Glacial History

Northwest Europe contains evidence of continental glaciation from the latest three cold stages, the Elsterian (Marine Oxygen Isotope Stages 14 and 12), Saalian (Stages 8 and 6) and Weichselian (Stages 4 and 2) (Bowen et al., 1986; Sibrava, 1986; Ehlers, 1996). The landscapes produced during these three cold stages differ markedly:

1. The Elsterian glaciation is characterized by extensive systems of very deep (up to 400 m) tunnel valleys. They form an anastomosing pattern of generally overdeepened and closed elongate depressions (Huuse and Lykke-Andersen, 2000). In The Netherlands and West Germany thrust moraines are absent. Elsterian thrust moraines are only known from the North Sea area (Bammens, 1986; Cameron et al., 1989; Laban, 1995), Central Germany (Eissmann, 1995) and Poland (Brodzikowski, 1995). Elsterian tills are found throughout the area and are interrupted only where they have been removed by post-Elsterian erosion and reworked into younger sediments. They contain commonly large proportions of locally derived Tertiary material. The rarity of thrust moraines in Northwest Europe may be due partly to removal by the more extensive Saalian glaciation. Yet, it is at least striking that Elsterian push structures have been rarely described as part of younger structures. In the area east of Leipzig and north of Dresden in central Germany a series of probably Elsterian age thrust moraines occur, which have been overridden by the Drenthe and Warthe advances of the Saalian glaciation (Eissmann, 1994).

2. The Saalian glaciation is characterized by large thrust moraines, particularly those belonging to the oldest and southernmost Rehburg line. Several thrust moraine lines are arranged along

Figure 8.1 Middle and Late Pleistocene margins of the Southern Scandinavian Ice Sheet in the Northern European Plains. A) Shaded relief map showing Elsterian (E), Saalian (S) and Weichselian (W) glacial limits. In central Europe, Elsterian and Saalian ice sheets were bounded by Variscan mountain chains. The Saalian margin in The Netherlands coincides with the northern fringe of NW-SE trending Central High (see also Fig. 8.19). Morphology of the Northern European Plains is strongly dominated by chains of end moraines, thrust moraines and till plateaux. Many of the present day major river valleys within this area follow ice-marginal drainage systems. Numbers indicate areas shown in detailed maps: 1 = central Germany (Fig. 8.16), 2 = Mecklenburg (Fig. 8.18), 3 = The Netherlands and western Germany (Fig. 8.19). B) Glacial limits and major end moraines: E = Elsterian, SD = Saalian, Drenthe glaciation, SW = Saalian, Warthe glaciation. Weichselian end moraines: L, Br = Leszno/Brandenburg, Pz, Fr = Poznan/Frankfurt, Pm = Pomeranian, G = Gardno. Correlation of end-moraine lines between western and eastern Europe is often uncertain because of different stratigraphical and dating methods. The map compiles data from Houmark-Nielsen (1983, 1987, 1989), Sj0rring (1983), Ter Wee (1983), Van der Wateren (1985, 1995), De Gans et al. (1987, 2000), Stephan (l987, 1995), Van den Berg and Beets (1987), Long et al. (1988), Brodzikowski (1995), Caspers et al. 1995), Eissmann (1995), Gozhik (1995), Khristophorova (1995), Knoth (1995), Macoun and Kralik (1995), Matveyev (1995), Mojski (1995), Müller et al. (1995), Raukas (1995), Rühberg et al. (1995), Skupin et al. (1993), Sejrup et al. (1998), Huuse and Lykke-Andersen (2000), Marks (2002). Location of major ice streams within the Weichselian glaciated area is based upon Boulton et al. (2001b), and location of those within the Saalian glaciated area is based upon the assumption that lobate moraines are produced by fast-flowing ice streams.

series of marginal positions. Some of these lie along deep glacial basins, up to 150 m below mean sea level, for example the Gelderse Vallei and the Ijssel Valley in The Netherlands (De Gans et al, 1987; Van der Wateren, 1981, 1985), the Nordhorn basin (Richter et al., 1951) and the Quakenbruck basin (Meyer, 1987; Van der Wateren, 1987, 1995) in Germany. Except for the North Sea area and the Hunze valley in the northern Netherlands (Van den Berg and Beets, 1987) extensive systems of deep tunnel valleys are not known from the Saalian, although it may be argued that the overdeepened glacial basins on the upstream side of thrust moraines are similar structures (Boulton and Hindmarsh, 1987). Shallow dry valleys running parallel to the assumed ice flow direction and relatively few drumlins are known to occur in the Saalian till covered areas (Schroder, 1978; Ehlers and Stephan, 1983; Rappol, 1984).

