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 luider 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 

 wliich subterranean drainage is well develoi)ed, 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, 

 illustrates 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 KXOBSTOyE CUESTA REGION. 

 General Stratigraphy. 



The general geologic structure and topograjihic condition of the area 

 here under discussion are shown on the general map. The map shows the 

 Knobstone cuesta south of the Muscatatuck River. The general lithologic 

 succession of rocks is shown by the geologic cross-section inserted in the 

 map in proper iwsition. 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 Knobstone 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 



