Malott: The ''American Bottoms'' Region 25 



the cherty layer within the Mitchell limestone group. The 

 writer is not in a position to discuss this point at present as 

 fully as could be desired. Further study will either prove 

 the Mitchell plain to be due to denudation down to a particu- 

 lar group of strata, or to be a true peneplain as described 

 by Beede. Locally the Mitchell plain has all the characters 

 of a true base-level plain, but it can be urged against this 

 interpretation that the plain is well developed to the east- 

 ward at elevations consistent with the level of the Kirksville 

 plain, while to the westward where the same Mitchell strata 

 have dipped down, the plain also is much lower than the 

 typical Mitchell level is supposed to be. 



Judging from the development in Brown County, Ind., of 

 a local gradation plain or peneplain below the level of the 

 Kirksville plain, in rocks of uniform composition and struc- 

 ture and several hundred feet in thickness, it must be in- 

 ferred that succeeding the Kirksville peneplanation there was 

 a period of crustal stability after an uplift of about 150 feet. 

 The Knobstone strata of Brown County are of such a char- 

 acter as to allow no benches to be formed as a result of local 

 dilferences in the operation of physiographic processes upon 

 different materials. The material is uniform over a wide 

 area and thru great thicknesses of rocks, and therefore favors 

 a uniform result. Moreover, the strata are of a shaly, silici- 

 ous nature such as favors steep slopes, but does not permit 

 the development of bluffs or cliffs. Weathering and erosion 

 are strictly mechanical. For these reasons the conditions for 

 the making and preservation of local peneplains are very 

 favorable. The writer expects to make the details relating 

 to this matter the subject of a later paper. The conclusion, 

 however, which may be stated now is : first, that there was 

 a short period of crustal stability after an uplift of about 

 150 feet, and that a gradation plane was then developed below 

 the Kirksville level in Brown County; and, second, that since 

 a wide area must have been subject to similar uplift followed 

 by a period of quiet, the ''American Bottoms" region probably 

 had approximately the same history. 



The absence of any topographic record of this short period 

 in the ''American Bottoms" region is tied up with the pres- 

 ence of a complex series of rocks upon which the physio- 

 graphic agencies had to work. The period was so short that 



4—16903 



