340 GLACIAL GEAVELS OF MAINE. 



cobbles, bowldei-ets, and bowlders up to 5 feet iu diameter begin 2 to 4 

 miles from the head of these streams and extend 15 or more miles down 

 the valleys. The size of the stones grows smaller as we go below the 

 principal terminal moraines. There is a large terrace in the valley of La 

 Plata River a little below where it emerges from the mountains, which is 

 composed in large part of very coarse matter and appears like a water- 

 washed terminal moraine. The bowlders were probably rounded by the 

 waves of a Tertiary lake. 



In the narrower part of the East Mancos Valley the plain of water- 

 rounded matter is 50 to 150 feet wide aud 3 to 8 feet deep. While the 

 stones of the moraines show unequal wear into subangular forms, and some 

 faces with little wear, these are polished quite equally on all sides and have 

 much rounder shapes. Some parts of the valley have been worked as 

 gold placers, and thus it has been revealed that underneath the gravel is 

 glaciated rock, hollowed out into numerous rather shallow potholes. The 

 miners affirm that most of the gold, which is quite coarse, is found in coarse 

 gravel near the bed rock, and not in the bottoms of the potholes and 

 hollows, but on level rock between them. This proves that the currents 

 were swiftest where the potholes and basins now are. The gravels are 

 most easily interpreted as due to swift subglacial streams, either beneath the 

 ice or in front of the ice as they rushed from the mouths of their tunnels. 

 The stream has eroded a portion of the original gravel deposit and 

 rearranged a portion as a new flood-plain. 



Summary. — The prfucipal valleys of La Plata Mountains were filled by 

 glaciers 600 or more feet deep. A very large proportion of the transported 

 matter was acted upon by the subglacial streams, with the result that for 

 many miles the valleys are strewn with frontal plains of glacial sediments, 

 though mixed probably with considerable stream wash. The moraines 

 are of less size than the ordinary for glaciers of such length. Hayden's 

 Atlas of Colorado shows no moraines among La Plata Mountains. My 

 exploration of the mountains was confined to two valleys. 



LAS ANIMAS VALLEY. 



Las Animas River rises in the heart of the San Juan Mountains and 

 flows southward into New Mexico, where it joins the San Juan River, 

 which it is the principal tributary. Its head waters occupy a radiating 



