LANDSLIDES AND ROCK AVALANCHES 



27£^ 



and the mining engineer look for gold 

 or other metalliferous deposits in certain 

 rock strata, and in ordinary mountam 

 formations these strata are fairly regu- 

 lar ; at least their positions can be deter- 

 mined. There may be rock faults, but 

 these the keen geologist can trace. How- 

 ever, it is evident that even men who are 

 supposed to see down a thousand feet 

 into the earth's crust must be perplexed 

 when the surface of a mountain slides 

 off. and two or three strata come tum- 

 bling down to pile up on the slopes and 

 the valleys to a depth of from ten to sev- 

 eral hundred feet. This chaotic condi- 

 tion of the rocks in a landslide area is 

 therefore the despair of the miner and 

 most tr}ing to even the experienced geol- 

 ogist. 



The failure to recognize the true sig- 

 nificance of the landslide phenomena and 

 to perceive their extent, have led to very 

 great loss of time, labor, and money in 

 prospecting of the Rico Mountains — a 

 portion of the San Juan — says Dr Whit- 

 man Cross, of the L'nited States Geo- 

 logical Survey. The reason that much 

 of the areas prospected have not been 

 recognized as landslide in character is 

 due to the fact that the great slides of 

 the San Juan region, such as that de- 

 scribed in the Himalayas, occurred long 

 ago, perhaps about the time of the Gla- 

 cial period, and many of the surface 

 traces have been obliterated to the casual 

 eye. 



Landslides are believed to be due gen- 

 erally to moisture, which, under favor- 

 able conditions, undermines foundations 

 and causes a breaking away of overlying 

 rocks. In the San Juan area the surface 

 rocks are volcanic and porous. These 

 are underlain by a likewise porous con- 

 glomerate which rests upon a sandy 

 shale. There is no drainage, and the 

 rains and snows sinking through the two 

 surface strata soften the shale and ren- 

 der it plastic. 



The earlier physical forms of the San 

 Juan Mountains were much bolder than 

 at present. High, narrow ridges must 

 have existed, but the slipping down of 

 billions of tons of their materials has not 



only lowered the ridges, but filled the 

 ravines, resulting in the present irregular 

 topography of low relief. 



Some of the landslide areas of Colo- 

 rado show that in the earlier days, quite 

 recent geologically, but probably scores 

 of centuries before the coming of man, 

 there must have been terrific times 

 among her mountains. There have been 

 thousands of slides, and some of them 

 of great magnitude. Possibly the great 

 saber-toothed tigers which ranged in the 

 plains below, and the giant sloths upon 

 which they preyed, along with other pre- 

 historic animals, heard the roar of the 

 descending rocks. But if so, man knows 

 it not, for the age of the older disturb- 

 ances can be but guessed. 



One of the greatest of the earlv con- 

 vulsions is known as the Silver i\Ioun- 

 tain Slide. It covers ten square miles, 

 and the amount of rock which crushed 

 down the mountain sides is beyond con- 

 jecture. On the northwestern slopes of 

 Red Mountain about six square miles 

 are covered with landslide debris, while 

 many other landslide areas cover from 

 one to five square miles. 



GLACIERS OF ROCK 



A singular feature of the San Juatj 

 region is the presence of rock streams, 

 veritable rivers of stone, which have 

 flowed down the mountain sides. When 

 seen from a distance they resemble gla- 

 ciers covered with debris. Ernest Howe* 

 describes these rock streams of which 

 Pierson Basin, in the Silverton quad- 

 rangle, is typical. 



"Nearly the whole floor of this large 

 basin." he says, "is covered by angular 

 rock debris to a depth of 50 to 100 feet. 

 The length of this rock stream is more 

 than three-fourths of a mile, while its 

 average breadth is about one-third of a 

 mile." with an estimated volume of ma- 

 teria of 13.000.000 cubic yards. "In 

 viewing this enormous mass of debris 

 from a distance, one is at once impressed 

 by its very peculiar form, which is like 

 that of a great tongue of some viscous 



* Professional Paper No. d-j, U. S. Geological 

 Survey. - - 



