THE WORK OF GROUND WATER 



73 



Fig. 50. — Conditions favoring landslides. The 

 strata AC and BD are clay or shale which, when wet, 

 are slippery, so that sliding is likely to occur. 



such a surface is developed upon a nearly pure limestone. It is a 

 desolate region in which vegetation is scanty, except in swallow 

 holes (dolines), where the small amount of insoluble matter yielded 

 by the rock accumulates and furnishes a soil for plants. The drain- 

 age is, for the most part, subterranean; and the surface is etched 

 out into a network of narrow channels between which blade-like 

 masses of rock rise. It is pitted with swallow holes and, where 

 important streams cross the karst land, they flow in deep gorges, 

 rather than in ordinary valleys. 



Landslides. — Landslides may result from a number of conditions, 

 one of which is often associated with underground water. Soil and 

 subsoil tend to move 

 down a hillside when 

 they become charged 

 with water. If this 

 movement is insensible 

 it is called " creep " ; if 

 sensible, " slumping " 

 or "sliding." Railroad 

 tracks may be gradually moved down hill and trees be tilted by the 

 slow movement of hillside creep. 



The conditions favorable for a landslide are a steep slope upon 

 which soil rests, or steeply dipping rock which has been undercut 

 at the base, artificially or by streams, so that the upper layers are 

 unsupported (Fig. 50). When, under either of these conditions, the 



soil or rock becomes 

 saturated with water, 

 its weight is increased, 

 and, moreover, the 

 water, acting as a 

 lubricant, lessens the 

 friction which pre- 

 viously prevented 

 the soil or rock from 

 sliding. Such was 

 the cause of the Mt. 

 Greylock, Massachu- 

 setts, landslide, in which a great mass of soil and glacial debris slid 

 down the steep mountain side after a period of excessive rainfall; 

 and of the landslide in Quebec, where the rock hillside slipped 



Fig. 51. — Diagram showing a valley which has been 

 deepened by glacial erosion, leaving steep slopes unsup- 

 ported on each side. Fractures may develop at AB, and 

 a portion of the side may slide into the valley. 



