Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 204

The origin of cirques involves at least two distinct processes: frost shattering and
glacial erosion. Today, most glaciologists support the nivation theory, which ascribes ini-
tial cirque development to freeze-thaw processes at the base of small snowfields. As
full glaciers develop, cirques become enlarged by plucking (Gordon 1977; Thorn 1979;
Thorn and Hall 1980; Kariya 2005).
The distribution, orientation, and elevation of cirques can reveal a great deal about
their development (Derbyshire and Evans 1976; Graf 1976; Federici and Spagnolo
2004). Cirques exist at lower elevations and are best developed on the windward side
of mountains, where precipitation is relatively heavy (Price 1981). As the snowline rises
toward the interior or toward more continental conditions, so do the elevations at which
cirques develop. Within this general pattern, however, cirques have preferred orienta-
tions. In the northern hemisphere, they are found primarily on slopes facing north and
northeast, while in the southern hemisphere they are found on south- and southeast-
facing slopes. This is largely in response to wind direction and shade. The prevailing
wind in middle latitudes is westerly, so exposed west-facing slopes are typically blown
free of snow which is then redeposited on east-facing slopes (especially in continental
climates with dry, powdery snow). Shade is also important, since protection from the
direct rays of the sun allows the snow to persist in areas where it might otherwise melt
(Alford 1980). This is the case even in mountains with oceanic climates receiving heavy
amounts of snowfall. For example, the present distribution of glaciers in the Cascades is
largely restricted to north-facing slopes, and cirque development follows the same pat-
tern.