Geology Reference
In-Depth Information
Cryoplanation terraces vary in form; they may be sickle-like, or elongate and relatively
narrow in shape. Their dimensions also vary; the smallest may be less than 50m in
maximum dimensions while others exceed 400-600 m in length and 150-200 m in width.
In central Siberia, some terraces are over 1 km in width and several kilometers in length,
and some bevel upland summits (Demek, 1969a, p. 42). The height of the scarp that limits
the upslope end of the terrace also varies. On gentle slopes the scarp height may be 1-2 m
but, in areas with greater overall terrace dimensions, the scarp height may exceed 10-20 m.
The angle of the terrace tread varies between 1° and 12°; usually the larger the tread and
the lower the inclination of the original slope, the smaller the gradient.
Lithology must play an important role in the development of these stepped profi les.
For example, on Ellef Ringnes Island, Canadian Arctic, D. A. St-Onge (1969) describes
the effects of snow and the different “nivation benches” that develop in gabbro, sandstone,
and shale bedrock. In areas of gabbro, the terraces are between 10 m and 15 m in width
and the risers, with slopes of between 25° and 35°, are between 2 m and 5 m in height. The
terraces form giant, near-horizontal steps and refl ect the predominantly coarse boulders
and relatively few fi nes which result from the disintegration of gabbro. Since the fi ner
particles are quickly removed by snowmelt percolating through the boulders, the terrace
is bounded by an apron of coarse angular debris. On adjacent sandstone the terraces are
more subdued in form because sandstone weathers to silt, sand, and sandstone aggregates,
all of which are relatively easily moved by wash. Thus, the terrace becomes an inclined
surface of 6-8°. Finally, in shale and siltstone, a variety of features develop, ranging from
large amphitheatre-like semi-circles to smaller hollows and ledges. These features refl ect
(i) the ease by which the soft shale is reduced to fi ne sand and silt, and (ii) the effective-
ness of wash in removing such material. A second example is provided by K. Hall (1997b),
who describes numerous small benches that occur on Alexander Island, in the humid
Antarctic Peninsula. They are interpreted in the context of near-horizontally-bedded and
extensively-jointed sedimentary rock that is being subject to dilation and thermal stress.
Cryoplanation is discussed further in the Chapter (pp. 244-246) and in Chapter 13
(pp. 332, 341).
9.3. MASS WASTING
Mass wasting is the term applied to the downslope movement of debris under the infl uence
of gravity. For convenience, mass-wasting processes in cold environments can be divided
into those that are slow and those that are fast. Slow mass-wasting processes are discussed
under the general heading of solifl uction, and then subdivided into frost creep and gelif-
luction. Rapid mass-wasting processes are discussed under the headings of active-layer
failures, debris fl ows and avalanches, and rockfalls.
While mass wasting is not unique to cold climates, mass-wasting processes are espe-
cially effective under periglacial conditions. There are several reasons. First, frost action
promotes rock disintegration and the resulting loose material is readily available for trans-
port by mass-wasting processes. Second, diurnal and short-term freezing of the ground
surface accelerates near-surface sediment movement. Third, the typically high moisture
content of the thawed active layer favors gravity-induced downslope movement. Fourth,
permafrost directly aids mass wasting by limiting the downward infi ltration of water into
the ground, thereby inducing high pore-water pressures in the near-surface. Fifth, the
permafrost table acts as a water-lubricated slip plane for movement of thawed material.
Finally, in many areas, the glacial legacy, consisting of over-steepened slopes and an
abundance of loose glacial debris, is conducive to the active development of slopes.
