James S. Aber
|Norway and Sweden||Denmark and Skåne|
Glaciation of Norway and central Sweden
Digital elevation model for northern Europe and the adjacent sea floor. Ice
sheets originated in the mountain plateaus of Norway and Sweden, and spread
outward in all directions into surrounding lowlands and onto shallow continental
shelves. Scandinavian ice sheets reached southward as far as the mountains of
central Europe during earlier glaciations. DEM derived from ETOPO5 database;
image processing by J.S. Aber.
|Small island in the Stockholm archipelago of eastern Sweden. The crystalline bedrock was sculpted by glacier erosion. Photo date 8/00, © J.S. Aber.|
The configuration of the ice sheet has been reconstructed on the basis of nunatak positions. A relatively thin, low-gradient ice profile results with the ice divide close to the modern drainage divide in western Norway. The ice sheet was thicker in the lowlands to the east and in fjord valleys to the west. This thin ice-sheet reconstruction contrasts with the traditional thick ice-sheet model--see Fig. 13-3. Nunataks in Scotland also indicate a thin ice cap during the last glaciation of the British Isles (McCarroll et al. 1995).
Western Norway was subjected to strong glacial erosion, and so few deposits predating the last deglaciation are found. Nonetheless, a few protected sites, such as coastal caves, do contain a record of multiple glacial and interglacial deposits. Most glacial deposits in western Norway date from late stages of the last glaciation. The Younger Dryas glaciation was a significant readvance or stillstand of the ice sheet during its general retreat, between 10,000 and 10,500 radiocarbon years BP. After this, the ice sheet retreated rapidly, although minor glacier stillstands occurred in some inner fjord valleys during the Preboreal.
|Glacier scoured bedrock landscape of western Norway, island of Askøy. This scene is typical of glacial zone 2, in which strong erosion removed most sediment and soil. Photo date 5/87, © J.S. Aber.|
|Overview of Fanafjord, near Bergen, western Norway. During the Younger Dryas glaciation, the fjord was filled by an ice tongue that reached the sea (to left). The margin of the ice tongue rested against the mountain side in foreground, along which a small lateral moraine was constructed. Photo date 6/87, © J.S. Aber.|
|View of lateral moraine on mountain side adjacent to Fanafjord. The grassy ridge marks the lateral moraine crest. Photo date 6/87, © J.S. Aber.|
|Glaciomarine terrace in Modalen, western Norway. This terrace was deposited as an ice-marginal marine delta during Preboreal retreat of the valley glacier. At that time the crust was still depressed and sea level was at the terrace top in this fjord valley. Photo date 5/87, © J.S. Aber.|
Along most of its margin, the Younger Dryas ice sheet terminated in the sea, because the crust was still depressed. This renewed glacial activity was apparently brought about by sharply colder climate in the North Atlantic region. One plausible theory is that rapid melting of the Laurentide Ice Sheet flooded the North Atlantic (via the St. Lawrence seaway) with cold fresh water that led to pack-ice freezing. Colder climate resulted in northern Europe, until the Atlantic returned to open-water, normal-salinity conditions.
Zone 3 glacial landscapes in central Sweden record a complex history of Vistulian glaciation. The area was completely glaciated during an early Vistulian advance that laid down till of local or western-derived lithology. The area was then deglaciated, and fossil-bearing deposits accumulated during the Jämtland interstade, dated around 55,000 radiocarbon years BP (Lundqvist and Mook 1981). The main Vistulian glaciation then moved in from the west and completely covered the area. During this glaciation, the ice divide migrated to the east of the region and finally split into local centers, resulting in complicated ice flow patterns.
|Relatively thick glacial sediments form a nearly continuous cover over the rolling landscape of central Sweden. Here sand and gravel is quarried for construction material, but the miners must work around huge erratic boulders. Grytan near Lake Storsjön, Sweden. Photo date 6/87, © J.S. Aber.|
The glaciation of northern Scandinavia was closely connected to ice sheets centered on the continental shelves of the Barents Sea and Kara Sea. It was long assumed these ice sheets behaved more-or-less synchronously with the Scandinavian ice sheet. However, recent field investigations in northern Russia have revealed quite a different story (Mangerud et al. 1999). Both the Barents Sea and Kara Sea ice sheets reached their maximum developments in the early and middle Weichselian, when the Scandinavian ice sheet was relatively small--see Fig. 13-4. During the late Weichselian, in contrast, the Scandinavian ice sheet expanded to its maximum and merged with the Barents Sea and British Isles ice sheets, but the Kara Sea ice sheet hardly existed--see Fig. 13-5. This suggests the inherent instability of these ice sheets, which were largely marine based.
