:i68 



frc(|uciitly tl)c" rc.iildiial vock back from the Kiiolistdiio cscariiiiient for 

 iiiih's. and forms an excelleut example of a strm-tural plain. Xewsom"' 

 makes it clear that the Harroclshiira,- has been stripp(Ml by erosion from the 

 nndi'rlyiny Knobstonc to the north of the ai'ca under discussion, and that 

 tills removal has pei'ndttcd in later times llie moi-e rajad dissection of this 

 ]»ortion of the area. 



The Salem limestone where typically developed is a calcareous freestone. 

 It is very massive and uubedded. and is not a well-jointed limestone. 

 These structural characteristics prevent it from having many sink-holes 

 formed in it directly. Topographically it is characterized by long gentle 

 slopes and fairly broad valleys. It is somewhat less resistant to denuda- 

 tional agents than the underlying Harrodsburg, but frequently the topog- 

 raphy of one merges into that of the other rather indistinctly. 



The Mitchell limestone is fairly resistant to mechanical denudational 

 agents. It is structurally characterized by its great number of thin beds 

 of very close and compact nature and its highly jointed condition. These 

 structural characteristics combined with its position above the base level 

 of the region of its outcro]) have been responsible for a wide area of subter- 

 ranean drainage whose perfection of development is probably not excelled 

 anywhere. It is pitted w'ith numerous sink-holes of all sizes and combina- 

 tions. Only the larger streams in this limestone belt are surface streams. 

 The outcrop of this limestone belt almost everywhere possesses a typical 

 karst topography. Its presence as a fairly resistant stone mechanically 

 and its disposition to drink np the waters which fall upon it by subter- 

 ranean drainage have caused to come into existence a wide structural 

 plain which has a westward dip somewhat less than the dip of the strata 

 which make up the lithologic unit. A structural plain of this kind is 

 expected to have an inclination less than the dip of the rocks where their 

 thickness is considerable and their resistance not extraordinary. The 

 removal of the overlying material took place rather progressively from 

 the east to the west. The eastern portion was exposed first and is therefore 

 older. While the western part was still protected by overlying strata, the 

 eastern portion was being reduced. The presence of outliers of the over- 

 lying clastic Chester several miles to the east of the general Chester scarp 

 is indicative of the method of the formation of the Mitchell plain.* 



The clastic Chester members over-lying the Mitchell limestone are made 

 up of unresistant shales and rather resistant sandstones with one or more 

 intercalated limestones. Sandstones predominate in the region here mapped. 

 The limestones are inconstant. The Chester of the region is found rather 



^Loc. Clt., p. 268. 



^Probably no cloarer prospiitation of tlio prinfiplos niidprlvhis the (lovclopinoiit 

 of sinkholes ami iiikIitut.iuihI (li-aiii,-i,i;v has bcon writtni than thr article l)y .T. \V. 

 Beede entitled, "Tho Cvclc ..f Sul.i<TraiM'aii I>r:iiiia.uv as mustratcd in tlic r.h.oin- 



ington, Indiana. Q\iadranj;lr". It.m lin-s ,>( thr Indiana Acadrniy (if Science for 



1910, pp. 89-111. Tliis article is a classi.- ,,f its kind. I'.eede re-ards the Mitclii'll 

 plain (which he names such) as a jieneplain. This idea is not eni|diasi/.ed liy the 

 writer, yet some evi<leiice appeai-s to show tliat the Mitclidl plaiTi jian c.diiciiles with 

 local peneplains, reiiresentin.^- one or more halls in re,^i(nial uplift. Iml as :i. whide 

 it is probably structural. See pp. 24-2(;. ■■The -AniericaK I'xdtouis' reuioii of 

 Eastern Greene County. Indiana — A Type Unit in Southern Indiana Physiography", 

 C. A. Malott, Indiana Univ. Studies No. 40, March 1919. 



