EROSIONAL HISTORY OF DRIFTLESS AREA 27 



crested divides," "even-crested uplands," and "even-crested 

 skylines." The term "even-crested summit areas" seems 

 to include every phase of the subject and to exclude inter- 

 in ediate plains, which might be included under some of the 

 other terms and which constitutes a separate point. 



Study of the principles involved in the formation of 

 topographies in which the highest elevations are flat-topped 

 and have about the same altitude, shows that such even- 

 crested summit areas constitute better evidence of more 

 than one cycle if the rocks involved are folded or tilted or 

 massive, than if they are horizontal or nearly horizontal 

 strata. 



In Regions of Folded Strata: A plain which bevels fold- 

 ed strata might be interpreted as recording the following 

 events: (1) the folding of the strata, forming a topography 

 of high relief with anticlinal ridges and synclinal troughs ; 

 (2) erosion of the surface until a large part is brought to 

 grade, leaving the surface relatively flat; (3) uplift of the 

 land relative to sea, renewed degradation by streams and 

 the relatively rapid removal of the non-resistant materials, 

 leaving the outcrops of the harder formations as ridges or 

 hogbacks, the tops of which are remnants of the peneplain 

 developed in the first cycle. 



It seems difficult to the writer to assign any other history 

 than that outlined above to topographies illustrating even- 

 crested summit areas in regions of folded strata. Other 

 possible interpretations may be mentioned. Tarr^ has ob- 

 jected to the idea that the more or less even-crested ridges 

 of the Appalachian region represent an ancient peneplain 

 and points out (1) that they are by no means of a common 

 level and (2) that elevations made of about equally resist- 

 ant rock, starting with their summits above timberline. 

 would be eroded rapidly and about equally to timberline, 

 and then acquire more or less uniform levels, all in a single 

 erosional cycle. It should be noted that these even-crests 

 are not at timberline. Other investigators- have proven. 



1. TaiT. R. S.. Am. Geol., Vol. 21, pp. 351-370. 



2. Davis, W. M., Am. Jour. Sci.. 1889, Vol. 37, p. 430 : Willis, Bailey, Phuaioqraphy 

 of the United States, pp. 169-202 ; Hayes and Campbell, Natl Groi/. Mag., Vol. 6, pp 

 65-126. 



