Origin of cirques and overdeepenings

Cirques and overdeepened basins in glacier beds, such as those in Figure 8.33, are similar in form. Both have steep headwalls and both tend to have beds with adverse slopes. We will discuss the headwalls first.

Headwalls have ragged surfaces, apparently resulting from fracture and removal of blocks of rock. This morphology suggests that they are eroded by glacial quarrying. As we have just discussed, quarrying appears to be a result of water-pressure fluctuations on time scales of hours to days. These fluctuations seem to be most pronounced close to areas of water input (Hooke, 1991). In the case of cirques, the water input is localized by the bergschrund, and in the case of overdeepenings, by crevasses that form over the convexities at their heads

(Figure 8.33). Thus, these water inputs and resulting pressure fluctuations occur at precisely the points where erosion is necessary to maintain the headwalls.

In the case of the headwall of an overdeepening, a positive-feedback process appears to be operating. Crevassing over a minor convexity in the bed, an initial perturbation, localizes water input and hence erosion. The erosion is concentrated on the downglacier side of the convexity. Thus, as the erosion progresses, the convexity is amplified, resulting in further crevassing.

The other defining characteristic of cirques and overdeepenings is the gentle adverse slope of their beds. The steepness of this slope may be limited by the ability of water to flow along the bed. For example, if k = 0.309 and p = 916 kg m—3 in Equation (8.12), it is easy to show (by using Equation (8.5)) that m = 0 when dzlds = —1.7 dH/ds. (This is left as an exercise for the reader. Note that the constant of proportionality is lower if the water is partially or completely saturated with air and a higher value of k is thus used.) In other words, when the adverse bed slope, dzlds, is 1.7 times the surface slope, all of the energy dissipated in the flowing water is needed to warm the water to keep it at the pressure melting point as the ice thins in the downglacier direction, and none is available to melt more ice (see Equation (8.10) and discussion of Figure 8.11c). Where the adverse slope is steeper, Equation (8.12) predicts, mathematically, that m should become negative, so water should freeze in the conduit, and Equation (8.24) then predicts that Pw > Pi. Indeed, measured water pressures in the main overdeepening of Storglaciaren are quite close to the overburden pressure (Figure 8.33).

Actually, field data suggest that the water, rather than remaining in the conduit, spreads out in a maze of linked water pockets with flow velocities that are too low to move significant amounts of sediment (Figure 8.22). If the adverse slope is steep enough, frazil ice (platelets of ice that form in the flowing water) may form and further inhibit flow. Because the subglacial drainage system is thus disrupted in overdeepenings, water is forced to follow englacial conduits through them (Hooke and Pohjola, 1994).

Where subglacial streams are thus not available to flush out the products of erosion, a layer of till must accumulate. Substantial amounts of this sediment can be entrained if frazil ice forms and is eventually incorporated into the glacier sole (Lawson et al., 1998). Continuity considerations suggest that the till layer will increase in thickness until the downglacier mass transfer by deformation within it and entrainment by frazil ice at its surface equals sediment production by erosion. Such a sediment layer would protect the bed throughout the downglacier reaches of an overdeepening, thus concentrating erosion at its head. This is probably why overdeepenings exist, and why their longitudinal profiles are characteristically asymmetrical with the deepest point at their upglacier ends (Hooke, 1991).

0 0