Geology Reference
In-Depth Information
borders of the discontinuous and sporadic permafrost zones. In high mountains, the
instability of rock slopes and the movement of ice-rich debris (rock glaciers) will undoubt-
edly increase (Haeberli and Burn, 2002). Slope failures in permafrost terrain are a geo-
technical hazard for many of the construction activities described in Chapter 14.
9.8. COLD-CLIMATE SLOPE EVOLUTION
Periglacial slope evolution involves a progressive and sequential reduction of relief with
the passage of time. Limited evidence suggests that this takes place by slope replacement
from below, with the formation of Richter denudation slopes.
The Richter denudation slope is thought to represent a balance between debris supply
and debris removal. Where the rate of weathering of a free-face and the debris at its base
is less than or equal to the ability of transportational processes to remove weathered
debris, a denudation slope forms below the free-face. Because weathering rates are
assumed to be uniform over Richter slopes, they decline at a constant and inherited angle.
Thus, over time, Richter slopes are replaced by (cryo) pediments as relative relief is pro-
gressively reduced.
The major complication to this simple model is that few regions exist where periglacial
slope evolution has managed to run its full course. This has been discussed in Chapter 2.
Nevertheless, two general and descriptive models are thought useful in our understanding
of periglacial landscapes. These are outlined below.
9.8.1. Cryoplanation
The periglacial literature contains numerous references to cryoplanation as a process
promoting low-angled slopes and level bedrock surfaces (Demek, 1978, pp. 148-149;
Dylik, 1957; French, 1976a, pp. 155-165; Péwé, 1970; Washburn, 1979, pp. 237-243).
According to Demek (1969a), cryoplanation terraces best develop in continental semi-arid
periglacial environments. According to Reger and Péwé (1976), cryoplanation terraces
require permafrost for their formation.
Following descriptions of bedrock (“goletz”) terraces and apparent “mountain plana-
tion” in Siberia (Jorré, 1933), Russian geologists proposed a cyclic model for the formation
of these fl at bedrock surfaces (Boch and Krasnov, 1943) (Figure 9.15). This model was
subsequently promoted in the European periglacial literature as “cryoplanation” (Demek,
1969a, b; Czudek, 1990; Czudek and Demek, 1973; Richter et al., 1963). The surfaces were
thought to initiate below structural benches, or initial slope irregularities, all of which
favor snow accumulation in lee positions. Then, frost action beneath the snow bank leads
to steepening and retreat of the slope and the formation of a frost-riven “riser.” Ultimately,
this leads to the development of a summit fl at as the riser is eventually consumed by an
adjacent terrace. In the fi nal stages, downwearing becomes the dominant mode of
evolution.
Recent Russian texts rarely mention cryopediments or cryoplanation (for example, see
Yershov, 1990; Kudryavtsev, 1978; Popov et al., 1985; Romanovskii, 1980). The reality is
that features previously identifi ed as cryoplanation terraces are probably controlled pri-
marily by lithology. Although the signifi cance of “cryoplanation” is debatable, the exist-
ence of these bedrock benches is not in question. In addition to Siberia, they also occur
in Alaska and Central Yukon (Cairnes, 1912; Eakin, 1916). Most are certainly erosional
