GROUND-WATER 209 



Creep, slumps, and landslides. When the soil and subsoil on a 

 slope become charged with water, they tend to move downward. 

 When the movement is too slow to be sensible it is called creep; 

 when rapid enough to be sensible, the material is said to slump 

 or slide. This may happen when the slope on which water-charged 

 mantle-rock lies is steep (Fig. 171). Some landslides have done 

 great damage. Where a stream's banks are high, and of unin- 



Fig. 169. Stalactites and stalagmites in Marengo Cave, southern Indiana. 



(Hains.) 



durated material, such as clay, considerable masses sometimes 

 slump from the bank or bluff into the river, or settle away slowly 

 from their former positions. This is a common phenomenon along 

 streams which have cut valleys in drift, and along shores of drift 

 on which waves are encroaching. The same phenomenon is com- 

 mon on a larger scale on the slopes of steep mountains. 1 



In creep and in landslides gravity is the force involved, and the 

 ground-water only a condition which makes gravity effective. 



1 tfcwMU has emphasized this point in 20th Ann. U. S. Geol. Surv., Pt. 

 u, pp. 193-202, and Cross, 21st Ann. U.S. Geol. Surv., Pt. II, pp. 129-150. 



