38 IOWA STUDIES IN NATURAL HISTORY 



cycles which might have effected a topography. Such 

 regions as the Piedmont Plateau or portions of the Lauren- 

 tian Shield, which are thought to have been land since the 

 beginning of the Cambrain period, have probably been 

 peneplained and uplifted many times, although the detec- 

 tion of so large a number of cycles would be extremely 

 difficult, if not impossible. On the other extreme, surfaces 

 fashioned by the Wisconsin ice sheet have probably nowhere 

 been eroded in more than one cycle, so short has been the 

 time since the retreat of the ice. 



The number of cycles of erosion which has affected a 

 given region is to be determined by the number of sets of 

 evidences of more than one cycle. For instance, in the 

 Appalachian mountains, even-crested summit areas, ante- 

 cedent streams, windgaps, intrenched meanders, associated 

 sets of straight and crooked streams, intermediate plains 

 and fluvial deposits on uplands, all are in evidence, and the 

 combination proves more than one cycle of erosion in the 

 history of the region. Obviously, even-crested summit areas 

 which are the remnants of an old peneplain, and an inter- 

 mediate plain representing an old peneplain, cannot belong 

 to the same set of evidences if they both occur in the same 

 region. Most of the antecedent streams and some of the 

 windgaps of the Appalachian region belong with the set of 

 evidences represented by the even-crested summit areas, 

 and most of the intrenched meanders and associated sets of 

 straight and crooked streams, and all of the fluvial deposits 

 are clearly correlated with the intermediate plain. There 

 seem to be two sets of intrenched meanders and two sets of 

 antecedent streams, one set of each related to the upper 

 plain and the other set to the intermediate plain. Thus, the 

 older set of evidences includes even-crested summit areas, 

 antecedent streams, windgaps and intrenched meanders, 

 and the younger set consists of intermediate plain, fluvial 

 deposits on divides, intrenched meanders, associated sets of 

 straight and crooked streams, antecedent streams and wind- 

 gaps. Each of these sets includes the proper combination 

 of evidences to prove more than one cycle, hence it is con- 

 cluded that the region is now in its third cycle, the first set 



