The distribution of tills

A scrutiny of geological maps showing the distribution of till shows that the till cover in central areas of the both the North American and European ice sheets is less extensive and thinner than in marginal areas. This may reflect the lesser erodability of shield rocks compared with fringing softer rocks, or it may reflect an almost inevitable consequence of outward transport and progressive till accumulation. Figure 2.11 shows a model of deforming bed transport by an advancing ice sheet and the net consequence of this mode of transport and deposition through an idealized glacial cycle (cf. Boulton, 1996b). A similar pattern would be produced by a lodgement mechanism but not by melt-out, which is not a continuous and cumulative process. Nor would we expect, if melt-out had been the dominant process of till deposition in Europe, to find the almost ubiquitous streamlining of drift surfaces that reflect active ice movement over the till surface, and which is reflected in Fig. 2.13. It is concluded therefore that the dominant process of till deposition must be a cumulative process of deposition beneath actively moving ice, implying either lodgement or deformation.

The keel-grooving mechanism for the creation of megascale lineations presented by Clark et al. (2003b) has been suggested by Tulaczyk et al. (2001b) not only to be a means of grooving preexisting sediments, but also of transporting them. It is difficult to understand, however, how bedrock-created keels could be a major means of longitudinal transport rather than transverse transport due to keel grooving, unless longitudinal transport was really produced by subglacial shear deformation.

Acknowledgements

The assistance of and discussions with Magnus Hagdorn and Sergei Zatsepin are gratefully acknowledged.

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