400 



"The Mitchell limestone, otherwise liiiown as the St. Louis, barren, 

 or cavernous limestone, is a bluish or grayish, hard, compact, even-grained 

 stone, generally having a conchoidal fracture. It is so compact as to make 

 it rather impervious. Intercalated layers of blue-gray shale are frequent. 

 Large concretions of chert are characteristic of certain horizons. When 

 the stone weathers, these masses of chert do not dissolve, but break into 

 more or less angular fragments which strew the ground over the Mitchell 

 area. In Indiana the formation is also characterized by the common pres- 

 ence of a genus of corals known as Lithostrotion or Lonsdaleia. In some 

 places, such as western Monroe or southern Crawford County, there is a 

 typical white oolite found near the top of the formation. 



"Analysis shows the Mitchell to be a very pure calcium carbonate, and 

 at Mitchell, Lawrence County, from which place the formation received its 

 name, it is extensively quarj.-ied for making lime and cement.- 



"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 Mammoth 

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

 inv^estigate the general adaptability of this limestone to cave formation. 



"The reasons of this adaptability 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. Vertical 

 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 sat- 

 urated instead of confining the water to joint-planes." 



The presence of joint-planes, its impermeability and its solubility, are 

 the qualities of the Mitchell limestone which make it favorable to the 

 development of caves and sinkholes. It is obvious that if a stone is im- 

 permeable and has joint-planes, the water will trickle down through these 

 joints instead of being absorbed by the rock. If the rock is soluble and the 



' In the southeru part of the State it reaches a thickness of 350 to 400 feet; in 

 the central part of its area, tliat is, in Lawrence and Monroe counties, the thickness 

 is from 150 to 250 feet, and from here gradually thins toward the north." 



