ROCK AVALANCHES 



1093 



Pholo by Whitman Cross, U. S. G. S. 



SLUMGULI-ION MUD FI^OVV 



At tlie head of SlumguUion Gulch, Colorado, 11,500 feet altitude, the underlying rocks, having become decomposed into mud, were unable 

 to hold the weight of the overlying rocks and the whole mass flowed down the gulch. The flow was six miles long, and dammed a fork of the 

 Gunnison River, creating a lake. The sparse tree growth shows that the flow occurred many years ago. 



such Himalayan rockslides have been recorded in the 

 last half century, while among the remote and unin- 

 habited regions of the great ranges numbers more are 

 of constant occurrence. The formations of the Colorado 

 landslide area point to many slides in the past even more 

 stupendous than these. Fortunately this wholesale 

 catastrophe era is ended for the mountains of the United 

 States although the recent Cimmaron slide shows that 

 all danger is not past, while the rock avalanches of the 

 Alps and Alberta indicate that man's mining operations 

 may precipitate such disasters. 



Apart from the study of landslides with reference to 

 the safety of human life, there is economic value in their 

 investigation as bearing upon man's search for the 

 precious metals. The geologist and the mining engineer 

 look for coal and for metalliferous deposits in certain 

 rock strata, and in undisturbed rock formations these 

 strata are fairly regular, at least their position can be 

 generally determined. There may be rock faults but 

 these the keen geologist can trace. However, it is evi- 

 dent that even a geological wizard or a John Hays Ham- 

 mond must be perplexed when the surface of a mountain 

 slides off and two or three strata come tumbling down 



and spread themselves over the slopes and valleys to 

 depths of from ten to one hundred feet. 



Doctor Cross, of the Geological Survey, says that the 

 faiku'e to recognize the true significance of the landslide 

 phenomena has led to very great loss of time, labor and 

 money in prospecting of the Rico Moimtains in Colorado. 

 The reason that much of the areas prospected have not 

 been recognized as landslide in character is because the 

 great slides of the San Juan region occurred long ago, 

 and many of the surface traces have been obliterated. 



Landslides are believed to be due generally to moisture 

 which undermines foundations. In the San Juan, the 

 upper strata are porous volcanic and conglomerate rock 

 and these rest upon a sandy shale. There is no drainage, 

 and consequently at times this unstable foundation be- 

 comes insufficient. The earlier physical formations of 

 the San Juan Mountains were much bolder than at 

 present. High, narrow ridges and peaks prevailed but 

 the sloughing of billions of tons of their materials has 

 greatly reduced the relief of the country. Many of the 

 rockslides of the San Juan region have taken the form 

 of veritable flows or rock streams. When seen from a 

 distance these resemble glaciers covered with debris. To 

 realize the terrific effect of recent landslides when asso- 



