48o POPULAR SCIENCE MONTHLY. 



TOPOGRAPHIC FEATURES DUE TO LANDSLIDES.* 



By ISRAEL C. EUSSELL. 



IT is well known that masses of rock sometimes break away from 

 steep slopes and descend as avalanches or landslides into adjacent 

 depressions. 



The topograph}' of a region where landslides occur is changed 

 in two ways — i. e., by the removal and by the accumulation of 

 material. The contours of a mountain, plateau, or other land form 

 that stands in relief, are altered by the removal of material; as, for 

 exam^^le, when a portion of the border of a plateau falls away, a 

 re-entering angle or curve is produced; or, when a rock avalanche 

 occurs on a mountain side, a high-grade gorge or depression may 

 result. The material composing a landslide comes to rest in ridges 

 and piles which have certain characteristic shapes. The most notice- 

 able feature in such instances is the backward slope of the surface of 

 the displaced material after it comes to rest. The surface of a land- 

 slide, whether composed mainly of a single block or of a heteroge- 

 neous mass of loose material, slopes toward the cliff from wliicli it 

 came. This backward slope tends to the formation of basins in which 

 water accumulates, and lakes and swamps result. The backward 

 slope referred to appears to be due to friction between the moving 

 mass and the rocks beneath, which retards the progress of the material 

 at the bottom and in front, so as to allow the material which comes 

 later and at a higher level to slide over it. In a heterogeneous mass 

 of fallen rocks there appear to be several planes of shear along which 

 differential motion has taken place. 



The changes produced by landslides are usually considered, even 

 by geologists, I believe, as of a local character and of minor impor- 

 tance in the topography of most regions. Recent studies by the 

 writer of the geography of the lava-covered region of Oregon, Wash- 

 ington, and Idaho, however, have shown that the phenomena referred 

 to furnish an explanation of the origin of a class of topographic forms 

 which occur not only on mountain sides and plateau margins, and 

 among the hills at the bases of such elevations, but in certain 

 instances in the minor features of broad and approximately level 

 plains. 



The landslides or avalanches which sometimes rush down the 

 sides of moimtains are frequently, and probably in most instances, 

 composed of loose rock and soil. The most frequent condition lead- 

 ing to such catastrophes is the saturation of the material with water. 

 Landslides usually follow heavy rains. The famous Willey land- 



* Published by permission of the Diieetoi' of the United States Geological Survey. 



