264 HOWE & CROSS GLACIAL PHENOMENA, SAN JUAN MOUNTAINS 



chaotic masses of hugh blocks and finer and more completely shattered 

 material on the crests of ridges between the modern streams or on isolated 

 mesas. Just west of the Uncompahgre river are noteworthy occurrences, 

 some of the material extending northward nearly 4 miles from the present 

 base of the high mountains. Another striking instance was found at the 

 extreme northwest end of the range, where landslide debris covers the top 

 of a hill whose nearly plane, though inclined, surface if extended would 

 meet the near-by spur of the mountains at a point not far below its 

 present summit. These relations are shown in plate 27. The landslide 

 material consists of late volcanic rocks entirely. The mountains im- 

 mediately to the east are made up wholly of early andesitic breccia and 

 intrusive andesite, although still farther east remnants of the late vol- 

 canics are preserved on the highest summits. These conditions seem to 

 indicate that at the time the landslides occurred a much bolder topography 

 existed in the higher mountains than prevails at the same points today, 

 and that the younger and higher formations which crowned the summits 

 have been more or less completely removed, possibly in part through the 

 agency of the very landslides under discussion. 



The relatively great distance from its source at which some of this 

 ancient landslide material has been found suggests that the detritus on 

 West Baldy and Horsefly may have had a similar origin ; but the difference 

 in plrysical character between this detritus and that of the landslides, 

 the greater variety of the materials represented in the West Baldy and 

 Horsefly occurrences, and finally the much greater distance from its 

 source at which, in the case of Horsefly at least, the material is found, 

 would seem to be sufficient reason for rejecting such an hypothesis. 



The possibility of water or torrential transportation of the Horsefly 

 material also seems unlikely. The detritus lies near, if not actually on, 

 the divide which separates the drainage areas of the Uncompahgre and 

 San Miguel rivers, and the remnants of the old topography show that at 

 the time the detritus was deposited the streams and their tributaries 

 flowed in very wide open valleys in essentially the positions that they 

 occupy today. If the Horsefly and West Baldy material is to be regarded 

 as contemporaneous with the kame-Kke deposits observed to the northeast 

 in the direction of Montrose, it is difficult to conceive how blocks 10 or 

 more feet in diameter could be carried far out on the western divide at 

 least 10 miles from the mountain front. 



It is known that the bedded volcanic rocks of the region once extended 

 some distance out over what is now the plateau country, and that they 

 have since been removed by erosion, but the detritus covering West Baldy 

 and Horsefly can hardly be regarded as representing what Shaler has 



