260 HOWE & CROSS GLACIAL PHENOMENA, SAN J UAN MOUNTAINS 



tains in general, and that this, the last stage of glaciation, undoubtedly 

 extended from Pleistocene into Recent times. Emphasis is laid on this 

 point in order to bring out by contrast the greater age of the other 

 deposits which are to be described. 



Occurrence of Drift older than the last Stage of Glaciation 



general character and distribution of the drift material 



i As was mentioned in the introduction, deposits of supposed drift and 

 gravels have been observed at many localities on the outskirts of the San 

 Juan mountains which are evidently older than the deposits of the last 

 stage of glaciation. Much of this detritus consists of water-worn and 

 stratified gravel clearly deposited, by streams and at elevations consider- 

 ably higher than those at which the streams now flow. Traces of these 

 gravels have been found many miles from the mountains and far beyond 

 the observed limits of the deposits of the last glacial stage. Some of the 

 detritus may be morainal, but if so the form of the deposits has been so 

 modified that often they may be overlooked, while the materials constitut- 

 ing them appear to be less fresh than those of the last moraines. 



The first definite information bearing on the origin of this detritus and 

 its evident greater age than that of the last stage of glaciation was found 

 in Uncompahgre valley and the territory immediately adjoining it in the 

 summer of 1904. The following account, supplemented by observations 

 made by Cross in 1905, is given in some detail, since the facts observed in 

 this locality have a direct bearing upon the occurrence of drift elsewhere, 

 in regard to the origin of which information has hitherto been lacking. 



REGION IN WHICH THE OLD DRIFT OCCURS 



After leaving the high mountains, the Uncompahgre river, as shown 

 by the sketch maps, figures 1 and 2, follows a course slightly west of north" 

 to its junction with the Gunnison river. Once beyond the limits of the 

 volcanic rocks, the stream enters an open valley flanked on the east by 

 Cimarron ridge, or "Tongue mesa" of the Hayden map, and on the west 

 by the Uncompahgre plateau. The main stream, together with its upper 

 branches, drains one of the highest parts of the San Juan mountains, 

 representing an area of about 100 square miles, composed largely of 

 Tertiary volcanic rocks — tuffs, agglomerates, flows, and intrusive porphy- 

 ries—through which the river has cut and exposed a great section of 

 Algonkian and later sediments which underlie the volcanics. From the 

 sources of the different streams to the broad open valley of the Uncom- 

 pahgre the topography is extremely varied. The line between the plateau 



v 



