Rock Avalanches 



Guv ElUOTT AllTCriHLI. 



"Beware the pint tree's withered Ijranch, 

 ISeware the awful avahuiche." 



BL'T the avalanche bringing duwn hundreds of 

 t(ins of packed snow, w'hich is feared li)' the 

 foresters and mountaineers of the West, is a mere 

 mimic phenomenon compared with the tremendous rock 

 a\alanches wdiich occur occasionally in \arious portions 

 of the North American continent. The snow avalanche 

 mav 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, 

 n-^any 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. Ivockslides of enormous magnitude have poured 

 down the mountain sides in Alaska and llrilish Culumbia, 

 hut in the \ery 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. Thev ma^• 



result from a breaking away of a rock mass — perh.aps an 

 er.tire 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 slo[)e of the mountain may have an underlying 

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

 in an exceptionally wet period will not stand the w^eight 

 of the overlving 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 destructi\e landslifle in the San Juan ]Moun- 

 tains was fortunatelv in an uninhal.iited area. It occurred 



ROCK STREAM IMOGEN BASIN 



'] lie crnml)liiiB nl tlie moiinl.iui |.r.,I . which resulte.l in this .Breat rod; flow, greatly reduced its hulk .iiul lowered its altitude liy jirnhahly several 

 hundred feet. This tongue .it •"tl.w" is tliree-Muarters of a mile long. It is a talus or "slide glacier" ;ind is hetueen the old ;tnd new 

 workings of the famous Camp liiid mine. 



low 



