272 HOWE & CROSS GLACIAL PHENOMENA, SAN JUAN MOUNTAINS 



as yet unknown and may prove to have been of a differential character 

 and marked by local warpings or movements in one locality which did 

 not occur at another. Any discussion of such matters based upon our 

 present knowledge is beyond the scope of the present paper. 



Summary. 



The events of the later stage of glaciation in the San Juan region are 

 recorded in a slight but characteristic modification of the topography, 

 and in an abundance of drift in the form of moraines and outwash gravels 

 which is oxidized but little and on which subsequent erosion has had 

 slight effect. Postglacial erosion has been insignificant in the higher 

 mountains, and it is believed that glacial conditions continued to exist 

 until comparatively recent times. 



The older detritus occurs farther from the mountains than the mora 

 recent drift and rests on the remnants of an old topography which was 

 deeply dissected before the time of the last stage of glaciation. The form 

 of the deposits suggests that they have undergone much modification by 

 erosion, and the materials composing them have been more or less decom- 

 posed by atmospheric agents. There is a marked contrast between the 

 appearance of this drift and that deposited by the last glacial ice. The 

 large size of individual boulders, the number of different rock types 

 represented in the material, and their distance from the source from 

 which they were derived suggest transportation and deposition by glaciers, 

 as the most plausible explanation of the origin of these deposits. 



Stratified deposits of water-worn gravels closely related to the older 

 drift in age and position extend far out from the mountains and are re- 

 garded as outwash deposits incident to the earlier glaciation. Between 

 these highest gravels and the Valley train of the last stage of glaciation 

 several intermediate gravel-covered terraces occur that are believed to 

 have been developed during the period of interglacial erosion which 

 accomplished the dissection of the old surface on which the early drift 

 was deposited. 



