Geology Reference
In-Depth Information
On upland surfaces, frost action produces extensive areas of angular debris, forming
blockfi elds. With time, continued retreat of the frost-riven “riser” leaves only residual
bedrock outcrops at the summit. At this stage, so-called “maturity” is reached in which
the pre-existing landscape has disappeared and the landscape is covered with a mantle of
frost-derived material. In “old age,” solifl uction degrades and fl attens the summits while
adjacent valleys and lowlands are progressively “plugged” by the accumulation of solifl uc-
ted debris.
In terms of slope evolution, the Peltier periglacial cycle of erosion represents an intui-
tive synthesis since it emphasizes many of the typical slope forms described earlier in this
chapter. It is too general, however, to provide anything but an overall framework within
which to view slope evolution. For example, no quantitative parameters are given for the
landscape changes which are thought to occur, there is no realistic discussion of the
manner in which frost shattering and mass movement infl uence slope form, and there is
a lack of attention paid to other processes, particularly running water.
9.8.2. Slope Replacement and Richter Denudation Slopes
In the extremely cold and arid regions of Antarctica, other landscape models have been
proposed to explain the development of rock slopes. They envisage slope replacement and
the formation of Richter denudation slopes (see Figure 9.3).
One of the fi rst, developed by R. Souchez (1966) and based upon observations in the
Sor Rondane Mountains of East Antarctica, emphasized the role of plastic deformation
and shearing failure. Under conditions of plastic fl ow, regolith movement was assumed
proportional to the angle of slope, and ground loss was proportional to convex curvature.
The resulting model was one in which slopes decline over time but remain predominantly
convex.
A second model, proposed by M. J. Selby (1974; Augustinus and Selby, 1990), is based
upon observations in the Dry Valleys of Southern Victoria Land. On steep upper slopes,
stress-release joints and slab failures are a consequence of gravitational loading. This leads
to headwall retreat. On lower slopes, if the rate of weathering exceeds the rate of debris
removal, talus cones or aprons accumulate. As the rock slope retreats, the rate of debris
supply is progressively reduced since the size of the headwall progressively decreases.
Ultimately, a Richter denudation slope extends upslope to eventually consume the upper
slope.
9.8.3. Rapidity of Profi le Change
It is sometimes assumed that slopes evolve more rapidly under periglacial conditions
than under non-periglacial conditions (Tricart, 1963; see 1970, p. 112). This remains
unproven, since the few data that are available suggest that weathering and slope pro-
cesses do not operate at signifi cantly faster rates in periglacial, as opposed to non-
periglacial, environments. For example, the rate of rockwall retreat under periglacial
conditions appears to be less than in rainforest, humid temperate, and tropical semi-arid
conditions, and limestone solutional activity is no greater in arctic regions than in other
arid or semi-arid areas. Indeed, although the impression gained from recently-glaciated
regions is one of relatively dynamic slope evolution, if one considers the arid, cold, and
never-glaciated regions of the world, a more subdued model of landscape evolution seems
more appropriate.
