342 GLACIAL GRAVELS OF MAHSTE. 



On tlie gentler slopes there is a body of drift that forms a complex problem. 

 Near the underlying rock almost all the stones show considerable attrition, 

 the original forms due to weathering and fracture having been somewhat 

 changed by a subsequent process of polishing. The coarser stones are 

 mixed with some fine matter, thus forming a mass somewhat resembling ihe 

 till of New England. Approaching the surface, we find an increasing pro- 

 portion of rain Avash and talus. Some of the worn stones are distinctly 

 glaciated. On steejj slopes where there has been much sliding and soil-cap 

 movement, the stones are subject to some wear, and thus the interpretation 

 of the sheets of drift on the wooded slopes of these mountains is difficult. 

 In many places there are bowlders in this drift that have plainly come from 

 a distance; hence, in part at least, it is of glacial origin. Naturally where 

 the snow covered the mountains almost to their summits there would be 

 much matter borne onward in the lower part of the ice. I leave it as an 

 open question how far the drift sheets of the gentler slopes of these moun- 

 tains were deposited subglacially and how far they are a lateral moraine, 

 left at the margin of the ice as it melted and sank to lower levels. 



The two forks of Mineral Creek come together at right angles. The 

 valley of the South Fork is the larger. At a time when the ice had 

 retreated from Las Animas Valley, also from the North Fork of Mineral 

 Creek, a glacier still continued to flow in the valley of the South Fork. It 

 flowed across the valley of the North Fork and abutted against Red Moun- 

 tain, where it deposited a terminal moraine 100 feet high near the railroad 

 from Silverton to Ironton. Above Silverton there are a number of terminal 

 (retreatal) moraines, more or less water-washed. All are small. 



About a mile north of the city of Durango a terminal moraine extends 

 across the valley of Las Animas River, here about one-third of a mile wide. 

 It forms a low ridge rising 10 to 30 feet above the sedimentary matter that 

 covers its flanks. It is thus proved that at one time the ice flowed to or 

 beyond Durango at an elevation of about 6,000 feet. I have not explored 

 the valley below that point sufficiently to know the extreme limit of the 

 ice. From Durango to Silverton it is 45 miles, and to the head of the Las 

 Animas 65 to 70 miles. Assuming that the ice surface was at the top of the 

 mesa near Durango and rose 1,500 feet above Silverton, we have an average 

 surface gradient of about 120 feet per mile. 



From where Las Animas River emerp-es from the mountains it flows in 



