Scribe - Regina

Subglacial Processes, Gary K.C. Clarke, 2005

Given the wide-ranging nature of this review paper, the discussion was somewhat fragmentary, so I apologize if this summary seems to be lacking in a coherent overall narrative.

Since several participants had expressed interest in self-organizing behaviors and spatial/temporal switches in their comments on the Google Groups post for the week, the discussion began there. As Ed Waddington pointed out, the classical view of a glacier is fairly continuous and involves no abrupt spatial or temporal switches; however, real glaciers and ice sheets are clearly affected by both. Some discussion ensued regarding the relationship between glacier flow and substrate: do fast-moving glaciers weaken the substrate, or does weak substrate lead to fast-moving glaciers, or both?

Building on the subject of till, Adam Campbell brought up the Deep Deformation puzzle that Clarke refers to in section 5.2.1, and the phenomenon of sliding movement deep below the surface of the till. There was general agreement that the classical expectation of till failure at or near the surface depended on having comparatively uniform till and an even water-pressure gradient, and neglected the possibility of weak layers, particularly at the point where till meets bedrock.

At this point we paused to define "fractally distributed till": till which shows a similar distribution of rock/grain sizes at every length scale. Bernard Hallet explained that such till is a natural result of the working of rock, and the length of time required to achieve it depended on the rate of strain.

The conversation turned here to bed topography, and its relative importance compared to surface topography. Bed topography may be only one-tenth as important as surface topography, according to a commonly used formula, but bed topography is frequently at least ten times more extreme than surface topography, meaning it cannot necessarily be neglected. Bed topography may also affect water at the bed, which in turn can cause surging if it builds up. The group concluded that cavities in the bed can form storage for subglacial water provided the sliding speed is sufficiently rapid, particularly in the lee side of a bed feature.