THE WORK OF GROUND WATER 



69 



Striking Effects of Ground Water 



Swallow Holes. — In limestone regions it is not unusual to find 

 many funnel-shaped depressions in the surface of the ground into 

 which water may flow. These are called " sink " or " swallow 

 holes " and may be very conspicuous features of the landscape 

 (Fig. 45). They are formed either (1) through direct solution 

 by surface waters along joints, in which case they are usually 

 more or less circular in outline ; or (2) by the falling in of the 



roof of a cavern, 



when they are often 



irregular in outline. 



Those formed in the 



first way are much 



more common than 



the latter, but are 



usually smaller. An 



example of sink holes 



formed by the falling 



in of a cavern roof 



occurred in the city 



of Staunton, Virginia, 



in 19 10, when four 



"cave-ins" occurred 



within three weeks, 



the largest of which 



was 60 by 90 feet. During the formation of this largest one three 



trees and portions of a dwelling house were engulfed. (3) In regions 



underlain by salt, local sinkings result from the solution and removal 



of the salt by underground water. 



After a swallow hole is formed, more or less of the material imme- 

 diately around the hole will be carried in by surface wash. More- 

 over, a large amount of water entering through the sink may cause a 

 rapid solution of the limestone in its immediate vicinity, resulting in 

 the formation of large basins locally called " prairies " or " coves." In 

 the United States these are well developed in Kentucky and Florida. 



If the bottoms of swallow holes become choked, small lakes or pools 

 come into existence. A striking example is shown in the history of 

 Alachua Lake, 1 Florida (Fig. 46). Previous to 1871 the waters of 



1 Florida Geol. Surv., Third Annual Report, 1910, pp. 62-67. 



Fig. 45. 



Small swallow or sink hol< 

 Switzerland. 



le Juras, 



