Geology Reference
In-Depth Information
In periglacial environments, the free face usually stands at angles in excess of 40°.
The rock is subject to disintegration and there is cliff recession through rockfall. The
talus slope beneath is at the repose angle of the coarser blocks, usually between 30°
and 40°. The junction between the talus slope and the footslope (basal) complex may
be abrupt or it may be a smooth concave profi le. The upper part of the footslope
varies in angle between
25° and 5-10°; the lower end has inclinations of between 2°
and 5°. Typically, the footslope is characterized by a micro-relief of lobes and
terraces.
This slope assemblage is well developed in regions that have experienced recent degla-
ciation, such as Svalbard and northern Scandinavia (André, 1993; Jahn, 1960, 1976; Rapp,
1960a, b). This slope assemblage can be regarded as an inherited, glacial form that is being
progressively modifi ed.
The cold-climate processes operating on this type of slope assemblage fall into three
groups: (1) rockfall and debris-cone (apron) accumulations, the result of mechanical
weathering of the free face, (2) debris fl ows, the result of melting snow and summer rain
on the talus slope, and (3) slow mass wasting (solifl uction and slopewash) on the lower
slope, the result of near-surface soil saturation. In terms of rapidity of profi le change, the
most important process appears to be either debris-fl ow activity associated with unusually
heavy summer rain (Jahn, 1976) or snow avalanches (André, 1993; Rapp, 1960b). Exam-
ples of debris-fl ow activity on talus slopes within this slope-form assemblage are illustrated
in Figure 9.2.
The visual impression of great thickness of talus is misleading because, except for
cones or fans, the boulder cover mantles a bedrock surface. Moreover, talus profi les are
usually concave, and not rectilinear as often thought, with higher angles occurring towards
the top of the slope. Talus is also coarsely stratifi ed, the result of either frost-coated clasts
sliding over each other (Hétu, 1995), or from relatively dry debris fl ows, (van Steijn et
al., 1995). Talus movement itself may be locally initiated by the impact of falling rock, or
when individual rock particles expand upon heating during periods of strong solar
insolation.
9.2.2. Rectilinear Debris-Mantled Slopes
These slopes develop upon consolidated bedrock and occur in regions characterized by
extreme cold combined with extreme aridity. Typically, they are at the angle of repose,
varying from 25° to 35-38°, and are covered with a thin veneer of loose material (Figure
9.3A). They are a kind of Richter denudation slope (Young, 1972, p. 107) in which debris
supply and debris removal are in some form of equilibrium.
This slope form has been described in the periglacial context almost exclusively from
the ice-free areas of Antarctica (Augustinus and Selby, 1990; French and Guglielmin,
1999; Iwata, 1987; Selby, 1971b, 1974; Souchez, 1966, 1967a). M. J. Selby (1971b, 1974)
concluded that slope debris is produced by retreat of exposed bedrock subject to salt
weathering enhanced by frost action. The debris is then removed by wind action. The
balance between weathering supply and removal results in rectilinear slopes at repose
angles. An essentially similar explanation is proposed by Iwata (1987), who observed that
these slopes occur most frequently on north-facing sides of bedrock ridges that are subject
to more frequent freeze-thaw action. In addition, they are best developed in gneissic rocks
possessing intensive joint systems but less well developed in adjacent granitic rocks where
joint density is lower (see Figure 9.3B).
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