156 FLORIDA GEOLOGICAL SURVEY SIXTH ANNUAL REPORT. 



low, following a prolonged drought, the support is withdrawn, and 

 the already partially disintegrated strata break and subside into such 

 cavities as have been formed by solution in the limestone. 



Where the strata lying above the limestones reach a consider- 

 able thickness their resistance to breakage is of itself sufficient to 

 prevent the formation of sinks. No specified thickness of strata can 

 ibe given through which sinks will not break, since this depends both 

 upon the character of the strata and the size of the cavities in the 

 underlying limestone. In the plateau section of Florida sinks have 

 broken through as much as 100 feet of materials, a large part of 

 which, however, are themselves more or less calcareous and phos- 

 phatic, and hence may have been rem.oved in part by solution before 

 breaking. 



Fig- 39- — Sketch indicating the relation between ground water level, 

 surface level, sinks and lake basins in central Florida from near Gainesville, 

 passing through Payne's Prairie and Orange lake and following the Ockla- 

 waha river to the St. Johns river. The surface level is represented by the 

 line "a"; the artesian water 'level by "b"; the top surface of the principal 

 water bearing limestones, chiefly, if not entirely the Vicksburg formation, 

 ■by "c." The verticail scale of the drawing is i in. equals approximately 

 100 ft.; horizontal scale, i in. equals 5 miles. The materials lying above the 

 water-bearing limestones include, in the vicinity of Gainesville chiefly 

 the Alum Bi'.uff formation, while east of Orange Lake, later formation 

 overlie the Alum Bluff. All of these formations dip toward the east, 

 although in this sketch the dip appears exaggerated, owing to the differences 

 between the vertical and horizontal scales. 



The surface level on the plateau, on the edge of which the city oi 

 Gainesville stands, is about 180 feet above sea. Upon going down this es- 

 carpment to Payne's Prairie, the surface level falls to from 60 to 80 feet above 

 sea, this elevation being maintained with little variation while crossing the 

 group of basins which include Payne's Prairie, Orange and other lakes. Be- 

 yond Orange lake the slope is gradual to the St. Johns river, which is 

 within a few feet of the sea level. 



