Geology Reference
In-Depth Information
9
Hillslope Processes and
Slope Evolution
A variety of slope forms exist in periglacial environments and none is exclusively “per-
iglacial” in nature. Their morphologies often resemble those found in temperate, warm,
and hot, semi-arid regions. Slopes evolve under cold-climate conditions primarily by
mass-wasting processes. Flow includes permafrost creep, soil (frost) creep, gelifl uction,
slopewash, debris-fl ow, and slush-fl ow. Slip involves active-layer failures and ground-ice
slumping. Fall involves avalanche and rockfall. Superimposed upon these processes is
control exerted by the presence of frozen or thawing substrate.
Slope evolution in periglacial environments probably involves a progressive and
sequential reduction of relief. Limited evidence suggests this is accomplished by
replacement from below with Richter denudation slopes being ultimately replaced by
low-angle (cryo) pediments.
9.1. INTRODUCTION
Our understanding of slopes in cold-climate environments is limited. Although signifi cant
advances have been made in recent years, the link between slope form and process is still
unclear. Moreover, as argued in Chapter 2, many periglacial landscapes are in disequilib-
rium with current cold-climate conditions. There is the inevitable problem, therefore, of
distinguishing between the effects of past and present processes. There is no slope form,
or slope assemblage, that may be regarded as uniquely “periglacial” in nature. A general
statement, made more than 40 years ago, remains valid today, namely: “
the similarities of
form in diverse climatic regions and the differences of form in similar climatic environ-
ments emphasise the need not of classifi cation but of understanding the interrelation of
climate, lithology, and process
” (Leopold et al., 1964, p. 383).
In this chapter, the various slope assemblages that are commonly encountered in per-
iglacial environments are described. The infl uence of lithology is explicitly considered.
The various mass-wasting processes that operate on cold-climate slopes are described.
The peculiarities of thawing and frozen slopes are outlined. A fi nal section contains
speculation concerning the nature of periglacial slope evolution.
9.2. SLOPE MORPHOLOGY
A number of cold-climate slope-form assemblages, identifi ed largely on the basis of the
author's personal experience, occur in periglacial environments (Figure 9.1). These are
described below.
