Geomorphic implications

Temperature distributions such as that in Figure 6.12 have implications for glacial erosion and deposition and the origin of some glacial land-forms. Erosion rates are likely to be highest where basal melt rates are low, and particularly where meltwater is refreezing to the glacier sole. Thus, we might expect to find that erosion was most intense some distance from the divide. Conversely, the formation of lodgment till by subglacial melting should be most prevalent beneath the ablation zone. Both are consistent with observation.

The calculations shown in Figure 6.12 also suggest that zones of frozen bed, a couple of kilometers wide, should develop along ice sheet margins in regions where mean annual temperatures are sufficiently low. This, indeed, seems to have been the case in North Dakota and adjacent areas of Alberta and Saskatchewan. Here, blocks of bedrock, tens to hundreds of meters on a side, became frozen to the base of the glacier and were moved outward a kilometer or so (Figure 6.16). Detachment may have been facilitated by high pore-water pressures in the unfrozen rock beneath the frozen zone. Upon deposition, these blocks formed hills. When the ice eventually receded, the basins from which the blocks were plucked became lakes (Moran et al., 1980).

In some places, subglacial frozen bed conditions persisted throughout the last glacial period. The best studied such areas are in western Sweden, in the divide region of the Weichselian ice sheet. These cold zones were probably a consequence of a combination of high accumulation rates, cold temperatures, and thin ice in the topographically high divide region. Relict periglacial landforms like patterned ground (Kleman and Borgstrom, 1994) and weathering features such as tors (Kleman and Hattestrand, 1999) are found in these areas. These features h<-2-3 km-H

Former ice sheet

L uniiozen

Lake

Former ice sheet

Figure 6.16. Hill-lake pair formed by thrusting at a frozen margin.

developed under ice-free conditions during the last interstadial. Around the edges of these zones of relict landscape, frozen bed conditions persisted on higher ground while lower areas were at the pressure melting point. Kleman and Borgstrom (1994) have described a distinctive set of landforms in such situations (Figure 6.17). Up- and downglacier from the higher areas the ice was sliding, whereas it remained frozen to the substrate over the hill itself. This led to longitudinal compression on the stoss side of the hill, and longitudinal extension on the lee side. Till dragged by the ice thus became stacked in transverse moraines on the stoss side. On the lee side it was pulled away, forming an abrupt scarp (Figure 6.17). Along the lateral boundaries, there is a narrow transition separating the relict surface from an area of glacially modified topography.

Ribbed moraine is another distinctive landform that is commonly present around the edges of areas where frozen bed conditions either persisted throughout a glaciation or perhaps developed during deglaciation (Hattestrand and Kleman, 1999). Ribbed moraines are anastomosing, somewhat sinuous ridges oriented transverse to ice flow (Figure 6.18a). The ridges typically consist of glacial drift similar to that in adjacent areas without ridges. The drift is usually till but may be glaciofluvial sediment or a combination of the two. In troughs between ridges, seismic investigations and limited exposures suggest that the drift sheet is generally thin or missing (Figure 6.18c). If one could decouple the ridges from the substrate and slide them together, they would fit remarkably well (Figure 6.18b). These characteristics suggest that the ridges were formed by pull-apart of a once-continuous drift sheet at the boundary between zones of thawed and frozen bed (Figure 6.18d). Hattestrand and Kleman have argued convincingly that this is the case. They have shown that ribbed moraine is confined almost exclusively to the core areas of late Pleistocene ice sheets. They find that the ridges are transverse to ice flow directions during deglaciation and not to flow directions during the Late Glacial Maximum, suggesting that they formed during deglaciation.

As our understanding of the origin and distribution of features such as hill-hole pairs, relict surfaces, and ribbed moraine improves, they will become increasingly valuable in constraining numerical models

Stoss side moraines

Lateral sliding boundary

Stoss side moraines

Lateral sliding boundary

Transverse till scarp

Stoss side moraines

Ice flow

Patterned ground Frozen

Extending flow

Unfrozen

Compressive flow

• Transverse till scarp

Figure 6.17. Schematic map (a) and cross section (b) through a low hill that remained frozen throughout a glacial advance and on which periglacial landforms are thus preserved. Stoss side moraines were formed in the zone of compressive ice flow on the stoss side of the hill, and transverse till scarps were formed on the lee side.

of Pleistocene ice sheets. Some modeling studies have already used the distribution of these features for this purpose (see, for example, Fastook and Holmlund, 1994; Moran et al, 1980) but much remains to be done.

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