Mountain landscapes shaped by frost action, gravitational transport, and
stream erosion alone generally have rounded summits, smooth slopes, and
V-shaped valleys. The jagged ridges, sharply pointed peaks, and deep
U-shaped valleys of the Tetons show that glaciers have played an
important role in their sculpture. The small present-day glaciers still
cradled in shaded recesses among the higher peaks (fig. 6) are but
miniature replicas of great ice streams that occupied the region during
the Ice Age. Evidence both here and in other parts of the world confirms
that glaciers were once far more extensive than they are today.
Glaciers form wherever more snow accumulates during the winter than is
melted during the summer. Gradually the piles of snow solidify to form
ice, which begins to flow under its own weight. Rocks that have fallen
from the surrounding ridges or have been picked up from the underlying
bedrock are incorporated in the moving ice mass and carried along. The
ability of ice to transport huge volumes of rock is easily observed even
in the small present-day glaciers in the Tetons, all of which carry
abundant rock fragments both on and within the ice.
[Illustration: Figure 6. _The Teton Glacier on the north side of
Grand Teton, air oblique view west. Photo by A. S. Post, August 19,
1963._]
Recent measurements show that the ice in the present Teton Glacier (fig.
6) moves nearly 30 feet per year. The ancient glaciers, which were much
wider and deeper, may have moved as much as several hundred feet a year,
like some of the large glaciers in Alaska.
As the glacier moves down a valley, it scours the valley bottom and
walls. The efficiency of ice in this process is greatly increased by the
presence of rock fragments which act as abrasives. The valley bottom is
plowed, quarried, and swept clean of soil and loose rocks. Fragments of
many sizes and shapes are dragged along the bottom of the moving ice and
the hard ones scratch long parallel grooves in the underlying tough
bedrock (fig. 7). Such grooves (_glacial striae_) record the direction
of ice movement.
The effectiveness of glaciers in cutting a U-shaped valley is
particularly striking in Glacier Gulch and Cascade Canyon (figs. 2 and
8).
The rock-walled amphitheater at the head of a glaciated valley is called
a _cirque_ (a good example is at the upper edge of the Teton Glacier,
fig. 6). The steep cirque walls develop by frost action and by quarrying
and abrasive action of the glacier ice where it is near its maximum
thickness. Commonly the glacier scoops out a shallow basin in the floor
of the cirque. Amphitheater Lake, Lake Solitude, Holly Lake, and many of
the other small lakes high in the Teton Range are located in such
basins.
The sharp peaks and the jagged knife-edge ridges so characteristic of
the Tetons are divides left between cirques and valleys carved by the
ancient glaciers.