472 THE JOURNAL OF GEOLOGY. 



About a mile above Durango, at the most distinct of the ter- 

 minal moraines thus far noted, the valley widens to about one 

 mile, and continues pretty broad for twelve miles or more north- 

 ward. The valley is here covered with rather fine sediment. 

 It is marked on Hayden's maps as alluvium, but the glacial char- 

 acter of the terraces near Durango is not recognized, though 

 deposits substantially the same, situated a few miles northwest 

 of Durango in the La Plata valley, are markedly morainal. 



The post-glacial history of the valley was as follows. The 

 terminal moraines near Durango formed a dam that held in a 

 lake. This lake was partially filled with sediments, and at the 

 same time the river was cutting down through the morainal bar- 

 rier. The outlet is now so low as to drain the lake, except there 

 are some low, marshy flats where the water stands only a short 

 distance below the surface of the ground. 



I have visited many of the tributary valleys of this river 

 above Silverton. Every cirque had its glacier that flowed down 

 into the larger valleys. The volcanic rocks of that region 

 weather readily, so that one seldom finds glacial scratches 

 except at recent excavations for roads and mines. It has there- 

 fore been a matter of considerable difficulty to determine the 

 depth of the glacier of the main valley. By degrees the esti- 

 mated depth increased until a few months ago, when I found 

 scratches well preserved on quartzite at a height estimated at 1,500 

 feet above the Las Animas river. This was near the Mabel mine, 

 about four miles southeast from Silverton, and not more than 

 500 to 8co feet below the top of the ridge which here borders 

 the valley on the east. The glaciated rock is situated on a long 

 gentle westward slope, while the scratches have a north and south 

 direction. Local glaciers would have flowed westward. These 

 scratches are therefore parallel with the movement in the main 

 Las Animas valley, under conditions where no local glacier could 

 have produced them. 



It thus appears that near Silverton (elevation of valley about 

 9000 feet) the Las Animas glacier was 1,500 or more feet deep, 

 while at Durango (elevation about 6000 feet) it had a thickness 



