Understanding glaciers and their role in the Earth system demands both an understanding of the way in which their properties are organized in the modern time plane, and how they change through time. The time dimension is important because of the long lag times (102-103yr) that may be required for dynamic changes to spread through the system and the possibility that there have been markedly different ice-sheet regimes through time (e.g. Clark, 1994).
In understanding the behaviour of glaciers in time and space, particularly the ice sheets that are by far the largest and climatically most influential part of the global glacier mass, it is important to combine evidence from two sources:
1 from modern glaciers, where we can directly measure properties and processes, and determine the magnitudes of pressures, forces and flow rates, but with the limitations that observations of time dependence have been restricted to a period of serious scientific study of little more than 50yr, that vital processes at the bed are difficult to observe, except at the margin and through limited borehole tests, and that it is difficult to assess the representativeness of these latter observations;
2 from deglaciated terrain, where we can characterize the sedimentary and geomorphological character of former glacier beds, and can create a partial chronology of glacier variation, but with the limitations that we have to guess the processes, infer them from the sediments, or use analogues from modern glaciers to account for them. The strongly polarized debate between Shaw & Munro-Stasiuk on the one hand and Benn
& Evans on the other (this volume, Chapter 8), about the origin of drumlins, exemplifies the problem when speculation is relatively unconstrained by definitive evidence.
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