52 UNIVERSITY OF COLORADO STUDIES 



two epochs of glacial extension have also been recognized.^ Though 

 quite likely most, if not all, of the Colorado glaciation yet recognized 

 was of Wisconsin age, as Cross and Howe, Siebenthal and Hole say, 

 the direct evidence is meager, and evidence of still earlier glaciation 

 may have been obliterated by more recent advances. In any event, 

 Hayden's statement'^ that the glacial period extended "back farther 

 into the past in the Rocky Mountains than geologists have accredited 

 it in other regions," and began in the Pliocene, seems unsupported 

 by evidence. The freshness of glacial phenomena, continued exist- 

 ence of glaciers and neve and other evidence show that the glacial 

 epoch extended down to the present time in Colorado and northward, 

 and that the retreat of the glaciers is still in progress. Though the 

 older glacial phenomena have been considerably affected by erosion 

 in places, especially by the removal of glacial debris and the cutting 

 through of moraines, the upper parts of the glaciated valleys have as 

 a rule been little affected by post-glacial erosion. 



Former Extent. — The area of Colorado is about 103,925 square 

 miles. It is difficult to say how much of it was once covered by glaciers, 

 as only an infinitesimal portion of the glaciated area has been accu- 

 rately mapped. Glaciers were confined to the crests of the higher 

 mountain ranges and the higher mountain valleys, seldom extending 

 up over the lateral divides. Along the crests of the ranges many of 

 the neve-fields united. Hence a map of the ancient glaciers and their 

 WCTe-fields would occupy a strip several miles wide along the greater 

 part of the crests of the several ranges whose peaks exceed 12,000 



'Emmons, S. F., "Abstract of Report on Geology and Mining Industry of Leadville, Lake County, 

 Colorado," Second Ann. Kept. U.S. Geol. Surv., for i88o-8i, p. 229, 1882; Geology and Mining Industry of 

 Leadville, Colorado, U.S. Geol. Surv., Mon., Vol. XII, p. 30, 1886; Emmons, S. F., and Irving, J. D., The 

 Downtown District 0/ Leadville, Colorado, U.S. Geol. Surv., Bull. No. 320, p. 13, 1907; Capps, S. R., and Leffing- 

 WELL, E. D. K., "Pleistocene Geology of the Sawatch Range, Near Leadville, Colorado," Journ. Geol., Vol. 

 XII, pp. 698-702, 1904; Westgate, Lewis C, "The Twin Lakes Glaciated Area, Colorado," Journ. Geol., 

 Vol. XIII, pp. 285-312, 1905; Cross, Whitman, and Howe, Ernest, "Geography and General Geology of 

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

 Howe, Ernest, and Cross, Whitman, "Glacial Phenomena in the San Juan Mountains, Colorado," Bull. 

 Geol. Soc. Amer., Vol. XVII, p. 267, 1906; Howe, Ernest, "Glacial Phenomena in the San Juan Mountains," 

 Science, N.S., Vol. XXIII, pp. 306-7, 1906; Siebenthal, C. E., "The San Luis Valley, Colorado," Science, 

 N.S., Vol. XXXI, p. 746, iQio; Capps, S. R., Pleistocene Geology of the Leadville Quadrangle, Colorado, U.S. 

 Geol. Surv. Bull. No. 386, 1909; HolE; Allen D., "Glacial Geology," Engineer Mountain Folio No. 171, 

 Geologic Atlas of the United States, U.S. Geol. Surv., p. 68, 1910. 



'Hayden, F. v., "Report"of F. V. Hayden, U.S. Geologist," Seventh Ann. Kept. U.S. Geol. and Geog. 

 Surv. Terr., for 1873, p. 53, 1874. 



