70 PHYSICAL GEOLOGY 



the principal stream of this region emptied into a sink or swallow 

 hole in the Alachua prairie. By the choking of this outlet a lake was 

 formed which, at its greatest extent, was eight miles long and in one 

 place four miles wide and of sufficient depth to permit a number of 



Fig. 46. — Diagrammatic section from Devil's Mill Hopper (northwest of Gainesville) 

 to Alachua sink, Florida. The Devil's Mill Hopper is 115 feet deep but does not 

 quite reach the level of underground water, D—E. B is Alachua sink, whose bottom is 

 filled with water to or a little above the level of underground water. C is a small sink 

 above the water table, which does not contain water. If the opening at the bottom 

 of the Devil's Mill Hopper becomes clogged, the sink will fill up to the surface and 

 become one of the deep, small, circular lakes frequently found in the region. (Modified 

 after Sellards.) 



freight steamers to ply upon it. After existing for about twenty 

 years the underground passage from the swallow hole was opened 

 again, and the lake gradually disappeared. 



Caverns. — Caverns occur in limestone regions and are usually 

 connected with swallow holes by more or less distinct passages. 

 They have been formed, with few exceptions, by the solvent power 

 of the water which poured through the swallow holes and joints, or 

 seeped through the rocks from the surface, and, to some extent, 

 through abrasion by the sediment carried by the subterranean 

 streams. Since water circulates most rapidly along joints and bedding 

 planes, it is in such positions that most rapid solution takes place, and 

 it is here that caverns occur. In certain spots, owing to the presence 

 of numerous open joints or to the solubility of the rock, large domes 

 are formed. Solution is usually most effective in forested regions, 

 since the humus affords a large and constant supply of carbon dioxide, 

 without which water is but slightly solvent. Caves may, however, be 

 formed by carbonated waters ascending from below; an example of 

 which is the interesting and extensive Wind Cave in the Black Hills 

 of South Dakota, which was formed by hot water coming up from a 

 great depth and gradually enlarging the joints and fissures in its 

 ascent. 



In regions of thick limestone, caves at different levels, called 



