Temporal and spatial distribution of drainage system types

Although we have been treating drainage systems on hard beds and soft beds separately, most glaciers rest on bedrock in some areas and on loose material in others. The character of the loose material is also quite variable. Till beneath valley glaciers commonly contains substantial amounts of sand and gravel, while that beneath Antarctic ice streams and that overridden by the Laurentide Ice Sheet in the midwestern United States is much finer. This is probably largely a consequence of the nature of the bedrock supplying the debris: resistant mountain ranges on the one hand and large areas of weak sedimentary rock on the other. The granulometry of the till affects the angle of internal friction and thus its strength (Chapter 7). Also affecting till strength is the effective pressure, which is typically higher beneath valley glaciers and lower beneath ice sheets. These two factors combined make till deformation more likely beneath ice sheets. From the point of view of a valley glacier, coarse till that is not deforming may look very much like rough bedrock and the drainage system developed on it may thus have characteristics of a system on a hard bed.

Thus, the nature of the drainage system must vary from place to place beneath a glacier and from one glacier to another. Where basal melting occurs, the melting is distributed, so some parts of any subglacial drainage system must be distributed. Some systems may be entirely distributed, while others may have an arborescent component. On short time scales, both types of system are likely to be altered or disrupted by flow of the ice, though the linked cavity system, stabilized by the location of cavities on the lee sides of bedrock obstacles, is less subject to such change. On longer time scales, the drainage system at a particular place must vary as the glacier profile changes, the bed is eroded, and subglacial materials are rearranged. The challenge, therefore, is to determine not what type of conduit system is present beneath a particular glacier, but rather the spatial distribution of the different types of system.

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