176 



again commenced to cut clown the surface, and probably during the Ter- 

 tiary period, it again reached partial peneplanation with a few monad- 

 nocks here and there standing above the general surface. It is this Ter- 

 tiary peneplain which gives the country its level appearance when viewed 

 from a distance. Since this second peneplanation, the country has probably 

 been relatively elevated to the present time. The western edge of the 

 Mitchell area is overlain by the Huron formation, which, by reason of its 

 hard and soft strata, has taken on a very rugged aspect. To the east of 

 this belt, level lies the central or slightly rolling area of the Tertiary pene- 

 plain, while to the east of this, the eastern edge again becomes rolling, 

 owing to the underlying Salem and Harrodsburg limestones. 



The Mitchell has a dip to the southwest which probably averages 20 

 to 30 feet to the mile. This affects surface streams, though these are very 

 few, owing to the extensive underground drainage. 



The general relief of the surface of the Mitchell area becomes greater 

 toward the Ohio River. This is probably due to at least three causes, 

 namely, the dip to the southwest, the increasing thickness of the forma- 

 tion, and the fact that the Ohio River is the largest stream draining the 

 area aud has cut down to the lowest level of any stream in the area under 

 consideration. In the vicinity of Wyandotte Cave the general level of the 

 upland is about 300 feet above the level of Blue River. 



The Mitchell limestone has long been known as the "Cavernous lime- 

 stone." Both the Wyandotte Cave of Indiana and the Mammoth Cave of 

 Kentucky occur in its strata. In three counties in the vicinity of Mam- 

 moth Cave, over five hundred caves are known to exist. These facts lead 

 us to investigate the general adaptibility of this limestone to cave forma- 

 tion. 



The reasons of this adaptibility are numerous. Besides the bedding 

 planes, two sets of vertical joint-planes exist, one set having a general 

 east and west direction and the other a north and south direction. Ver- 

 tical joint-planes are probably more numerous in this, than any other of 

 the Mississippian limestones. Owing to the fact that the Mitchell is rather 

 impervious and often of a lithographic nature, the down flowing water is 

 forced to follow the joint and bedding planes. The underlying Salem lime- 

 stone contains joint-planes but is porous enough to become thoroughly 

 saturated instead of confining the water to joint-planes. 



The Mitchell limestone has a great thickness of rocks of nearly uniform 

 texture. It is composed of nearly pure calcium carbonate, which renders 



