EOCENE GLACIATION IN SOUTHERN COLORADO 685 



The locality at which this most recent discovery of Eocene till 

 in Colorado was made is about 20 miles southeast from Pagosa 

 Springs and in the south-central portion of the Summitville quad- 

 rangle of the United States topographic atlas. 



The deposit is exposed in the valley walls of White Creek where 

 that stream is dissecting the surface of V Mountain. The best 

 exposures may be reached by trail from the Blanco Basin, following 

 the base of the bold mountain escarpment just east of V Mountain 

 to a large lake held in by recent landslides, and thence westward 

 half a mile to the junction of the two upper forks of White Creek. 



The ridge between the two upper forks of White Creek and that 

 west of the west fork are composed of this ancient till, but on their 

 surfaces there are fragments of the later Tertiary volcanics that 

 have fallen or been washed from the mountains to the east. 



The till is composed of stones ranging up to 5 feet in diameter 

 imbedded in a clay matrix. Many of the stones are distinctly 

 striated, and most of them are subangular and beautifully polished 

 and planated. The notable character of this till, however, is the 

 abundance of stones that have come from the pre- Cambrian forma- 

 tions, now nowhere exposed near this locality, and the many bowl- 

 ders known to have come from the Cutler or Dolores formations 

 of Permo-Triassic age which must also be buried in the core of the 

 range. Of equal significance is the absence of stones from the later 

 Tertiary volcanics. These two points make it clear that the ice 

 which deposited this till formed and accomplished its work during 

 the time when the pre-Cambrian core of the range and the upturned 

 Paleozoic and Mesozoic formations were exposed at the surface, 

 and before the later Tertiary lavas and tuffs were present. 



The stones in this till consist of granites, quartz, quartzites, 

 schists, gneisses, jaspers, red sandstones from the Cutler or Dolores 

 formations, and conglomerates from one or the other of those 

 formations. There are also many porphyries and some bowlders 

 of a tuff -breccia, just as there are in the type section of the Ridgway 

 till. These igneous and volcanic rocks were derived from an earlier 

 series of intrusives and eruptives and are quite distinct in age from 

 the later volcanics which constitute the mass of the present moun- 

 tains. 



