370 STEPHEN R. CAPPS, JR. 



posed of angular fragments mingled with a matrix of mud, which 

 moves as a viscous fluid. The material of the rock glaciers is for the 

 most part coarse and angular, and instead of a semi-liquid filling of 

 mud, the interstitial openings are filled with solid ice, except in the 

 surface portions, where there is no filling at all. 



There is also an opportunity for interesting comparisons with the 

 rock slides of the San Juan Mountains of Colorado, so well described 

 by Cross and Howe^ in the Silverton Folio and by Howe^ in a recent 

 publication. At first glance there seems to be a great similarity 

 between the rock streams of the Colorado Mountains and the rock 

 glaciers of the area under discussion. Both are composed of angular 

 talus from high mountains, and show some striking similarities 

 in appearance and in surface configuration. I am convinced, how- 

 ever, that the rock glaciers of the Nizina region are not formed in the 

 way in which Mr. Howe^ explains his rock streams, by a flow down 

 the slopes "with a sudden violent rush that ended as quickly as it 

 started." No opportunity has so far been had to make a series of 

 observations extending over a considerable period of time to prove 

 conclusively that these rock glaciers are in motion, or to determine 

 the rate of movement. There are a number of facts, however, which 

 seem to lead inevitably to this conclusion. 



■Jn the Silverton Folio, published in 1903, Cross and Howe state: 

 "The larger rock streams, however, must owe their origin to glaciers; 

 no other agencies could transport such vast quantities of rock waste 

 so far from their sources." Later, Howe has published his opinion 

 that the rock streams of the San Juan Mountains are really landslides, 

 which occurred in a sudden violent rush of material. In this opinion 

 Cross now agrees with him. 



In his description of the great Elm landslide, Heim^ has pointed 

 out that sudden landslides may have a form remarkably similar to 

 that which is developed by slow movement, and it is well to keep this 



1 Whitman Cross and Ernest Howe, U.S. Geol. Sur. Folio, No. 120, Silverton. 



2 Ernest Howe, "Landslides in the San Juan Mts., Colo.," professional paper, 

 U.S. Geol. Sur., 67, 1909. 



3 Ihid., p. 54. 



4 Albert Heim, "Der Bergsturz von Elm," Zeitschr. Deulsch. Geol. Gesell., 1882, 

 98. 



