252 HOWE & CROSS GLACIAL PHENOMENA, SAN JUAN MOUNTAINS 



Bighorn mountains, Wyoming. More recently, Capps and Leffingwell * 

 have drawn similar conclusions from studies in the Arkansas valley, in 

 Colorado. 



Evidence of the last stage of giaciation is obvious in the San Juan 

 mountains, and abundant proofs have been found throughout the region 

 examined that the ice disappeared in relatively recent times.f What may 

 be traces of an earlier stage have been observed at a number of localities, 

 but until lately satisfactory evidence in this connection has been lacking. 

 In the course of the regular field work of the U. S. Geological Survey in 

 the Ouray quadrangle on the northern side of the San Juan mountains, in 

 the summer of 1904, this evidence was found in the TJncompahgre valley, 

 and during the field season of 1905 further data were obtained in 

 adjacent regions. 



In presenting the results of these recent observations, the whole subject 

 of giaciation in the San Juan province must necessarily be considered. 

 The character and extent of the last stage of giaciation have been de- 

 scribed in several published reports of the Survey, including the Telluride, 

 La Plata, Silverton, Needle Mountains, and Rico folios, and a special 

 report on the geology of the Rico mountains, X to which reference should 

 be made for details omitted in this paper, the primary purpose of which 

 is to present the recently observed facts and to discuss their significance 

 in relation to the phenomena observed in other parts of the San Juan 

 mountains. 



The last Stage of Glaciation 



CHARACTER AND EXTENT 



The topography of the higher San Juan mountains everywhere shows 

 the influence of glacial erosion, yet the amount of this erosion was slight, 

 merely producing a modification of the earlier topography. Many of the 

 larger valleys are U-shaped. In the pre-Cambrian areas, especially in the 

 upper Animas valley, roches moutonnees are well preserved, and in the 

 higher mountains the majority of the streams rise in typical glacial 

 cirques. The glaciers which accomplished this modification were of the 

 valley type, supplied by the snow fields in the many cirques at their heads, 

 1 but no evidence has been found of an ice sheet or cap covering the whole 



* S. R. Capps and E. D. Leffingwell : Pleistocene geology of the Sawatch range, near 

 Leadville, Colorado. Journal of Geology, vol. xii, 1904, pp. 697-706. 



t R. C. Hills : Extinct glaciers of the San Juan mountains, Colorado. Proc. Colorado 

 Sci. Soc, vol. i, pp. 39-46. 



G. H. Stone : The Las Animas glaciers. Journal of Geology, vol. i. 1893, pp. 471-475. 



t Whitman Cross and A. C. Spencer: Geology of the Rico mountains. Colorado. 21st 

 Ann. Rept. U. S. Geol. Survey, part ii, 1900, pp. 7-165. 



