Environmental Engineering Reference
In-Depth Information
Figure 15.12
Late Pleistocene (Devensian) permafrost
landforms on British mountains.
Source: After Ballantyne and Harris (1994).
debris, together with more specialized processes associated with nivation and rock
glaciers. They are indicative of the generally slow and seasonal nature of movement in
the permafrost environment and the ability of interstitial ice formation to arrest mobile
materials and form cross-slope turf-banked and stone-banked lobes on solifluction
terraces. Ice melt over a frozen substrate is, however, a recipe for rapid movement and
summer debris flow is a more hazardous form of
geli
fluction.
Debris joins the supply of superficial deposits susceptible to the most widespread and
related permafrost processes of sorting, ice wedging and cryoturbation (Figure 15.13).
They do little more than rework or ornament the land surface but they also contribute
indirectly to permafrost mass wasting and denudation. They are most effective in
relatively thick superficial materials on flat and usually low-lying ground but also
develop on plateau detritus. Sorting occurs through the preferential frost heave of stones
towards the surface through a finer matrix at rates of 10-50 mm a
−1
. Small ice lenses
freeze first beneath stones with higher thermal conductivity than the surrounding loose
matrix, or grip the stones during downward migration of the freezing plane. Frost
push
or
pull
is reinforced as further ice crystal growth in the subjacent void maintains the upward
progress of the stone, which invariably rotates so that its longest axis is vertical. Sorting
is completed as gravity preferentially draws stones down the gentle slopes of frost-heaved
