399 



Fig. 3. Map sbfiv.ins- tlip formation of sinis: holes on tlie periphery of a valley. 



The accompanyiuy map (Fig. Ill) beautifully illustrates this point. 

 It Is based ou data from the Bloomington quadrangle of the United States 

 Topographic Survey. A deep gorge from the southeast cuts well into the 

 old peneplain, thus tapping the water table. The water on plain around the 

 periphery of this valley "sinlis" into the limestone and comes to the sur- 

 face near the bottom of the gorge. 



In the area under discussion, tlie Ohio river and its tributaries supply 

 the surface drainage. Although any sort of limestone may develop sink- 

 holes, the Mitchell is the sinkhole and cave-forming limestone par excel- 

 lence. Its qualities in relation to cave formation have been discussed by 

 Green ('08). He summarizes them as follows: 



