Geology Reference
In-Depth Information
Box 10.2
CIRQUES
Cirques usually start as depressions excavated by
streams, or as any hollow in which snow collects and
accumulates (
nivation hollow
). Snow tends to accu-
mulate on the leeside of mountains, so cirques in the
Northern Hemisphere tend to face north and east.
In the steep terrain of alpine regions, it is usual for
cirques to show poor development and to slope out-
wards. In less precipitous terrain, as in the English
Lake District, they often have rock basins, possibly
with a moraine at the
lip
, that frequently hold lakes
(
tarns
). Despite their variable form and size, the ratio
of length to height (from the lip of a mature cirque
to the top of the
headwall
) is surprisingly constant,
and lies within the range 2.8 : 1 to 3.2 : 1 (Manley
1959). The largest known cirque is Walcott Cirque,
Victoria Land, Antarctic, which is 16 km wide and
3 km high. Some cirques have a composite character.
Many British mountains have
cirques-within-cirques
.
In Coire Bà, one of the largest cirques in Britain, which
lies on the east face of Black Mount, Scotland, sev-
eral small cirques cut into the headwall of the main
cirque.
Cirque staircases
occur. In Snowdon, Wales,
Cwm Llydaw is an over-deepened basin with a tarn and
sheer headwall. The headwall is breached partway up
by Cwm Glaslyn, a smaller cirque, which also holds a
tarn. And above Cwm Glaslyn lies an incipient cirque
just below the summit of Y Wyddfa. It is unclear if such
staircases represent the influence of different snowlines
or the exploitation of several stream-cut hollows or
geological sites.
Plate 10.8
Roche moutonnée, known as Lambert Dome, Yosemite, California, USA.
(
Photograph by Mike Hambrey
)
