an average thickness of perhaps less than 40 feet. The Mitchell limestone 
is a group of limestones totaling some 350 feet in thickness in the region. 
It consists of about 220 feet of St. Louis limestone at the bottom, about 90 
feet of Fredonia Odlite (representing the St. Genievieve), and about 4u 
feet of Gasper Odlite (distinctly of Chester age) at the top. The Mitchell 
limestone, though composed of several geologic units, is really a great 
lithologie unit of compact, thin-bedded, highly jointed limestone layers with 
occasional thin bands of shale and impure limestone horizons. The lime- 
stone in places contains considerable chert. Near the top of the St. Louis 
chert is quite conspicuous and bears numerous colonies of the coral Litho- 
strotion canadense. The uppermost strata outcropping in the region are the 
clastic members of the Chester series above the Mitchell limestone. The 
sandstones and shales, however, contain one or more members of limestone. 
The total thickness in the region is approximately 200 feet. 
Regional Dip. 
The above lithologic series dip to the west or probably a little south of 
west at the north of the area, at the rate of about 30 feet to the mile. Local 
dips may be much more or less than this amount, due to anticlinal or terrace 
flexures, such as the geologic cross-section shows. This regional dip causes 
each lithological unit to outcrop along its strike extending almost north 
and south. The lowest unit outcrops farthest east and the others follow 
in order to the west. No particular unit has an areal outcrop proportional 
in width to its thickness with respect to any other unit. This is because of 
the difference in resistance of the units and the topographic condition. The 
topographic condition of the area occasionally allows a thinner unit to 
occupy a wide area while a thicker unit may have a relatively narrow 
outerop. 
Areal Outcrop of Lithologic Units. 
For the details of the areal outcrop of the above geologic or lithologic 
units, reference must be made to a geologic map. It may be stated here 
parenthetically that the writer is firmly convinced that no detailed phy- 
siographie study of the driftless area of southern Indiana can be adequately 
made without the aid of detailed lithologic knowledge of the region. This is 
essentially a geologic study, and its expression is found in the geologic map 
and the stratigraphic column. A general idea of the representative outcrop 
may be gained by reference to the geologic and topographic cross-section 
on the general map. This map also shows the position of Knobstone escarp- 
ment by means of hachure lines, following Newsom’s map.’ This escarpment 
is composed of the Knobstone strata. The lowland area to the east consists 
of both the lower softer portion of the Knobstone strata and the unresistant 
New Albany shale. At the south the Mississippian limestone belt begins 
at the very top of the escarpment and extends westward. In the middle por- 
tion of the area only the Harrodsburg limestone extends as far east as the 
1J. F. Newsom, A Geologic and Topographic Section Across Southern Indiana. 
26th Annual Report, Ind. Dept. of Geol. and Natural Res. 1901. 
