by glaciers.The process is most active near the head of a glacier, where the ice
deepens and flattens the gradient of the valley. Small hills or knobs in valleys
overridden by glaciers are rounded and smoothed by abrasion.
Glaciers flowing down mountain peaks gouged large pits called cirques,
from French meaning “circle.” They are semicircular basins or indentations
with steep walls high on a mountain slope at the head of a valley.The expan-
sion of adjacent cirques by glacial erosion creates arêtes,horns, and cols. An
arête, from French meaning “fish bone,” is a sharp crested, serrated, or knife-
edged ridge that separates the heads of abutting cirques. It also forms a divid-
ing ridge between two parallel valley glaciers. A col is a sharp-edged or
saddle-shaped pass in a mountain range formed by the headward erosion
where cirques meet or intercept each other. When three or more cirques
erode toward a common point, they form a triangular peak called a horn, such
as the famous Matterhorn in the Swiss Alps.
The glaciers extended far down the valleys, grinding rocks on the valley
floors as the ice advanced and receded. In effect, a river of solid ice embedded
with rocks moved along the valley floors, grinding them down like a giant file
as the glacier flowed back and forth over them.The advancing glaciers left par-
allel furrows called glacial striae on the valley floors as they sliced down
mountainsides. Miles from existing glaciers are large areas of polished andFigure 199Dome
Glacier cascading from the
Columbia Ice Fields,
showing the moraine-
covered tongue of the
Athabaska Glacier and
abandoned lateral and
recessional moraines,
Alberta, Canada.
(Photo by F. O. Jones,
courtesy USGS)
Historical Geology