GROUND-WATER 



These limestone sinks (Figs. 26 and 23) as they are called, are con- 

 spicuous in the cave region of Kentucky, and are well known in 

 many other limestone districts. Some limestone sinks are made in 

 other ways. 



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. 27). Some 

 landslides have 

 done great damage. 

 Where a stream's 

 bank are high, and 

 of unindurated ma- 

 terial, such as clay, 

 considerable masses 

 sometimes slump 



Fig. 25. Stalactites and stalagmites in Marengo from the bank into 

 Cave, southern Indiana. (Hains.) the river, or settle 



away slowly from 



their former positions. The same thing takes place 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. 



ORE-DEPOSITS 



Many ore-deposits are but a special result of the chemical 

 work of ground-water, and are of interest because of their industrial 

 value. An ore is a rock that contains a metal that can be extracted 

 profitably, though the term is often extended to include unwork- 



1 Russell has emphasized this point in 2oth Ann. U. S. Geol. Surv., Pt. II, 

 pp. 193-202, and Cross. 2ist Ann. U. S. Geol. Surv., Pt. II, pp. 129-150. 



