THE WORK OF GROUND-WATER. 



231 



of the original cave. This is so common that regions of limestone 

 caves are often affected by frequent sinks formed in this way. 

 They are a conspicuous feature of the landscape in the cave region of 

 Kentucky, and are well known in many other Hmestone districts. 

 They are known as limestone sinks. (Fig. 206 and Fig. 2, PL XVII.) 



Under certain circumstances caves may give rise to striking fea- 

 tures of another sort. If for any reason the roof is destroyed at the 

 two ends of a cave, remaining intact over the middle, the latter part 

 constitutes a natural bridge. Natural bridges also originate in other 

 ways (pp. 151, 153). 



Fig. 209. — Landslide topography on Badger Mountain, Washington. The slumping 

 material in this case is basalt. 



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. The common 

 downward incHnation of trees growing in such situations, the result 

 of the more rapid creep of the surface as compared with the deeper 

 part of the soil, is both an expression of the movement and of its slow- 

 ness. Other factors besides ground-water are involved in creep (see 

 p. 112). 



When the movement is 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. 207). Great landsUdes 

 of this sort have been recorded, and some of them have done great 

 damage. Where a stream's banks are high, and of unindurated mate- 

 rial, such as clay, considerable masses sometimes slump from the bank 



