Geology Reference
In-Depth Information
small joints and fractures that are inevitably present. Eventually, the shear strength of the
material is reduced below the level at which it is capable of countering the stresses imposed
by gravity.
Like the other rapid mass movement processes described earlier, rockfall is episodic
and varies in magnitude from year to year, as well as spatially across the free-face. It is
diffi cult to establish rates of debris production and rockwall retreat without extensive and
long-term observations. Unfortunately, our understanding of rock weathering under cold-
climate conditions is still incomplete (see Chapter 4). For example, a recent study attempted
to measure weathering and rockfall occurrence on a cliff consisting of sandstone and shale
near Longyearbyen, Svalbard (Prick, 2003). During 8 months of continuous monitoring,
traps were placed beneath the free-face to collect rockfall debris. At the same time, sensors,
inserted into bedrock to depths of 40 cm, measured rock temperatures and, at an adjacent
locality, tablets of sandstone and limestone, exposed to the atmosphere, were weighed at
regular intervals and non-destructive determinations of the Young's modulus of elasticity
were carried out (Prick, 1997). It was found that the appropriate combinations of cold
temperature and high rock moisture thought conducive to frost disintegration were largely
lacking and there was little evidence for thermal shock. Moreover, the elasticity measure-
ments suggested that the limited frost shattering that was observed was probably the result
of simple wedging along joints in the sandstone because no changes in elasticity were
observed in the sandstone control tablets. One is tempted to conclude that rockfall activity
is largely controlled by the inherited mechanical properties of the rock in question.
A number of studies have attempted to measure the rate of rockwall retreat under cold-
climate conditions (Table 9.6). Rates, which range from 0.003 to 2.50 mm/year, are all gross
approximations and should be treated with caution. For example, R. Souchez (1967b)
computed the volume of material lying dow nslope of a raised beach. T his involved assump -
tions concerning the shape of the talus, its thickness, and the particle size and porosity of
the debris. Thus, his volumes may be too large and retreat rates too high. On the other
hand, A. Rapp (1960a) estimated the volume of material in fresh rockfalls and then aver-
aged that amount over the whole area of the rockwall in question. This leads to an under-
estimate of retreat since rockfalls do not necessarily occur uniformly over the whole of the
free-face. Finally, the methods that have been used to arrive at mean recession rates (mm/
year) vary in nature, reliability, and precision. For example, M.-F. André (1993) derived
rates at Kongsfjord from debris arriving at the foot of snow avalanches, while those from
Wijdefjord and Ossian Sarsfjellet were computed using a standard lichen growth curve.
Bearing these considerations in mind, the available evidence suggests that rates of
rockwall retreat of between 0.3 and 0.6 mm/year are probably typical for most lithologies.
If correct, there is little support for the conventional view that weathering and slope retreat
is appreciably faster in periglacial environments than in other environments. When reces-
sion rates are compared to those for humid temperate and subtropical semi-arid environ-
ments (see Table 9.6), it is clear that retreat rates in periglacial environments are, in
general, at least one order of magnitude lower.
9.6. SLOPEWASH
Slopewash refers to a group of processes that include both surface wash (the downslope
transport of weathered material over the ground surface by running water) and subsurface
wash (the set of processes associated with water movement and sediment transport within
the regolith) (Lewkowicz, 1988b, p. 354). In non-permafrost regions, the primary instiga-
tor of slopewash activity is snowmelt, rather than rainfall. In permafrost regions, the
