567 
sion of them is not intended here. Such would call for a detailed topo- 
graphic map of the entire region. The topographic condition is very inade- 
quately presented on the general map. The elevations given here and 
there, the presence and position of the Knobstone escarpment, the drainage 
lines, and the inserted geologic and topographic cross-section show the main 
elements. Where some considerable detail is given, a topographic map 
has been prepared. This small bit of the larger area is presented in detail 
to reveal a particular condition. The explanation of the present topo- 
graphic condition of the area emphasizes lithology, since the lithologic 
units largely control the topography. The time factors or elements will 
be little more than mentioned, except where the explanation offered is 
one which has never been brought to the attention of physiographers pre- 
vious to this presentation. 
Influence of Lithology. 
The New Albany black shale and the sandy shale and muddy sandstones 
of the Knobstone group belong to the class of non-resistant rocks. They 
weather very rapidly by alternate freezing and thawing. The fragments 
given up by the freezing and thawing method of weathering are readily 
carried away by the streams of running water. These rocks also are 
easily corroded by running water. Streams even of small size in these 
rocks have reached grade close up to their sources. As a_ result 
of this, streams heading in a region where these rocks are relatively 
high above local base level have a very precipitous descent at their very 
headwaters, but soon take on a relatively flattened gradient within a 
short distance from their oyver-steepened heads. The upper part of the 
knobstone, however, consists of a relatively large amount of massive im- 
pure sandstone. It is much more resistant than the lower Knobstone 
rocks. Only very locally, however, does this upper portion resist weather- 
ing sufficiently to stand as vertical cliffs. It may be said that the great 
thickness of Knobstone rocks with the corresponding rather wide areal 
outerop does not result in the formation of cliffs. But the outcrop con- 
sists of quite steep slopes, often attaining 30 degrees or more from the 
horizontal.” 
The Harrodsburg limestone immediately over-lying the Knobstone is 
much more resistant to mechanical denudation, and the interstream spaces 
are often gently rolling tracts. Where the streams have cut through the 
Harrodsburg capping of the -Knobstone, tongue-like upland tracts are 
bordered by the steep slopes leading abruptly to the Knobstone valleys. 
The limestone is soluble in meteoric waters and since it is bedded and 
jointed, sink-holes are quite common. The Knobstone-Harrodsburg contact 
is consequently a spring horizon of some importance. The Harrodsburg is 
“For weathering and erosion of Knobstone rocks, see the following references: 
J. F. Newsom, A Geologie and Topographic Section Across Southern Indiana, 26th 
Annual Report, Ind. Dept. of Geol. and Natural Res. 1901, pp. 265-275. G. H. Ash- 
ley, Geology of the Lower Carboniferous Area of Southern Indiana, Ind. Dept. of 
Geol. and Natural Res. 1902, pp. 54-58. E.R. Cummings, The Geological Conditions 
of Municipal Water Supply in the Driftless Area of Southern Indiana, Proceedings 
of the Ind. Acad, of Science, 1911, p. 114-124. 
