22 



Indiana University Studies 



fails to maintain the qualities which a type unit in southern 

 Indiana physiography should possess. 



Whenever the physiographer searches over a region, 

 whether in the field or by means of its representative, the 

 detailed topographic map, he fixes upon certain salient forms 

 which represent results that may be expressed in terms of 

 denudational units. For example, monadnocks standing on 

 bases which may now be no broader than the broader parts 

 of inter-stream tracts bespeak the former existence of a pene- 

 plain. Trenched valleys below such a peneplain show that 

 uplift of the region initiated another cycle of erosion ; and 

 local benches and sags in the older ridge may indicate a halt 

 or series of halts in the general uplift of the region. Features 

 of this sort may conceivably exist in a region, regardless of 

 the nature of the rocks upon which the features are de- 

 veloped. Nevertheless it must be agreed that the nature of 

 the material upon which these characters are impressed is 

 a controlling factor, and that some rocks are favorable for 

 preserving and revealing such historical monuments while 

 others are not. We shall see how the region of the "Ameri- 

 can Bottoms" reveals periods of peneplanation, followed by 

 uplift and other complicating associated processes. 



The Kirksville Peneplain. The broad ridge beginning in 

 the region of Cincinnati and trending northwest and then 

 west between Beech Creek valley and the ''American Bot- 

 toms", and the branch of this ridge running between the 

 ''American Bottoms" and Cliffy Creek valley, are fairly broad 

 with gentle undulations, and are never flat over any con- 

 siderable area. The general elevation of this Cincinnati- 

 Ridgeport ridge and its branch south of the "American Bot- 

 toms" is consistently 800 feet above sea level, but neverthe- 

 less it slopes gently toward the west. By consulting the 

 geologic map one may see that the generally accordant level 

 of the ridges is not due to denudation down to a certain re- 

 sistant stratum, but that to the east the top of the ridge is 

 in rocks of Chester Age, while to the west it is in rocks of 

 Pennsylvanian age. It is proper to mention here the fact 

 that this level persists over a series of high ridges to the 

 east of this region, and that it is developed on various strata, 

 and furthermore that it gradually rises to the east, but at a 

 moderate rate, probably not more than 5 feet to the mile. 



