THE WORK OF GROUND WATER 



75 



odically until a gentle slope is formed. During a single year (191 1) 

 nearly 36 per cent, of the total material excavated had been brought 

 in by landslides. 



Landslides may dam streams, forming lakes (Fig. 53) or rapids. 

 Lake Oechenen, in the Kandersteg valley of Switzerland, and the Cas- 

 cades of the Columbia River were formed by landslides broken from 

 the high mountains a few centuries ago. The rounded hills and basins 



Fig. 54. — Landslide topography which has much the appearance of a moraine. 

 Kandersteg valley, Switzerland. 



sometimes produced by landslides are very similar in appearance to 

 those formed by glaciers (Fig. 54). Moreover, the heterogeneous clays 

 and angular bowlders of which they are composed resemble glacial 

 debris. The rocks of landslides, however, instead of being scratched, 

 as is true of glacial bowlders, often show impact marks, formed by the 

 striking of one rock against another in their violent descent down the 

 mountain side. 



Concretions 



Although concretions are usually of little geologic importance, 

 they occur so frequently in the rocks and sediments of the earth 

 and excite so much interest that they deserve some attention (Fig. 55). 



