Geol.-Vol. I.] TURNER— ORIGIN OF YOSEMITE VALLEY. 289 



general effect of glaciation is to reduce inequalities, especially when the whole 

 surface is covered by an ice sheet. When the ice is forced by topography 

 to take the form of tongues on the border of an ice-sheet, or when the glacier 

 is but a tongue, owing to its formation in a valley, the action may intensify 

 the topographic differences. 



5. Glacial Cirques. 



At the head of glaciated can3'^ons there are usually amphi- 

 theaters with steep walls. These have been called cirques. 



Where the rock is intersected by horizontal and vertical 

 joints or other structural lines, the deepest cirques are 

 formed, with nearly vertical walls and nearly horizontal 

 floors. Where the rock is relatively unjointed, as on the 

 slope east of Royal Arch Lake, which is an incipient cirque 

 bordered by local moraines, the floor of the cirque slopes 

 upward and the walls are seldom high. Such slopes with 

 massive rock are usually undergoing exfoliation. Incipient 

 cirques like those at Royal Arch Lake are very common 

 apd were mostly formed late in the glacial history of the 

 range, by residual or "hanging" glaciers. A much better 

 example of an incipient cirque than that at Royal Arch 

 Lake may be seen on the north slope of a rounded granitic 

 hill (altitude 9,202 feet), which lies seven miles a little west 

 of north from the summit of El Capitan. This slope is evi- 

 dently undergoing exfoliation, and the ice has eaten out a 

 shallow amphitheater with steep walls; but the general 

 inclination of the sloping floor of the amphitheater is evi- 

 dently determined chiefly by the exfoliation partings. 



The cirques of the Sierra Nevada are referred to at some 

 length by Russell in his monographic paper on Mono Val- 

 ley.^ As Professor Russell shows, a number of geologists 

 regard the cirque as a product of ice erosion. According 

 to Helland,'^ Lieutenant Lorange, a Norwegian Engineer, 

 "formed the opinion that the cirques and some fjord-val- 

 leys of Norway were formed by glaciers. Under the gla- 

 ciers in cirques, where a space intervened between the 

 bed of the cirque and the ice, he saw a great many stones, 



1 Quarternary History of Mono Valley, California. Kighth Annual Report, U. S. Geol. 

 Surv., Part I, 1S89, pp. 352-355- 

 - Quart. Journ. Geol. Soc. Loudon, Vol. XXXIII, 1877, pp. 142-176. 



