Rock Avalanches 



Bv Guy Elliott Mitciikll 



"Beware the jjiiiL tree's withered branch, 

 Beware the awful avalanche." 



BUT the avalanche bringing down hundreds of 

 tons of packed snow, which is feared by the 

 foresters and mountaineers of the West, is a mere 

 miniic phenomenon compared with the tremendous rock 

 avalanches which occur occasionally in various portions 

 of the North American continent. The snow avalanche 

 may sweep a trail some scores of feet wide for a dis- 

 tance of a mile or more down the mountain side, shatter- 

 ing to kindling wood, it is true, every tree in its path ; rock 

 avalanches, however, have scalped entire mountain faces, 

 many feet deep and thousands of acres in area, removing 

 millions of tons of rock and soil, covering entire valleys 

 with the debris, damming streams and forming sizeable 

 lakes. Rockslides of enormous magnitude have poured 

 down the mountain sides in Alaska and British Columbia, 

 but in the very heart of the United States in the mag- 

 nificent San Juan Mountains of Colorado are probably 

 the most extensive American rock slide areas. 



Rock or land slides are of several sorts. They may 



result from a breaking away of a rock mass perhaps an 

 entire portion of a mountain of unstable equilibrium as 

 in the case of the great Frank rock slide later referred 

 to when the falling mass sometimes smashes to frag- 

 ments and flows down the slope with incredible swiftness, 

 or the slope of the mountain may have an underlying 

 stratum of sand, or slippery clay, or other material which 

 in an exceptionally wet period will not stand the weight 

 of the overlying mass, or the slide may be surficial 

 the removal of a few feet of mud. In any case the 

 destruction in the affected area is usually complete, while 

 in a rock slide of first magnitude objects may be buried 

 by a flow of broken rock to a depth of 100 or more feet. 

 And when one of these unstable areas gets ready to slide, 

 not all the engineering resources in the world could stop 

 it, nor does it take more than a few seconds to do its 

 work, leaving a sweep of waste of a hundred times 

 greater magnitude than the most terrible avalanche of 

 snow and ice. 



The last destructive landslide in the San Juan Moun- 

 tains was fortunately in an uninhabited area. It occurred 



KOCK STREAM IMOGEN BASIN 



The crumhling of live mountain peak, which rcsullid in this great rock flow, greatly reduced its bulk and lowered its altitude by probably several 

 hundred feet. This tonj(ue of "flow" is ihrci-(|\iartcrs of a mile long. It is a talus or "slide glacier" and is between the old and new 

 workings of the famous Camp Bird mine. 



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