26 



Indiana University Studies 



its impress upon the variable strata must have been feeble, 

 and a farther uplift and continued denudation have largely 

 erased any such evidence as would clearly and conclusively 

 show that there was in this region a stage of crustal stability 

 below the Kirksville and above the present grade level of the 

 streams. That there was such a period, the well-developed 

 gradation plane in the Knobstone rocks of Brown County con- 

 clusively testifies. The Mitchell plain of the limestone area 

 gives a similar but more equivocal testimony, as above noted. 

 In failing to record such a condition, the region of the ''Ameri- 

 can Bottoms" fails by so much of being a representative unit 

 of southern Indiana physiography. 



Further Uplift and Stream Trenching. So far as all ap- 

 pearances are concerned in the ''American Bottoms" region, 

 the uplift following the development of the Kirksville pene- 

 plain might have been commensurate with the difference 

 between the Kirksville level and the grade level of the present 

 streams. This difference is something like 225 to 250 feet, 

 making proper allowance for the relief of the Kirksville pene- 

 plain itself. It is assumed that the present divides, which 

 are the sole representatives of the Kirksville, were also the 

 divides of the peneplain before uplift, and that the valleys 

 of the latter were lower than the divides by an average of 

 perhaps some 25 to 30 feet to the mile, a reasonably low 

 estimation of regional slope where the drainage lines are 

 small and their distance from the divides short, as is the case 

 in the region under consideration. By way of comparison, 

 it may be noted that the difference of level between the Kirks- 

 ville plain and the present graded stream level in the Knob- 

 stone region of Brown County is about 275 feet, measured 

 at places where the present streams are comparable in size 

 to those of the region under discussion. This shows no dis- 

 parity in the figures. It may be concluded, therefore, that 

 the stream trenching which is characteristic of the unglaci- 

 ated portion of southern Indiana is excellently represented in 

 the "American Bottoms" region. 



Valley FilliRg. The question may well arise whether 

 stream trenching below any local peneplain level, or below 

 the Kirksville, as it is represented in the "American Bottoms" 

 region, may not have gone deeper than the present base level. 

 If so, the total uplift has been greater than is evidenced by 