3. The Weichselian glaciation is characterized by generally smaller thrust moraines than those of the Saalian. Tunnel valleys are not as numerous and on the average about half as deep as the Elsterian tunnel valleys. Drumlins have been reported from Denmark, Schleswig-Holstein (Stephan, 1987) and Poland (Karczewski, 1987).

In Germany subdivision of the glaciations had traditionally been based on morphostratigraphy, distinguishing various ice-marginal lines, which were assigned to 'stages'. Each of these, particularly within the Weichselian glaciated area, was subdivided into end moraines. According to Ehlers et al. (1995) and Ehlers (1996) the Brandenburg/Leszno, Frankfurt/Poznan moraines probably belong to the same ice (re)advance as they are represented by one single till unit. The end-moraine lines define different stages in the retreat after 20 ka BP. In other cases such end moraines could be shown to have formed during an ice sheet advance and not necessarily marked a major stagnation of the advance, for example the Older Saalian Rehburg line of thrust moraines. It must be borne in mind that some of these moraines may be diachronous, which makes correlation with similar end-moraine lines elsewhere problematic (Van der Wateren, 1995).

Figure 8.1a is a shaded relief map of Northwest, Central and part of Eastern Europe showing the Middle and Late Pleistocene margins of the Southern Scandinavian Ice Sheets in relation to topography. The high topography in the south comprises Variscan mountain chains — including the Teutoburger Wald, Harz Mountains, Thuringerwald, Erzgebirge and Sudetes - and the Alpine chain of the Carpathian Mountains. The Elsterian and Saalian ice sheets terminated against these natural barriers. The plains north of these mountains, underlain by Cenozoic sediments, are largely shaped by successive overriding ice sheets. Major end-moraine lines with extensive plateaulike till plains on their proximal side, stand out in the map. Large valleys have been carved by ice-marginal drainage, which originated from south-to-north running rivers that were redirected parallel to the ice margins. Examples of these are the Vistula River valley in Poland (~53°N, 15°-20°E), the Elbe River (~54°N, 10°E) and Weser River (~53°N, 10°E) in Germany and the Rhine and Meuse Rivers in The Netherlands (~52°N, 5°-6°E). The smaller Weichselian ice sheet terminated near the northern margin of the sedimentary basins, in the plains previously overridden by the Elsterian and Saalian glaciations. Except in Eastern Europe it was less confined by topographic barriers.

Figure 8.1b shows the margins of the ice sheets, as well as major end moraine lines within these margins (marking advances, or readvances during retreat), based upon a compilation of data from various sources. This map of the southern Scandinavian glaciated area must be regarded as tentative. The limits of the Elsterian, Saalian and Weichselian ice sheets are generally well established. However, detailed correlation of individual end-moraine lines between different countries — particularly between those of Eastern and Western Europe — remains problematic in places. This chapter therefore focuses on the western and central parts of the area glaciated by the Southern Scandinavian Ice Sheets.

The Elsterian glaciation completely changed the regional drainage of the Northern European Plains. Many rivers that previously drained northward into the Baltic basin were redirected along the ice-sheet margin into westward directions. Large ice-marginal lakes developed next to the Variscan highlands, for example the ice-dammed lake occupying the Elbe River valley south of Dresden in central Germany (Fig. 8.1b). This pattern repeated itself during subsequent glaciations, for example the dammed Weser River system in Lower Saxony alongside the Drenthe ice margin in western Germany (Van der Wateren, 1994b, 1995), and extensive tracts of ice-marginal valleys and lakes succeeding the last glacial maximum in Poland (Marks, 2002). Ice-marginal lake deposits are major components of the lithostratigraphy of many thrust moraines (Ruegg, 1981; Van der Wateren, 1995).

Boulton et al. (2001b), using a combination of satellite image analysis, existing geological data and numerical ice-sheet modelling, recognized patterns of flow-parallel lineations and ice-marginal features. These allowed them to make the following palaeo-glaciological inferences.

1. Shape and dynamic behaviour of the ice sheet show a strong spatial asymmetry.

2. Ice-sheet margins are commonly diachronous. Ice margins continued to advance in some areas while in others retreat set in.

3. As has been suggested previously (Ehlers, 1990), this asymmetry stems from a clockwise rotation of the main ice divide. According to Boulton et al. (2001b) this is the result of changing accumulation patterns across the ice sheet with relatively warm and moist conditions prevailing on the western flank, while cold and dry conditions dominate the areas to the east and on the leeside of the divide. Climatic warming leads to decay of the western part of the ice sheet while the eastern part may continue to grow.

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