A great variation in both daily and annual temperatures results in
minute amounts of contraction and expansion of rock particles. Repeated
changes in volume produce stress and strain. Although the rocks in the
Tetons are very dense, they eventually yield; a crack forms. Water which
seeps in along this surface of weakness freezes, either overnight or
during long cold spells, and expands, thereby prying a slab of rock away
from the mountain wall. Repeated _frost wedging_, as the process is
called, results eventually in tipping the slab so that it falls.
[Illustration: Figure 4. _Talus at the foot of the jagged
frost-riven peaks around Ice Floe Lake in the south fork of Cascade
Canyon. Photo by Philip Hyde._]
What happens to the rock slab? It may fall and roll several hundred or
thousand feet, depending on the steepness of the mountain surface.
Pieces are broken off as it encounters obstacles. All the fragments find
their way to a valley floor or slope, where they momentarily come to
rest. Thus, rock debris is moved significant and easily observed
distances by gravity.
None of this debris is stationary. If it is mixed with snow or saturated
with water, the whole mass may slowly flow in the same manner as a
glacier. These are called _rock glaciers_; some can be seen on the south
side of Granite Canyon and one, nearly a mile long, is in the valley
north of Eagles Rest Peak.
The countless snow avalanches that thunder down the mountain flanks
after heavy winter snowfalls play their part, too, in gravitational
transport. Loose rocks and debris are incorporated with the moving snow
and borne down the mountainsides to the talus piles below. Trees,
bushes, and soil are swept from the sites of the slides, leaving
conspicuous scars down the slopes and exposing new rock surfaces to the
attack of water and frost. Battered, broken, and uprooted trees along
many of the canyon trails bear silent witness to the awesome power of
snowslides.
These are some of the methods used by Nature in making debris and then,
by means of gravity, clearing it from the mountain slopes. There are
other ways, too. A weak layer of rock (usually one with a lot of clay in
it), parallel to and underlying a mountain slope, may occur between two
hard layers. An extended rainy spell may result in saturation of the
weak zone so that it is well lubricated; then an earthquake or perhaps
merely the weight of the overlying rock sends the now unstable mass
cascading down the slope to the valley below. The famous Lower Gros
Ventre Slide (fig. 5) was formed in this way on June 23, 1925.