368 
frequently the regional rock back from the Knobstone escarpment for 
miles, and forms an excellent example of a structural plain. Newsom?® 
makes it clear that the Harrodsburg has been stripped by erosion from the 
underlying Knobstone to the north of the area under discussion, and that 
this removal has permitted in later times the more rapid dissection of this 
portion of the area. 
The Salem limestone where typically developed is a calcareous freestone. 
It is very massive and unbedded, 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 outcrop have been responsible for a wide area of subter- 
ranean drainage whose perfection of development is probably not excelled 
anywhere. It is pitted with numerous sink-holes of all sizes and combina- 
tions. Only the larger streams in this limestone belt are surface streams. 
The outerop of this limestone belt almost everywhere possesses a typical 
karst topography. Its presence as a fairly resistant stone mechanically 
and its disposition to drink up the waters which fall upon it by subter- 
ranean drainage haye 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 
8Loc. Cit., p. 268. 
‘Probably no clearer presentation of the principles underlying the development 
of sinkholes and underground drainage has been written than the article by J. W. 
Beede entitled, ‘The Cycle of Subterranean Drainage as Illustrated in the Bloom- 
ington, Indiana, Quadrangle’. Proceedings of the Indiana Academy of Science for 
1910, pp. 89-111. This article is a classic of its kind. Beede regards the Mitchell 
plain (which he names such) as a peneplain. This idea is not emphasized by the 
writer, yet some evidence appears to show that the Mitchell plain part coincides with 
local peneplains, representing one or more halts in regional uplift, but as a whole 
it is probably structural. See pp. 24-26. “The ‘American Bottoms’ region of 
Eastern Greene County. Indiana—A Type Unit in Southern Indiana Physiography”, 
Cc. A. Malott, Indiana Univ. Studies No. 40, March 1919. 
