THE PENEPLAIN 851 



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acter of the old peneplain surface is easily seen. As the valleys 

 become widened and the interstream portions reduced, the old 

 peneplain level is less and less readily recognized, the uniform 

 agreement in height of the interstream ridges being the most con- 

 spicuous feature. It can, however, be shown that uniform height 

 of interstream ridges may also be brought about in a country where 

 the original surface was very diverse, if the streams are uniformly 

 spaced. (Shaler-27.) This is especially true if the streams are of 

 approximately equal power, and the rate of erosion is thus more 

 or less uniform. 



Age of the Peneplain. It is evident that the peneplain is of 

 later age than that of any of the strata affected by the erosion in 

 the formation of the peneplain. In the case of folded strata which 

 have become peneplaned the commencement of peneplanation may 

 be regarded as simultaneous with the folding, and since in a strongly 

 folded region even the latest strata deposited may be involved in 

 the folds and so protected from complete erosion, we may not be 

 far wrong in considering that folding and peneplanation begin 

 shortly after the deposition of the youngest stratum involved. It 

 must, however, be borne in mind that folding of strata without 

 fracture takes place at some distance below the surface (see Chap- 

 ters XIX and XX), and that therefore a series of perfect folds in 

 any given series of strata suggests that these strata were at con- 

 siderable depth below the surface at the time of the formation 

 of the folds. Under such conditions, when perfectly folded strata 

 are found near the surface of a peneplain, it is not likely that the 

 later strata, deposited before the commencement of folding, are 

 included within the folds.. Thus within some of the strongly folded 

 strata of the Hudson River group in Albany County, N. Y., only 

 middle and earlier Ordovicic strata are involved so far as known, 

 though there is every reason for believing that the folding did not 

 take place until late Ordovicic, if ngt early Siluric time. 



In the case of horizontal strata which have been peneplained, 

 the latest preserved stratum is not to be regarded as the last one 

 deposited before elevation and erosion, for this would allow no 

 removal of strata by erosion during the peneplanation. In general 

 we may consider that the amount of rock removed from a given 

 region during a stated period of elevation and erosion is propor- 

 tional to the distance of that region from the point where erosion 

 was replaced by deposition, /. e., from the seashore or piedmont 

 plain of the period. Exceptions to this must, however, be recog- 

 nized where local conditions limited or accentuated erosion, as in the 

 case of a warped surface where some portions of a given formation 



