EIO GEANDE AND SAN MIGUEL VALLEYS. 343 



a valley 1 to 2 miles ^^dde whicli extends for 12 miles and then suddenly 

 narrows to one-third of a mile. It is at this point that the terminal (or 

 retreatal) moraine near Durango is formed. North of this point a sheet of 

 water-rounded matter containing many bowlderets and bowlders extends 

 for many miles up Las Animas Valley. At the melting of the ice at this 

 point the moraine and overwash plain formed a barrier or dam across the 

 valley, and in the broad valle}^ to the north there gathered a shallow 

 lake. Into this temporary lake there came a broad sheet of sand and silt. 

 It is now practically drained b}^ the river cutting down through the dam at 

 Durango. 



The valley of Las Animas River for many miles in the mountains con- 

 tains a body of water-rounded glacial gravel, now deeply eroded, so that 

 in many places a little terrace here and there is all 'there is left of a deposit 

 once 30 to 70 feet deep. In other places, as at Silverton, this gravel plain 

 is still well developed. 



It is thus proved that a glacier 1,500 or more feet deep originated in 

 Las Animas Valley and flowed 70 or more miles southward. For many 

 miles it was a mile or more in breadth. It left rather scanty moraines for 

 a glacier of its size, but a very large amount of water-transported matter. 

 Its distal extremity reaches 37° 15' north latitude or less, an elevation 

 somewhat below 6,500 feet. 



Glaciers occupied the upper valleys of Los Pinos, San Juan, Navajo, 

 Chama, and other rivers of the western slopes of the San Juan Mountains, 

 but I have not explored them sufficiently for notice here. 



UPPER RIO GRANDE VALLEY. 



A very large glacier must have occupied the upper Rio Grande 

 Valley. A large number of basins and valleys open down into it from the 

 Continental Divide, all well glaciated. My explorations were near the 

 head waters, and do not permit description of the lower end of this large 

 glacier. 



VALLEY OF THE SAN MIGUEL RIVER. 



The main river flows in a l)0x canyon deeply eroded in sedimentary 

 rocks which are nearly horizontally bedded. Approaching the mountains, 

 we find the branches occupying valleys eroded down through sheets of 

 volcanic lavas and tuffs into sedimentary beds, while in the higher cirques 



