THE LAS ANIMAS GLACIER. 473 



of about 350 feet and a breadth of one -fourth mile or more. 

 Its extreme length was more than sixty miles, probably about 

 seventy miles. The average slope of the upper surface was 

 eighty-three feet or more per .mile. For fifteen miles its breadth 

 was one or more miles. 



From the terminal moraines near Durango, the valley of the 

 Las Animas is for several miles southward covered by a plain of 

 water -washed material, from coarse gravel up to bowlders three 

 to five feet in diameter. Some of these have glacial scratches, 

 though most have been so much rolled and polished as to pre- 

 serve no distinct scratches. The lower terraces at Durango are 

 of this character. They are typical of the overwash gravels found 

 in many of the Rocky mountain valleys. The subglacial streams 

 poured out their load of sediments in the valley in front of the 

 ice, where they were mixed with some material dropped directly 

 from the ice, and hence not rolled far enough to obliterate the 

 glacial scratches. More or less of this glacial gravel is found in 

 all the wider parts of this valley and its tributaries above Silverton 

 until we reach within five or ten miles from the heads of the val- 

 leys. During the retreat of the tributary glaciers they poured 

 out much less glacial gravel after they came to be ten miles or 

 less in length, and what there was is usually but little water-worn. 



Since the above was written further exploration reveals the 

 fact that a large glacier originated on the eastern slopes of the 

 La Plata mountains, and flowed southeastward down the valley 

 of Junction creek and joined the Animas glacier in the northern 

 part of Durango. Five hundred or more feet above the creek it 

 left a lateral moraine on the top of the narrow ridge which 

 borders the valley on the south. The moraine consists chiefly 

 of the eruptives and metamorphosed sediments found in the La 

 Platas, and but little of the local rocks. 



The drift terraces near Durango are found at different levels. 

 The lowest terrace is that above described, and consists of 

 glacial gravel mixed with matter that has been but little rolled. 

 The higher terraces have the appearance of ordinary valley 

 terraces as seen from the river, but in some cases do not extend 



