ABSTRACTS AND DISCUSSIONS OF PAPERS 39 



After the retreat and disappearance of the early Tertiary ice, stream erosion 

 continued, and the western portion of the San Juan Mountain area was reduced 

 to a surface of slight relief which may be thought of as a peneplain. After 

 the deposition of the Telluride conglomerate on this peneplain there was fur- 

 ther erosion in the range, and then came the three great epochs of volcanism — 

 the San Juan, the Silverton, and the Potosi. During these epochs of volcanism 

 a great volcanic plateau was developed. By this time the Miocene epoch had 

 been reached and possibly passed, and with the quieting down of volcanic 

 activity began the erosion and dissection of the volcanic plateau. During this 

 period of dissection another generation of San Juan Mountains was carved, 

 this time out of volcanic debris and great lava flows. 



The San Juan Mountains that were first carved out of this great volcanic 

 plateau should then be thought of as surmounting those of today. Perhaps, 

 if replaced, they would rise 3,000 or 4,000 feet above the present summits. 

 They rose above the present summit peneplain. 



With the redoming of the area, which involved the warping or doming of 

 the summit peneplain, another cycle of erosion was begun. Valleys were again 

 formed, and in these valleys snows collected which in time formed glaciers 

 that advanced to the lowlands bordering the range. These earliest Pleistocene 

 glaciers retreated and disappeared. The range continued to be uplifted, and 

 the streams were so rejuvenated that they cut great canyons below the broad 

 troughs occupied by the Cerro glaciers. Again, climatic changes favored the 

 formation of ice among the summits, and that ice (the Durango glaciers) 

 descended through the main canyons to the foothills and later retreated and 

 disappeared. The canyons were still more deeply cut into the mountain mass, 

 and then climatic conditions favorable for glaciation once more returned and 

 the Wisconsin or third series of Pleistocene glaciers formed and descended 

 through the great canyons, nearly as far as those of the Durango stage. These 

 glaciers have now disappeared, and there is no true glacier ice remaining in 

 the region today ; but the streams are vigorously dissecting the mountain mass 

 to still greater depths. The Black Canyon of the Gunnison has been largely 

 cut during and since Pleistocene time. The vigor of the stream work is illus- 

 trated in many a sharp V-shaped notch cut below the depth of ice-action. 

 High among the mountains are the remarkable landslides and great accumu- 

 lations of talus. 



The studies suggest somewhat continuous mountain growth in this region 

 during late geologic time. 



Presented in abstract extemporaneously by the senior author. 



DOMINANTLY FLUVIATILE ORIGIN, UNDER SEASONAL RAINFALL, OF THE 

 OLD RED SANDSTONE 



BY JOSEPH BARBELL 



(Abstract) 



The old red sandstones of the British Isles have been commonly inter- 

 preted as deposited in great lakes. Walther has, however, urged that these 

 formations are desert deposits; Goodchild regards them as laid down partly 

 in large inland lakes, partly as torrential deposits, partly as old desert sands. 

 IV — Bull. Geol. Soc. Am., Vol. 27, 1915 



