258 HOWE & CROSS GLACIAL PHENOMENA, SAN JUAN MOUNTAINS 



the waters of the stream held in check by the ice-dam across the mouth 

 of the valley caused by the Uncompahgre glacier. 



VALLEY TRAIN 



Distinct gravel-covered terraces occur on both sides of Uncompahgre 

 river below the great moraine, and also in Cow creek for 7 miles above its 

 mouth. The surface of these terraces is about 50 feet above that of the 

 present flood-plains of the streams. The gravels are water-worn, strati- 

 fied, and of only moderate coarseness, individual boulders seldom reach- 

 ing one foot in diameter, and that only near the moraine ; their lithologic 

 character is appropriate to the drainages in which they occur, those of 

 Cow creek being largely of volcanic material, while the ones of the Un- 

 compahgre are of sedimentary and volcanic rocks mingled. The relation 

 of this low Uncompahgre terrace to the great moraine shows clearly that 

 the gravels represent outwash deposits or valley train laid down by 

 waters which issued from the foot of the glacier, and are essentially con- 

 temporaneous with the moraine itself. The gravels of the Cow Creek 

 terrace are of the same age and doubtless of similar origin, although their 

 relation to a terminal moraine is not evident. A part of the Cow Creek 

 terrace is shown in the foreground in plate 25, figure 1. 



Deposits of similar character and even more extensive occur in Animas 

 valley below the terminal moraines. The terraces covered by them are 

 shown in plate 31 in their relation to the moraines and to still higher 

 terraces, to be discussed later on. It will be noted that the view shows two 

 low terraces beyond the river which blend in some places. It is believed 

 that these levels are due to the outwash of gravel from the glacier, cor- 

 responding in the main to different positions of the ice-front, perhaps 

 those of the two moraines. 



About 10 miles south of Durango the gravels cover the broad valley of 

 the Animas from the bases of the high mesas east and west of it, shown in 

 plates 28 and 29, and are probably not less than 25 feet thick. The size of 

 the boulders near the terminal moraine is a conspicuous feature of the 

 valley train. Certain of them, representing the pre-Cambrian rocks of 

 the Needle mountains, are from 1 to 3 feet in diameter; but such large 

 blocks are not found at any great distance from the terminal moraine, 

 most of the gravels being well rounded and seldom more than 2 or 3 

 inches in diameter. This terrace level, like that of the Uncompahgre 

 valley, is about 50 feet above the present level of the rivers flood-plain. 

 Similar gravels have been found in the drainage of the San Miguel and 

 Dolores rivers and are abundant along the Mancos, La Plata, Florida, and 



