Conclusions

Mapping glacitectonic styles is a powerful way to reconstruct former ice sheets and to identify the glaciological conditions existing at the time, if the following reservations are made. Not all thrust moraines are equally useful for a reconstruction of ice-sheet margins (Van der Wateren, 1995). If the subsurface geology is the main factor controlling the position of a line of thrust moraines, individual ridges do not have to be synchronous and the use of this line for an ice sheet

Figure 8.21 Profile across the Lamstedter Berge. Black: Clay and silt, Tmio = Miocene clay and silt, E = Elsterian (Lauenburg) clay, H = Holsteinian (interglacial) clay and silt; white: Saalian glacifluvial sand and gravel. Till of the Drenthe 1 stage (SDI, white triangles, grey shading) is incorporated in the nappe structures. The thrust moraine is covered by Drenthe 2 stage till (SD2, black triangles, stipple shading). Like the Hamburg-Blankenese thrust moraine the Lamstedter Berge are built of sub-horizontal nappes measuring several square-kilometres (style D). (After van Gijssel, 1987).

Figure 8.21 Profile across the Lamstedter Berge. Black: Clay and silt, Tmio = Miocene clay and silt, E = Elsterian (Lauenburg) clay, H = Holsteinian (interglacial) clay and silt; white: Saalian glacifluvial sand and gravel. Till of the Drenthe 1 stage (SDI, white triangles, grey shading) is incorporated in the nappe structures. The thrust moraine is covered by Drenthe 2 stage till (SD2, black triangles, stipple shading). Like the Hamburg-Blankenese thrust moraine the Lamstedter Berge are built of sub-horizontal nappes measuring several square-kilometres (style D). (After van Gijssel, 1987).

reconstruction is rather pointless. Stable margins are identified by the presence of stacked till sheets, outwash fans and dump end moraines, either undeformed or as main constituents of thrust moraines.

The combination of glacitectonic analysis with other methods, such as sedimentology, sediment petrology, fabric analysis and stratigraphy, offers great scope to improve regional reconstructions of ice movement. This method of kinetostratigraphy has been most successfully applied in Denmark (Berthelsen, 1978, 1979; Houmark-Nielsen and Berthelsen, 1981; Houmark-Nielsen, 1987, 1994).

Conditions promoting the formation of thrust moraines, as discussed in previous sections, are:

1. structure of the underlying Cenozoic sedimentary basins, particularly the depth of a detachment layer (clay, lignite)

2. palaeogeography, particularly the occurrence of ice-marginal valleys containing accumulations of coarse-grained sediments

3. glaciology — stagnation of the ice margin during (re)advance and an ice thickness sufficient to provide the potential energy to move thrust sheets (Van der Wateren, 1995).

Many of the largest European thrust moraines are horseshoe-shaped around lobate margins. Whether this is an indication of surging is debatable, but they clearly formed around the termini of ice streams. Map patterns of glacitectonic features suggest that most of these ice streams were rather long-lived and therefore not really surges.

Although permafrost has been commonly cited as a prerequisite for the formation of thrust moraines (e.g. Richter et al., 1951; Boulton and Caban, 1995; Boulton et al., 1999), this does not appear to be the case. Aber (1988) emphasized that thrust moraines and related structures may form under water as well as subaerially. Similar but larger structures, such as accretionary wedges, form in subduction zones below several thousands of metres of sea water (e.g. Byrne, 1994).

The observation that many thrust moraines contain subaquatic sediments from lakes in ice-dammed ice-marginal river systems is another argument against permafrost as a precondition for pushing. Although tempting, equalling the dimensions of thrust moraines with the thickness of the permafrost layer (Boulton et al., 1999) may lead to erroneous palaeoclimatic conclusions.

Apparently massive as well as laminated tills in the area glaciated by the Southern Scandinavian Ice Sheets are mainly produced by deformation of the unlithified sediments that fill the sedimentary basins. This can be demonstrated most readily where the tills can be correlated with relatively in situ sediments cropping out nearby.

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