582 



»S7M/:l/.l/,'r .l.\7> COXCLrSION. 



Tliis iiii])(M- (•(informs to tlie i)riiici]»l(' that pliysiograpliy is oxplaiiatory 

 ratlKT than a mass of iloscriptivc matter. TIk" map, especially the topo- 

 graphic map, takes the place largely of the descriptive matter. The purpose 

 of the paper is to show specifically the responsibility of the geological 

 structure in the development of topographic form, especially in a regional 

 way. and also to show the importance of the combined geologic structure 

 and topographic condition in drainage adjustment. It deals with the knob- 

 stone cuesta region lying between the Muscatatuck and Ohio Rivers near 

 the eastern margin of the driftless area of southern Indiana. The details 

 defining, describing, and explaining the geologic structure and topographic 

 conditi(m take up a relatively lai*ge proportion of the paper. Details are 

 given showing how the particular lihtologie units with their regional west- 

 ward (lip are important conditioning factors in giving rise to topographic 

 forms, and that the topographic condition in the stages of youth and ma- 

 turity are lagely dependent up(m lithology and structure. Other condition- 

 ing factors scarcely less important are the so-called time factors, such as 

 regional elevation and depression, warping and tilting, ghiciation, and cli- 

 matic changes. Active factors, weathering and erosion, are given no detail, 

 but their activity is tacitly assumed and occasionally referred to directly. 

 The development of the Scottsburg Lowland, or the Eastern Lowland of 

 Newsom. is given an explanation somewhat different from any heretofore 

 advanced. A local peneplaih after having been normally developed is sub- 

 jected to further ero.sion by a peculiar sort of rejuvenation brought about 

 by glaciation. The Ohio River has been formed from a number of smaller 

 streams which were near the margin of the glacial ice at its farthest ad- 

 vance. This stream dispossessed a minor stream in the vicinity of New 

 Albany, and on account of its ability to reacli a much lower gi'adient than 

 the smaller stream, intrenched itself in the local peneplain developed in the 

 area of soft rocks. Tributary streams have since partially destroyed 

 the local peneplain rejuvenated in this manner. Such a rejuvenation is 

 here called static rejuvenation. 



The peculiarity of the streams flowing east and north from the Knobstone 

 escarpment is described. Blue River is discussed in some detail, since it 

 is representative of all the streams on the back-slope of the cuesta. Its 

 peculiar, fairly uniform gradient demands explanation. It is shown that 

 such a gradient is the result of a complex set of conditions, in which lith- 

 ology, uplift, and static rejuvenation play their part. 



Finally the details of the piracy of Muddy Fork of Silver Creek are 

 given. It is shown that this piracy is a direct result of the geologic and 

 topographic condition along the Knobstone escarpment. This piracy is 

 not an instance of a single case, but consists of successive piracy 

 wherein a large number of tributaries belonging to a single stream system 

 are annexed to the drainiige system of an invading stream. It is noted that 

 the ccmditions are highly favorable for the piracy to continue, and that 

 eventually the larger jiart of ^Nluddy Fork of Blue River will be taken over 



