REGION OF THE OLD DRIFT 261 



country of moderate relief and the high rugged mountains is a sharp 

 one, several peaks of the border zone, having elevations in the neighbor- 

 hood of 14,000 feet, rising abruptly from 7,000 feet often without inter- 

 vening foothills. Most of the streams head in glacial cirques far above 

 timber line, drop into deep canyons which open out into U-shaped 

 valleys as they leave the higher mountains, and then, quite as suddenly, 

 enter broad valleys eroded in soft Cretaceous shales. 



The topography of the region in which the deposits of old drift occur 

 is characterized by the presence of broad valleys or drainage basins which 

 have been dissected during more than one period of erosion. Before dissec- 

 tion the region was one of very moderate relief, with low divides between 

 the streams, and it is upon these old surfaces, or those developed during 

 the intermediate periods of erosion, that the gravels and drift occur which 

 are believed to be older than the last stage of glaciation. Near the 

 Uncompahgre river the relation of the old surface to the flood-plains of 

 the present streams and to certain intermediate levels is well shown. 

 These features are brought out in plate 25, which, with the aid of the 

 accompanying descriptions, should give a clear idea of the topography. 



THE DEPOSITS OF EARLIER DRIFT 



West of the Uncompahgre river and north of Dallas creek is an elevated 

 region whose almost plane surface is inclined slightly to the northeast ; it 

 is the extreme southeastern extension of the Uncompahgre plateau, which, 

 as shown by the Hayden map, extends nortlrwest for nearly 70 miles. 

 About 10 miles due west from the junction of Dallas creek with the Un- 

 compahgre river (figure 2), Horsefty peak, a small hill, whose elevation 

 is about 10,000 feet, rises 600 feet from the surface of this plateau. The 

 peak itself is the culminating point of a line of low hills which extends in 

 a northerly direction from the divide at the head of Dallas creek to a little 

 beyond Horsefly, where the line turns somewhat to the west and the hills 

 rapidly decrease in elevation; they form the crest of the divide between 

 the Uncompahgre and San Miguel rivers. These hills are covered so 

 thickly by pebbles, boulders, and blocks of volcanic material, often 10 or 

 15 feet in diameter, that in many places the hills have the appearance of 

 being entirety composed of the detritus (see plate 26). The material was 

 derived almost entirely from known late volcanic flows and breccias of the 

 mountains and appears to have been once partly rounded or subangular, 

 but has been much modified in form through weathering. In addition to 

 the late volcanic rocks, there is a little granite and Algonkian quartzite, 

 probably derived from an early Tertiary conglomerate, which in age imme- 

 diately precedes the volcanic rocks and which is known to occur in the 



