36 KINDLE 



limestone has a thickness of not less than fifteen hundred feet,' 

 and owing to inclination of its strata probably extends several times 

 that depth below the surface at Staunton. In this, as in most other 

 limestone regions, subterranean streams play an important role 

 in the drainage system. In the Carboniferous limestone region 

 of Kentucky the location of hundreds of sub-surface stream chan- 

 nels is marked by caves mouths and sink holes. "In three 

 counties in the vicinity of Mammoth Cave, over five hun- 

 dred caves are known to exist." 4 In considerable areas with 

 which the writer is familiar in the limestone region of southern 

 Indiana, more than 90 per cent of the rainfall is carried off 

 by underground streams. Such streams collect the surface 

 waters largely through the agency of irregular hopper-shaped 

 depressions called limestone sinks or sink holes. In the earliest 

 stage of the development of a sink hole the rainwater seeps through 

 a crevice or joint in the limestone to a lower level. Gradually the 

 passage way thus begun increases in size. At the same time the 

 corrosion of the limestone goes on at the surface rather more rapidly 

 in the immediate vicinity of the crevice than elsewhere. This 

 solvent action of the surface waters, either alone or in conjunction 

 with the breaking down of cavern roofs, in time develops the 

 inverted cone-shaped depressions so characteristic of most limestone 

 regions. The limestone sinks, though apt to be irregular in shape, 

 generally approach more or less closely a circular outline at the top. 

 In the Shenandoah valley these depressions generally attain a 

 depth of from 20 to 80 feet and a width of from 60 to 300 feet. 



It happens not infrequently that the outlets connecting the bot- 

 tom of the sink hole with the subterranean stream become choked 

 up and the sink becomes a pond or small lake. Nearly all ponds in 

 limestone regions have had this origin. Such a pond, if the outlet 

 at the bottom remains closed for a sufficiently long period, eventu- 

 ally becomes silted up or filled with marl and is transformed into a 

 marsh, and sometimes even into dry land, through the complete 

 filling of the depression and the reestablishment of surface drainage. 

 Such a cycle is liable to be interrupted at any time through adven- 



3 Folio U. S. Geol. Surv., No. 14. 



4 Caves and cave formations of the Mitchell limestone; F. C. Greene, Proc. Ind. 

 Acad. Sci., 1908, p. 176. 



