Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 202

submerged beneath glacial ice. When the landscape is submerged beneath a very thin
mantle of ice, it is actually somewhat protected, since temperatures under the ice re-
main below the freezing level and surfaces are not subject to intense frost shattering.
The rugged topography revealed in Figures 4.41b and c, however, is believed to develop
under a partial ice cover or in a situation where the higher areas are mantled by thin ice
frozen to its bed; in the valleys, the base of the thicker ice remains at the pressure melt-
ing point and flows rapidly, causing considerable erosion (Sugden and John 1976; Sug-
den et al. 2005). Frost and mass-wasting processes attack the exposed surfaces while
glaciers occupy the valleys and slope depressions to carve, deepen, and sculpt the topo-
graphy into a distinctive landscape (Cotton 1942, 1958; Flint 1971; Embleton and King
1975; Phillips et al. 2006). The dominant features of this type of landscape are cirques,
glacial troughs, horns, arêtes (sawtooth ridges), tarns, and paternoster lakes (strings of
lakes in rocky basins), and hanging valleys.
“Few landforms have caught the imagination of geomorphologists more than the gla-
cial cirque (corrie)” (Sugden and John 1976; Benn and Evans 2010). A cirque is a semi-
circular, bowl-like depression carved into the side of a mountain where a small glaci-
er has existed (see Figs. 4.30 and 4.42 for examples). Cirques are typically located at
the heads of valleys, but may develop anywhere along a mountain slope. They vary in
size from shallow basins a few meters in diameter to the huge excavations several kilo-
meters deep and wide found from Antarctica to the Himalaya. A well-developed cirque
usually contains a headwall, a basin, and a threshold. The headwall is the steep and
smoothed bedrock surface at the back of the cirque, extending concavely upward to the
ridge. The basin is a circular or elongated depression at the base of the headwall and
is frequently overdeepened as the result of glacial erosion. It rises to a threshold, a lip
or slightly elevated rampart at the outlet end of the basin. The threshold, composed of
either bedrock or depositional material, results from the decreasing glacial mass and
rate of movement at the periphery, so that the intensity of erosion is less and deposition
occurs. If the cirque was occupied by a valley glacier with ice flowing downward and
away into the valley, a threshold may not form. Thresholds are typical of cirques occu-
pied by small glaciers (either during their formation or afterward, when a large glacier
has shrunk). The presence of a threshold produces an enclosed basin where water col-
lects, often forming a tarn (lake) as described below.