ROCK GLACIERS 545 



The interstices between the blocks are occupied by ice at a 

 short distance beneath the surface, and the freezing and rethawing 

 are in large part responsible for the present movement. 



The original accumulation of these rock glaciers was believed 

 to be due to a large extent to transportation of the rock masses by 

 true glaciers and their deposition along the path of the glacier, 

 rather than at the front in a moraine. Some modern ice glaciers 

 of this region pass downward into rock glaciers, and furnish a sug- 

 gestion of the mode of origin of those accumulations. Rock streams 

 thought to owe their origin in part to ice glaciers were described by 

 Howe (24) from the San Juan Mountains in Colorado, while others 

 observed in ,Veta Peak, Colorado, were found to be wholly inde- 

 pendent of ice work. In the San Juan Mountains these streams are 

 found in the valleys in the higher peaks, and they are tongue-like, 

 having a length ranging up to one kilometer and a width up to 600 

 meters, while the thickness ranges from 20 to 40 meters. The ma- 

 terial is a mixture of large and small angular blocks, the largest 

 having a diameter of 5 meters. A sharp demarcation exists between 

 the stone streams and the talus of the rock walls, from the foot of 

 which the stream starts. The rock stream of Veta Peak is remark- 

 able in that it consists of fragments of a whitish porphyry derived 

 from the summit of the mountain, and, although the rock stream 

 passes down between banks of red shale and sandstone for a dis- 

 tance of 1,700 feet or more, not one particle of any other kind of 

 rock than porphyry is to be seen. The porphyry fragments are of 

 approximately uniform size, fine material being for the most part 

 wanting. Only a few large fragments occur, the average size being 

 from I to 2 feet in diameter, only one over 6 feet having been noted. " 

 The length of the stream is something over a mile, while the drop 

 of the surface from the head to the foot of the stream in that 

 distance is about 2,200 feet. The width in the lower part of the 

 stream is 500 feet, but its two feeding branches are each about 1,000 

 feet wide above the junction. The depth is estimated at from 130 

 to 300 feet. 



Rock Slides and Falls. 



These are common in nearly all mountain regions. They may 

 occur in the surface soil and debris which cover the slope, and in the 

 decomposed rock masses of the mountainside, or they may occur in 

 fresh rock. In the former case, saturation by water is commonly 

 a preliminary occurrence, the slide occurring when the inertia and 

 friction of the rock masses are overcome. Such slides leave con- 



