402 WALLACE W. ATWOOD AND KIRTLEY F. MATHER 
farther west. The probable form of the San Juan glacier which 
advanced down the Rio Grande Valley is indicated on Fig. 3. 
At the northwest margin of the range the San Juan glacial drift 
is associated with the valley of the Uncompahgre River which has 
its source in the mountains south from Ouray. In its upper course 
the stream flows through a deep canyon cut between some of the 
highest mountain peaks of the range, but below Ouray the valley 
widens and above the broad open floor there are terrace remnants 
characteristic of the valleys in the plateau country bordering the 
mountains. 
Between Ridgway and Montrose the western slope of the valley 
is formed by the eastern escarpment of the Uncompahgre plateau. 
To the east, in the low country, the valley is bordered by low hills 
and mesas. ‘Ten miles due west from Ridgway the divide between 
the Uncompahgre and the San Miguel rivers is marked by a line of 
hills rising above the surrounding plateau surface. The highest of 
these is Horsefly Peak, a hill composed of Mancos shale held up 
above the surrounding mesa by a protective capping of a heavy 
bowlder deposit. As noted by Howe and Cross’ Horsefly Peak and 
the hills adjacent to it 
are covered so thickly by pebbles, bowlders, and blocks of volcanic material, 
often 10 or 15 feet in diameter, that in many places the hills have the appear- 
ance of being entirely composed of the detritus. 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 immediately precedes the volcanic 
rocks and which is known to occur in the mountains to the south and east. 
The mass of the hill beneath the gravel is composed of Cretaceous shales 
resting on the Dakota sandstone, which forms the capping formation of the 
plateau, and it is probable that many of the moraine-like hillocks and depres- 
sions resembling kettle-holes may be due to the uneven erosion of the shales, 
but in a few places it is possible that true morainic forms exist, although much 
modified by erosion and weathering. 
Similar deposits occur on West Baldy, 53 miles south of Horsefly 
Peak, as noted by Howe and Cross, as well as on South Baldy, a few 
miles to the southeast, and on several hills rising above the level of 
Hastings’ Mesa to the southwest, as shown in Fig. 4. At each of 
t Bull. Geol. Soc. Am., XVII (1905), 261-62. 
