Geology Reference
In-Depth Information
tion surfaces which truncate geologic boundaries. Some workers regard pediments as part
of the semi-arid landscape evolution model proposed by King (1953) and earlier authors
(e.g. Johnson, 1932). Others regard such surfaces as being essentially basal weathering
surfaces, or “etchplains” (Büdel, 1970). The cause of planation is not easy to understand
but most workers in hot semi-arid regions generally agree that pediments develop through
the action of rills, gullying, rainwash, and sheetfl ooding coupled with backwearing of steep
upper slopes (Leopold et al., 1964, pp. 496-498).
The traditional interpretation of pediments in cold-climates relies upon the assumed
activity of cryogenic processes (Czudek and Demek, 1970, 1973; Priesnitz, 1981) whereby
frost action extends the pediment upslope by a process of scarp retreat. However, in arid
regions, such as central Siberia, interior Yukon Territory, and central Alaska, it seems
prudent to downplay the role attributed to frost. A more reasonable assumption is that
cold-climate processes act on these slopes within the constraints imposed by a pre-existing
pediment morphology. One is tempted to believe that pediments are not typical cold-
climate slope forms but are inherited elements in the landscape.
9.2.5. Stepped Profi les
Closely related to pediment-like surfaces are large terraces or steps carved in bedrock.
They usually occupy higher elevations in the landscape (Figure 9.6; also see Figure 9.5A).
Terms used to describe these features include “goletz” terraces, altiplanation terraces,
nivation terraces, and equiplanation terraces. All are now generally referred to as cryo-
planation terraces. Like cryopediments, these slope profi les occur mainly in the unglaci-
ated regions of northern North America and northern Russia (Boch and Krasnov, 1943;
Czudek, 1990; Demek, 1969a, b; Lamirande et al., 1999; Lauriol and Godbout, 1988;
Lauriol et al., 1997; Reger and Péwé, 1976). Cryoplanation terraces have also been
described from South America (Grosso and Corte, 1991). In Antarctica, they have not
been explicitly recognized although the literature variously refers to “block terraces,” “fl at
erosion surfaces,” “pavements,” “step-like topography,” and “stepped slopes” (Hall,
1997b, p. 182; Matsuoka, 1995).
Figure 9.6.
Bedrock (cryoplanation) terrace developed upon argillite and chert of Pre-Cambrian
age, Buckland Hills, Barn Mountains, Northern Yukon Territory, Canada. The helicopter provides
scale.
