262 HOWE & CROSS— GLACIAL PHENOMENA, SAN JUAN MOUNTAINS 



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 depressions 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, 5% miles south of Horsefly 

 peak, a noteworthy feature of their occurrence being the abundance of late 

 volcanic rocks and intrusives and the small amount of the early breccias. 

 These intrusives are found in place about 8 miles to the south, and, 

 together with the early andesitic breccias, make up a large part of the 

 mass of Mears peak west of mount SnefMs. The late volcanic rocks, 

 belonging to the Potosi volcanic series of recent reports, now occur only as 

 thin remnants of flows capping the highest peaks and constitute but a 

 small part of the range of mountains forming the northwest buttress of 

 the San Juan. The abundance of such material in the West Baldy and 

 Horsefly detritus has much significance in considering the age and origin 

 of these deposits, subjects which are discussed in a later paragraph. 



Ten miles north of Horsefly peak and at an elevation of about 8,000 

 feet, a small kame-like hill rises, probably not more than 100 feet, above 

 the gently inclined surface of the Uncompahgre plateau and about mid- 

 way between the crest of the plateau, and the Uncompahgre river. This 

 hill is made up of rounded boulders and gravel, mainly of dark volcanic 

 rock, some of the larger boulders being several feet in diameter. The hill 

 is isolated and such igneous material is not commonly scattered over the 

 surrounding country. Other similar knolls occur between this point and 

 Montrose. Southeast of Montrose, along a road leading toward the end 

 of the Cimarron ridge, there are small remnants of boulder and gravel 

 deposits consisting mainly of volcanic debris. These, gravels form low 

 knolls, rising above the graded surface of the Cretaceous shales, and are 

 conspicuous, since they are of different composition from the hills of 

 shale which may also be noted here and there. The largest boulders of 

 volcanic material occurring in the knolls are probably not more than 2 

 feet in diameter. 



The slopes extending from Cimarron ridge to Cow creek, whose course 

 is nearly parallel to the trend of the ridge, as shown in the sketch map. 

 figure 2, are more or less thickly covered with gravels and boulders. The 

 debris lies in disordered heaps, modified somewhat by erosion, but in 

 places retaining a form which suggests ice deposition. The material con- 

 sists of pebbles and boulders derived from the agglomerates of the ridge 



