. Annual report. Florida Geological Survey; Geology -- Florida. 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 low, following a prolonged drought, the support is withdrawn, and the already partially disintegrated strata break and subside into such cavities as have been formed by solution in the limestone.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 characte
. Annual report. Florida Geological Survey; Geology -- Florida. 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 low, following a prolonged drought, the support is withdrawn, and the already partially disintegrated strata break and subside into such cavities as have been formed by solution in the limestone.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 in part by solution before 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 "; 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
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