|Stevns Klint, eastern Denmark. Danian (lower Paleocene) limestone forms a resistant ledge over upper Cretaceous chalk in lower part of cliff. Soft chalk underlies much of the western Baltic; chalk was highly suspectible to glacial erosion and ice-push deformation. This cliff is the stratotype for the Cretaceous/Tertiary boundary. Photo date 5/79, © J.S. Aber.|
A new method of glacial stratigraphy, called kineto-stratigraphy, has proven useful for working out the sequence of glacial advances and glaciotectonic deformations of Denmark (Berthelsen 1978). A kineto-stratigraphic unit consists of all glacial features--erosional, deformational and depositional--associated with the advance and retreat of an ice lobe. The directional, or kinetic, character of these features is the unifying element that relates to the regional pattern of ice movement--see Fig. 13-7. On this basis, a detailed stratigraphy is established for Quaternary glacial and interglacial strata in Denmark (Houmark-Nielsen 1987, 1999)--see Fig. 13-8.
|Huge chalk slabs (megablocks) at Hvideskud near the southern end of Møns Klint. This cliff is about 50-60 m high here. The chalk megablocks are deformed together with Quaternary sediment (brown zones). The chalk was transported by ice movement from the south. Photo date 10/86, © J.S. Aber.|
|Chalk masses thrust and folded into vertical positions near the center of Møns Klint. The pinnacle of Sommerspiret stood > 100 m above sea level, and chalk is deformed to a depth of 20-40 m below sea level. Deformed Quaternary sediment (brown) can be seen at the base of cliff. The chalk pinnacle fell down during a storm in the early 1990s. Photo date 10/86, © J.S. Aber.|
|Small glaciotectonic fold in till (gray) and sand (brown), island of Møn, Denmark. The nose of the isoclinal fold is dug out, and the pen indicates the fold axis. Ice movement was from right to left, at right angle to the fold axis. Such directional indicators are important for kineto-stratigraphy of glacial deposits. Photo date 5/79, © J.S. Aber.|
Directional features are many and varied: striations and grooves, till fabrics, drumlins, ice-push structures and faults, and indicator erratics. The use of indicator erratics, which was popular early in this century, has experienced a renewal with development of new quantitative techniques and presentation methods (Smed 1989, 1993).
Danish indicator erratics are derived from diverse source rocks in southern Norway and Sweden, the Baltic depression, and southwestern Finland. The composition of indicator erratics for a particular till at a given site may be plotted with circle diagrams according to the abundance and source of each erratic. The pattern of circles marks the path of ice movement that transported erratics to the site; this procedure is demonstrated for two tills from Ristinge Klint, southern Denmark--see Fig. 13-9. The patterns must, of course, be interpreted with caution. The sources of certain erratics are equivocal, and some erratics could have been reworked from older glacial deposits.
The western Baltic region of southern Denmark, Skåne and northern Germany shows great variations in ice movement directions during Vistulian glaciation. Ice advances seemingly came from all directions during different phases, particularly during late phases of glaciation. Interpretation of this complicated scenario is highly controversial, but two main models are now in place.
|Space-shuttle photograph of western Baltic region. High-oblique view; north toward top--Sweden to upper right, Denmark to left, Germany at bottom. Late glacial Baltic ice streams flowed along the Baltic basin toward the west (left), and a marginal ice dome may have formed over the southwestern Baltic region. NASA Johnson Space Center, Imagery Services, STS045-613-04, 3/92.|
|Space-shuttle photograph of the islands of Fyn (top), Langeland (right), and Ærø (center), southern Denmark. Near-vertical view of westernmost Baltic Sea and Danish islands. Lagerlund (1987) proposed that this region was the site of a marginal ice dome that developed during late phases of Vistulian glaciation. NASA Johnson Space Center, Imagery Services, STS045-87-25, 3/92.|
|Space-shuttle photograph of eastern Denmark (center) and Skåne, southern Sweden (right). Low-oblique view toward west showing Baltic Sea. The outline of Danish islands indicates the shape of ice lobes that advanced from the Baltic depression during final phases of the last glaciation. The island of Møn is visible in lower left portion. Dislocated chalk masses in Møns Klint were pushed from the northeast and the south; glaciotectonic deformation may have taken place in an interlobate position (Jensen 1993). NASA Johnson Space Center, Imagery Services, STS045-152-134, 3/92.|