399 
je [SRE ONES 
Fig. 38. Map showing the formation of sink holes on the periphery of a valley. 
The accompanying map (Fig. IIL) beautifully illustrates this point. 
It is based on data from the Bloomington quadrangle of the United States 
Topographic Survey. A deep gorge from the southeast cuts well into the 
oid peneplain, thus tapping the water table. The water on plain around the 
periphery of this valley ‘‘sinks” into the limestone and comes to the sur- 
face near the bottom of the gorge. 
In the area under discussion, the 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: 
