270 HOWE & CROSS GLACIAL PHENOMENA, SAN JUAN MOUNTAINS 



Although it has been impossible to trace these three principal levels 

 around the western side of the San Juan mountains, there can be little 

 doubt that the ones on the south are to be correlated with the similar ones 

 of the Uncompahgre river. The relation of the lowest terrace to the last 

 stage of glaciation and directly to the moraine north of Durango is clearly 

 shown, and also the relation of this terrace to the one next above, known 

 locally as the Florida mesa, is essentially the same as that existing between 

 the lowest and intermediate terraces of the Uncompahgre drainage. That 

 the remnants of a still higher terrace shown in the isolated hills and mesas 

 along the lower Animas river may correspond to the highest terrace of the 

 Uncompahgre seems not unlikely, but this level has not been recognized 

 in the immediate vicinity of the mountains, and no early drift correspond- 

 ing to that of the Uncompahgre region has been thus far observed on the 

 southern side of the San Juan either in the Animas drainage or that of the 

 other south-flowing streams, such as the Florida and La Plata, that have 

 been examined. On the whole, it seems most probable that the origin of 

 the terraces and of the gravels which cover them on the south side of the 

 San Juan mountains was analogous to or actually the same as that of the 

 terraces on the north. 



Comparison with similar Deposits elsewhere in the Kocky 

 Mountain Province 



At least two series of glacial formations have been recognized by Salis- 

 bury and Blackwelder * in the Bighorn mountains of Wyoming. The 

 second stage was apparently altogether similar to the later stage in the 

 San Juan mountains, but probably somewhat more drift was deposited, 

 or at least is now preserved. The same freshness of the materials is 

 notable in each region, and in each case but a small amount of erosion has 

 taken place since the disappearance of the ice. It is interesting to note 

 that five small glaciers still exist in the Bighorn mountains. The occur- 

 rence of older drift in that region is more noteworthy than in the San 

 Juan region, as it is more abundant, and the morainal characteristics of 

 glacial deposits often well shown. The deposit in Wyoming referred to 

 is very discontinuous, and the greater part, as in the San Juan region, is 

 restricted to hilltops. Evidence was also found, although conclusive proof 

 was lacking, that in Wyoming a still earlier period of glaciation occurred. 

 Gravels and large boulders 15 to 25 feet in diameter were observed on 

 isolated spots 8 or 10 miles away from their source, and their decomposed 

 condition was most noteworthy. On the whole, from the authors' descrip- 



* Op. cit. 



