68 



Types of Aquatic Environment 



V 



Solution Lakes and Ponds Of very different charac- 

 ter are the lakes whose basins are produced by the 

 dissolution of limestone strata and the descent of the 

 overlying soil in the form of a "sink." This is erosion, 

 not by mechanical means at first, but by solution. It 



occurs where beds of soluble strata 

 lie above the permanent ground 

 water level, and are themselves 

 overlaid by clay. Rain water 

 falling through the air gathers 

 carbon dioxide and becomes a 

 solvent of limestone. Percolat- 

 ing downward through the soil it 

 passes through the permeable 

 carbonate, dissolving it and 

 carrying its substance in solution 

 to lower levels, of ten flowing out 

 in springs. As the limestone is 

 thus removed the superincum- 

 bent soil falls in, forming a sink 

 hole. The widening of the hole, 

 by further solution and slides 

 results in the formation of the 

 pond or lake, possibly, at the 

 beginning, as a mere pool. 



The area of such a lake is doubtless gradually 

 increased by the settling of the bottom around the 

 sink as the soluble limestone below is slowly carried 

 away. Its configuration is in part determined by the 

 original topography of the land surface, and in part by 

 the course of the streamflow underground : but its bed 

 is unique among lake bottoms in that all its broad 

 shoals suddenly terminate in one or more deep funnel- 

 shaped outflow depressions. 



Lime sinks occur over considerable areas in the south 

 ern states, and in those of the Ohio Valley, but perhaps 



FIG. 17. Solution lakes of 

 Leon County, Florida, 

 (after Sellards). 



The white spots in the lakes indi- 

 cate sinks. 



A. Lake lamonia; area at high 

 water 10 sq. mi. 



B. Lake Jackson; area 7 sq. mi. 



C. Lake Lafayette; area 3K 

 sq. mi. 



D. Lake Miccosukee; area 1 % sq. 

 mi.; depth of north sink 28 ft. 

 Water escapes through this sink 

 at the estimated rate of 1000 

 gals, per minute. 



O. Ocklocknee River; S, St. 

 Mark's River; T, Tallahassee. 



