During lake phase V the outlet lay at the Vas senden threshold and the regression continued at a slow rate over the whole lake. The tilting continued as seen from the present difference in elevation between the two thresholds. Probably a decelerating tilting continued up to the present (Bakkelid 1979). A time-gradient curve for the shorelines Investigation of tilted !akes has the advantage that the shoreline gradients can be estimated directly between the outlet threshold (base leve! for all lake shorelines) and terraces or other thresholds surrounding the lake at different levels (Fig. 4). Extensive preliminary work constructing shore displacement curves, which is needed for esti mating the marine shore level gradients, is there fore unnecessary. Besides, the method is not restricted to areas below the marine limit. The main potential of the method is to yield tilt data from areas above the marine limit. Combining marine and lacustrine based data the research areas can be extended considerably. Sem The time-gradient curve from Jølstravatnet (Fig. 10) describes postglacial tilting of the land surface in an area above the marine limit. The curve is based on the following data and assump tions: (l) The curve is constructed with the assumption that tilting through time can be described by an exponential function (Grønli 1941a, b; Norrman 1964; Bergqvist 1977). (2) The present shoreline gradient is zero. (3) The gradient at the start of the drainage across the Skei threshold is l. Om/km and the sup posed age of this event is 9500 BP. Fig. 9. X-ray photograph of laminated gyttja (Bed B) and the (4) The gradient for the end of the drainage transition to detritus gyttja (Bed A) in core XI. (Minerogenic across the Skei threshold (5970 ± 70 BP) is layers are dark.) calculated to be 0.14 m/km taking into NORSK GEOLOGISK TIDSSKRIFT 70 (1990) Tilting of lake Jølstravatnet, W. Norway 57 SHORELINE GRADIENT M/KM 1,2 1,1 1,0 T-3598A 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 +----.�------�----r----.----.-----.----.----=r�--� 10 9 8 7 6 5 4 3 2 , o TIME 3 (14( YEARS B.P. X 10 l LAKE ri PHASE l ll1 m l NI l Fig. JO. The approximate time-gradient curve for the Jølstravatnet area. consideration the lake level changes in Jøl areas. This may indicate differences in the stravatnet and the channelling properties at crust/mantle properties as the rate of ice-load the thresholds (Klakegg 1981). removal (deglaciation) during Preboreal time (5) It is assumed that no faulting or other tectonic is at least as high in the Møre-Trøndelag area disturbance has occurred. Furthermore, as further south (Reite et al. 1982). Altema small-scale deviation from the idealized curve tively, the difference may be explained by may have been caused by hydroisostasy, neo larger gradients of the total Weichselian ice glaciation and the fact that radiocarbon years load in the Møre-Trøndelag area. are not identical to calendar years. (2) Apparently there is a convergence between the Jølstravatnet curve and the curve from the A comparison between the Jølstravatnet time Hordaland coast from ca. 8000 BP. This is gradient curve and two marine based curves from important as it indicates a mode of uplift western Norway (the Hordaland coast (Kaland which appears as a tilting of approximately 1984) and Møre-Trøndelag (Svendsen & Man straight lines for the last ca. 8000 years in this gerud 1987), (Fig. 11)) indicates some interesting part of western Norway. trends in the glacioisostatic rebound of western (3) The difference between the Hordaland and South Norway: the Jølstravatnet curves in early Holocene indicates bent or broken Preboreal/Boreal (l) The early Holocene gradients of the Møre shorelines in western Norway. The 9500 BP Trøndelag shorelines appear to change more sborelines for example appear as a marked slowly than the gradients of Jølstravatnet and bent line between a slightly concave Younger the Hordaland coast, giving higher Holocene Dryas (10,500 BP) shoreline and a straight shoreline gradients than in the other two 8000 BP shoreline in a preliminary combined 58 O. Klakegg & N. Rye NORSK GEOLOGISK TIDSSKRIFT 70 (1990) /Mere- Trøndelag 0,5 Fig. Il. Three time-gradient curves from western Norway (Fig. 1): The Møre-Trøndelag curve (Svendsen & Mangerud 1987), the curve of the 10 3 years B.P. Hordaland coast (Kaland 10 1984) and the Jølstravatn curve. shoreline diagram (Fig. 12). Dating uncer ern) curve appears to be slightly delayed rela tainties may explain some of the apparent tive to the total deglaciation of the area, as gradient difference between the 9500 BP opposed to the Møre-Trøndelag area where shorelines of the Hordaland coast (0.52 m/ a similar differencebetween eastern and west km) and Jølstravatnet (l. O m/km). However, ern gradients (Fig. 11) apparently disappears some hending caused by a delayed degla synchronously with the melting of the last ice ciation (ice-load removal) in the eastern bodies in that area. (Jølstravatnet) areas relative to the western (Hordaland coast) area seems likely. Acknow/edgements. - Valuable comments and constructive ( 4) The convergence between the Hordaland criticism have been given by Eiliv Larsen, Arne J. Reite, Jan coast (western) and the Jølstravatnet (east- Mangerud and Eivind Sønstegaard. Irene Lundquist drafted the m.as.l. �...._ Jelstra _..J vatnet -.., 90 ro-- 9500 B.P. BO Jiordalanll..J !+- Nordfjord -.1 70 ro-- coast ...., / 10500B J 60 /'' / 50 / ---- ? / " f\1.11'..- // ' 30 BOOOB.P. � Fig. 12. A tentative shoreline � - / /// - 20 / -- -- diagram of western Norway - - - :::: ::: - combining data from the 0 2 10 .5� / �""' Hordaland coast (Kaland -- 0.35mfkm 1984), the Nordfjord area t-0 10 20 30 40 50 (Klakegg & Nordahl-Olsen 1985) and Jølstravatnet. NORSK GEOLOGISK TIDSSKRIFT 70 (1990) Ti/ting of lake Jølstravatnet, W. Norway 59 liguresand Janne Grete W. Hagen typed the manuscript. Peter Klakegg, O. 1987: Skei. Quaternary map AQR 083084 - Padget corrected the English text. To these persons we proffer l: 20000. Norges geologiske undersøkelse. our sincere thanks. Klakegg, O. & Nordahl-Olsen, T. 1985: Nordfjordeid. Descrip Manuscript received September 1988 tion of the Quaternary geological map 1218 I - l: 50000. Norges geologiske undersøkelse Skrifter 71, 1-29. Kræmer, R. 1977: Isavsmeltingsstudier og anvendt kvar tærgeologi i Naustdal, Sunnfjord. Unpubl. cand. real. thesis, References Univ. of Bergen. Aarseth, I. & Mangerud, J. 1974: Younger Dryas end moraines Mangerud, J., Larsen, E., Longva O. & Sønstegaard, E. 1979: between Hardangerfjorden and Sognefjorden, western Glacial history of western Norway 15000-10000 BP. Boreas Norway. Boreas 3, 3--22. 8, 179-187. Aksdal, S. 1986: Holocen vegetasjonsutvikling og havni Norrman, J. O. 1964: Vætterbæckenets senkvartæra strand våendringer i Florø, Sogn og Fjordane. Unpubl. cand. scient. linjer. Geologiska Foreningen i Stockholm Forhand/ingar 85, thesis, Univ. of Bergen. 391-413. 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