EXTINCT AND EXISTING GLACIERS OF COLORADO 45 



or so-called benches, been at all satisfactorily explained, as the result 

 of "sapping" at the bottom of crevasses and the bergschrund. John- 

 son suggested in 1899^ that the ice protects the bottom from changes 

 in temperature except where crevasses and the bergschrund reach the 

 bottom. So at the bottom of these openings disintegration of the rocks 

 by freezing and thawing would be going on rapidly, while elsewhere 

 this important geological agency would be inoperative. Thus, the 

 moderate change in grade which caused a crevasse would be developed 

 into a sub-glacial clijBf and the bergschrund would also form a cliff. 

 Surface water dropping into the openings and flowing away under 

 the ice greatly faciHtates the work by direct erosion, as well as by 

 carrying off the debris from the disintegration of the rock. These 

 cliffs slowly recede up stream, those formed by the crevasses making 

 the characteristic benches, and that formed by the bergschrund devel- 

 oping into a cirque. This idea is accepted by most geologists^ and 

 enlarged upon, though it is objected to by some,^ especially in Europe, 

 partly because of a supposition that the ice on both sides of crevasses 

 would prevent much change in temperature. In examining the 

 bergschrund of small glaciers in Colorado, I have been convinced, 

 however, that this does go on and is an immensely potent factor in 

 the sculpturing of our mountains. With this idea in mind, one can- 

 not view the crest of the Continental Divide in Colorado without 

 being impressed with the importance of this agency. 



Snow-Bank Talus Ridges/ — Ridges composed of angular rock, 

 simulating moraines in form, are common in the higher mountains, 



» Johnson, Willard D., "The Work of Glaciers in High Mountains," Science, N.S., Vol. IX, pp. 112-13, 

 iSgg. 



' Matthes, Francois E., "Glacial Sculpture of the Big Horn Mountains, Wyoming," Twenty-first Ann. 

 Rept. U.S. Geol. Sum., for 1899-1000, Part II, pp. 173-90, 1900; Daly, Reginald A., "The Accordance of 

 Summit Levels among Alpine Mountains: The Fact and Its Significance," Journ. Geol., Vol. XIII, pp. 118-19, 

 190s; Gilbert, G. K., "Systematic Asymmetryjof Crest Lines in the High Sierra of California," /owrn. Geol., 

 Vol. XII, pp. 579-88, 1904; Cross, Whitsian, and Howe, Ernest, "Geography and General Geology of the 

 Quadrangle," Silverton Folio, No. 120, Geol. Atlas 0/ the United States, U.S. Geol. Surv., p. 24, 1905; Russell, 

 Israel C, "Glaciers of Mount Rainier," Eighteenth Ann. Rept. U.S. Geol. Surv., lor i8g6-Q7,FsLTtll, p. 382, 

 1898. 



3 Bonney, T. C, Presidential Address, British Association for the Advancement of Science, Science, N.S., 

 Vol. XXXII, pp. 323-3S, 1910. See also Davis, W. M., "Glaciation of the Sawatch Range, Coloia.do,'\BiUl. 

 Museum Compar. ZooL, Vol. XLIX, pp. 6-7, 1905. 



« Howe, Ernest, Land-slides in the San Juan Mountains, Colorado, U.S. Geol. Surv., Prof. Paper, No. 

 67, pp. 3S-36, igog. 



