686 WALLACE W. ATWOOD 



The lithological character of this drift is distinctly different 

 from that of the three Pleistocene drift deposits which are so com- 

 monly found in the foothill regions bordering the San Juan Moun- 

 tains, and which are all present in this immediate district. The 

 Pleistocene glacial deposits are characterized by the stones of the 

 later Tertiary volcanics and usually contain very little that could 

 not have been derived from those volcanics. 



In one exposure near the junction of the two forks of White 

 Creek on V Mountain a pebble-clay till is exposed which resembles 

 the upper member of the Ridgway till at the type locality. This 

 pebble clay contains many stones less than one-quarter of an inch 

 in diameter and a few cobblestones and small bowlders. The best 

 striae were found on stones that were taken from this pebble-clay 

 phase of the till. 



Beneath this exposure of Eocene till is the Mancos shale, and 

 in this respect the conditions are identical with those at the north- 

 west base of the range. Upstream from the best exposures of the 

 till an andesitic rock cuts the Mancos shale and appears to be at 

 the base of the till for some little distance. On the slopes above 

 this deposit of till there are beautifully waterworn pebbles of pre- 

 Cambrian rocks similar to those that characterize the Eocene 

 glacial deposit. They appear to have come from the complete 

 disintegration of a conglomerate. Such a conglomerate overlies 

 the Ridgway till at almost all of the known localities. 



This section is somewhat less satisfactory than many of those 

 described in the first report on Eocene glaciation in the San Juan 

 Mountains, for it is not at present overlain by the later Tertiary 

 volcanics. The lithologic character of this deposit determines 

 its age. 



