Geology Reference
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(A)
(B)
Figure 9.2. Debris-fl ow activity on talus slopes. (A) At Longyearbyen, Svalbard, slopes are formed
in near-horizontal beds of Tertiary-age sandstone, siltstone, and shale. The free face (upper part of
the slope) is developed within massive sandstone while the talus slope (lower part) is within shale
and siltstone (Dallmann et al., 2001). (B) At Sleepy Mountain, northern Yukon Territory, Canada,
the free face is developed in Cretaceous-age sandstone and quartzite that overlies softer shale and
siltstone, covered by talus, which forms the lower concavity (See also Figure 2.5).
9.2.3. Convexo-Concavo Debris-Mantled Slopes
A third slope form is characterized by a relatively smooth profi le with no abrupt breaks
of slope. There is a continuous or near-continuous veneer of rock debris but no widely
developed free face or bedrock outcrop. Maximum slope angles range from 10° to as high
as 25-30°, depending upon lithology. In areas of variable rock resistance, bedrock out-
crops may be present in the upper sections and indistinct mounds of coarser, less weath-
ered debris may constitute the summits of rounded interfl uves (Figure 9.4). In form, the
slope profi les extend over the complete range of convexo-concavo forms, from dominantly
convex to dominantly concave.
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