Geology Reference
In-Depth Information
Figure 9.4.
Smooth convexo-concavo debris mantled slopes with bedrock outcrop, in Cretaceous-
age shale, Prince Patrick Island, NWT, Canada.
to J. Büdel (1960), the convexities and concavities refl ect solifl uction and slopewash proc-
esses while the steeper middle section is a backwearing debris slope subject to weathering
and gravitational processes. The maximum slope angle, which may range from 10° to
25-30°, is essentially one of repose and refl ects lithological factors. Thus, it can be regarded
as a modifi ed Richter slope. Second, many convexo-concavo slopes have an abrupt break
of slope at the junction between the main part of the slope and the lower concavity. This
phenomenon is similar to the “pediment junction” of hot arid regions. One explanation
is that snow banks remain longest in depressions and at the foot of lee slopes (e.g. see
Figure 2.4). Thus, in relation to the total slope profi le, there is an increase in slopewash
intensity at this location, which, in turn, leads to the progressive removal of fi ne material.
This idea, sometimes termed “nivation,” is discussed more fully later in this chapter.
Third, it is not uncommon for more resistant bedrock to outcrop and disrupt the smooth
convexo-concavo profi le. These bedrock outcrops, surrounded by frost-derived debris, are
commonly referred to as “tors.” They occur upon a wide range of lithologies, including
sandstone and dolerite (Derbyshire, 1972), gneiss (Dyke, 1976), shale (Figure 9.4), quartz-
ite, and dolomite (see Figure 9.5A).
In view of their supposed cyclic (see below) and climatic signifi cance (see Part III), it
is useful to distinguish between tors that occur on slopes and those that form summit ele-
vations. Usually, slope tors are surrounded by coarse debris and are structurally control-
led. They form through enhanced disintegration of adjacent rock and corresponding slope
retreat. However, other factors besides lithological variability may also be important. For
example, in Southern Victoria Land, Antarctica, tors are developed predominantly upon
west- and northwest-facing slopes (Derbyshire, 1972). This suggests they may also refl ect
relatively high solar radiation received by these slopes. However, differential mechanical
weathering may not be the only process responsible. On Ellef Ringnes Island, in the
Canadian High Arctic, tor-like features (“relief ruinifore”) have developed in sandstone
as the result of wind erosion (St-Onge, 1965).
9.2.4. Pediment-Like Slopes
Extensive low-angled surfaces are reported from many of the more continental and arid
periglacial environments. These include the unglaciated regions of central Siberia, interior
