Cirque
CIRQUE (Lat. circus, ring), a French word used in physical geography to denote a semicircular crater-like amphitheatre at the head of a valley, or in the side of a glaciated mountain. The valley cirque is characteristic of calcareous districts. In the Chiltern Hills especially, and generally along the chalk escarpments, a flat-bottomed valley with an intermittent stream winds into the hill and ends suddenly in a cirque. There is an excellent example at Ivinghoe, Buckinghamshire, where it appears as though an enormous flat-bottomed scoop had been driven into the hillside and dragged outwards to the plain. In all cases it is found that the valley floor consists of hard or impervious rock above which lies a permeable or soluble stratum of considerable thickness. In the case of the chalk hills the upper strata are very porous, and the descending water with atmospheric and humous acids in solution has great solvent power. During the winter this upper layer becomes saturated and some of the water drains away along joints in the escarpment. An underground stream is thus developed carrying away a great deal of material in solution, and in consequence the ground above slowly collapses over the stream, while the cirque at the head, where the stream issues, gradually works backward and may pass completely through the hills, leaving a gap of which another drainage system may take possession. In the limestone country of the Cotteswold Hills, many small intermittent tributary streams are headed by cirques, and some of the longer dry valleys have springs issuing from beneath their lower ends, the dry valleys being collapsed areas above underground streams not yet revealed. In this case the pervious limestone is underlain by beds of impervious clay. There are many of these in the Jura Mountains. The Cirque de St Sulpice is a fine example where the impervious bed is a marly clay.
The origin of the glacial cirque is entirely different and is said by W.D. Johnson (Journal of Geology, xii. No. 7, 1904) to be due to basal sapping and erosion under the bergschrund of the glacier. In this he is supported by G.K. Gilbert in the same journal, who produces some remarkable examples from the Sierra Nevada in California, where the mountain fragments have been left behind "like a sheet of dough upon a board after the biscuit tin has done its work"; so that above the head of the glaciers "the rock detail is rugged and splintered but its general effect is that of a great symmetrical arc." Descending one of the bergschrunds of Mt. Lyell to a depth of 150 ft., Johnson found a rock floor cumbered with ice and blocks of rock and the rock face a literally vertical cliff "much riven, its fracture planes outlining sharp angular masses in all stages of displacement and dislodgment." Judging from these facts, he interprets the deep valleys with cirques at their head in formerly glaciated regions where at the head there is a "reversed grade" of slope, as due to ice-erosion at valley-heads where scour is impossible at the sides of the mountain but strongest under the glacier head where the ice is deepest. The opponents of ice-erosion nevertheless recognize the very frequent occurrence of glacial cirques often containing small lakes such as that under Cader Idris in Wales, or at the head of Little Timber Creek, Montana, and numerous examples in Alpine districts.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
