Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 265

of the rate of weathering and denudation. Movement within the talus may also be sub-
stantial because of rockfall, snow avalanches, mudflows, running water, creep, and the
removal of material from below by stream action. The average rate of surface movement
ranges from about 5 to 20 cm yr −1 in the Canadian Rockies, although measurements of
such features are circumstantial; individual rocks may move great distances, while oth-
ers remain motionless (Gardner 1973). Rates of talus shift ranging from zero, from 6 to
111 cm yr −1 in the Canadian Rockies (Gardner 1973), and up to 22 cm yr −1 in northern
Sweden (Rapp 1960) have been reported. There is also considerable regional variability
because of the local environment, slope orientation, gradient, and rock type.
In some areas, talus is now inactive. Evidence for this includes lack of new material
from above, dense concentration of lichens, weathering rinds on rock surfaces, infilling
of rock voids by fine material, and encroachment by vegetation. Inactive talus can ob-
served at lower elevations in some mountainous areas that were active in the past dur-
ing colder climatic conditions. Although talus formation is best developed in cold climat-
ic regimes, it may also form in other environments, so its interpretation as evidence of
a cold climate must be made with caution (Hack 1960).
Rock Glaciers
Rock glaciers are an important component of high mountain systems, often serving as
a visible indicator of mountain permafrost (Fig. 5.17; Barsch et al. 1979; Barsch 1996;
Haeberli et al. 1999, 2006; Haeberli 2000). They consist of unconsolidated but frozen,
ice-supersaturated debris that creeps or flows downslope at a rate of 1-100 cm yr −1
and exhibit a variety of forms, most typically tongue shaped and lobate. Their surface
topography is quite variable, but some can display a sequence of transverse and lon-
gitudinal furrows, as well as a steep front slope that rests near the angle of repose
(Wahrhaftig and Cox 1959; Benedict 1973; Haeberli 1985; Martin and Whalley 1987;
Vitek and Giardino 1987; Barsch 1996). Humans have used rock glaciers as a source for
construction material, a backdrop for residential areas, dam abutments, drill sites, shaft
and tunnel portals, and a water source for urban areas (Burger et al. 1999; Giardino
and Vick 1987).
A rock glacier's internal structure is thought to be a three-tiered system, with a top
layer of rock fragments covering a second ice-cemented or ice-cored interior that over-
lies rock deposited and overridden by the top layers (Humlum 2000). Rock glaciers'
periglacial or glacial origins are commonly debated, relating to whether or not rock gla-
ciers have an ice-cemented (periglacial) or ice-cored (glacial) internal structure. Sup-
porters of the periglacial model believe that cemented ice in the form of interstitial ice
(pore ice) or segregated ice (ice lenses) produces creep (Wahrhaftig and Cox 1959; Hae-
berli 1985; Barsch 1996). Glacially derived or ice-cored rock glaciers form when debris
from a rockfall covers a glacier, or when a glacier experiences excessive ablation during
a stagnant period, allowing moraine or rock debris to melt out and occupy the surface
(Wahrhaftig and Cox 1959; Outcalt and Benedict 1965; Potter 1972; Whalley 1974; Be-
nedict 1973; White 1976; Ackert 1998; Potter et al. 1998). Abundant evidence supports
each model, creating controversy (Whalley and Martin 1992; Clark et al. 1998). Some
researchers have accepted an intermediate viewpoint, advocating both models in cer-
tain circumstances. Rock glaciers are sometimes considered part of a landscape con-