Page 278 - Fundamentals of Geomorphology

GLACIAL AND GLACIOFLUVIAL LANDSCAPES 261

Box 10.2

CIRQUES

Cirques usually start as depressions excavated by Victoria Land, Antarctic, which is 16 km wide and
streams, or as any hollow in which snow collects and 3 km high. Some cirques have a composite character.
accumulates (nivation hollow). Snow tends to accu- ManyBritishmountainshavecirques-within-cirques.
mulate on the leeside of mountains, so cirques in the In Coire Bà, one of the largest cirques in Britain, which
Northern Hemisphere tend to face north and east. lies on the east face of Black Mount, Scotland, sev-
In the steep terrain of alpine regions, it is usual for eral small cirques cut into the headwall of the main
cirques to show poor development and to slope out- cirque. Cirque staircases occur. In Snowdon, Wales,
wards. In less precipitous terrain, as in the English Cwm Llydaw is an over-deepened basin with a tarn and
Lake District, they often have rock basins, possibly sheer headwall. The headwall is breached partway up
with a moraine at the lip, that frequently hold lakes by Cwm Glaslyn, a smaller cirque, which also holds a
(tarns). Despite their variable form and size, the ratio tarn. And above Cwm Glaslyn lies an incipient cirque
of length to height (from the lip of a mature cirque just below the summit of Y Wyddfa. It is unclear if such
to the top of the headwall) is surprisingly constant, staircases represent the influence of different snowlines
and lies within the range 2.8 : 1 to 3.2 : 1 (Manley or the exploitation of several stream-cut hollows or
1959). The largest known cirque is Walcott Cirque, geological sites.

Plate 10.8 Roche moutonnée, known as Lambert Dome, Yosemite, California, USA.
(Photograph by Mike Hambrey)