364 
erosion cycle. But to say that a region is in youth, maturity, or old age is 
usually insufficient, since most regions with any but the softest rocks have 
more than one erosion cycle represented. If the region is limited in area, 
its topographic condition may be signified by the statement of the particu- 
lar stage represented in the erosion cycle. But the terms youth, maturity, 
and old age are really only first glance terms when applied to any region 
of considerable areal extent. It will be seen that the form, size and rela- 
tionships of the relief features are largely dependent upon rock structure 
and the denudational agency which actively produced them. The broader 
details included under the term topographic condition (details embraced 
in the statement of the stage of the erosion cycle, but never specifically 
stated) are such as the local base level, the elevation of areas above base 
level, the size of the streams, proximity to major streams, presence of major 
and minor divides or watersheds, and the regional topographic forms pres- 
ent which are dependent upon the type of rock and which may involve spe- 
cial physiographic processes. The meaning of the last mentioned detail 
of topographic condition may be clarified by an illustration. A region of 
limestone rock whose surface is well above the potential base level and in 
which subterranean drainage is well developed, is characterized by specific 
topographic forms. The outcrop of the Mitchell limestone, the so-called 
Mitchell plain with its disappearing streams and its sink-hole topography, 
ilustrates the condition. Solution by descending meteoric waters concen- 
trated along the joints and bedding planes of the limestone rock largely 
gave rise to the particular topographic forms present, and these forms are 
dependent upon the type of rock. 
GEOLOGIC STRUCTURE OF THE KNOBSTONE CUESTA REGION. 
General Stratigraphy. 
The general geologic structure and topographic condition of the area 
here under discussion are shown on the general map. The map shows the 
Knobstone cuesta south of the Museatatuck River. The general lithologie 
succession of rocks is shown by the geologic cross-section inserted in the 
map in proper position. This section extends from just north of Jefferson- 
ville west to Marengo. Something like 100 feet of New Albany shale 
overlie the Devonian limestones which outcrop mainly east of the mapped 
area, and are present in the region mainly below drainage level. Succeed- 
ing the Devonian New Albany shale comes the Knobstone group of sandy 
blue-gray shales and muddy sandstones, the latter coming into prominence 
towards the top. The Kneobstone group has a thickness of approximately 
500 feet. The Knobstone and suceeding formations discussed in this paper 
belong to the Mississippian period. The Harrodsburg, Salem and Mitchell 
limestones follow in order. The Harrodsburg limestone is rather silicious, 
and is usually more crystalline than the other limestones. It consists 
of thin to massive layers characteristically unevenly bedded, having a total 
thickness of about 90 feet. The Salem limestone is quite massive, and is 
typically a calcareous freestone. In the region under discussion it has 
