EROSIOXAL HISTORY OF DRIFTLESS AREA 29 



regions of folded strata. In a previous article by the writer' 

 an upland plain in northwestern Illinois was conceived to 

 be (1) an original marine plain of deposition; (2) a marine 

 plain of erosion; (3) a structural plain; or (4) a true pene- 

 plain. Only in case such upland plains can be proven to 

 be true peneplains do they constitute proof of more than 

 one cycle in the erosional history of a region. For the de- 

 tailed discussion of the characterizing features of plains 

 formed in the four ways outlined above, readers are re- 

 ferred to the article cited. A plain of marine deposition 

 should be parallel with the rock strata and should not have 

 on its surface deposits of any sort younger than the marine 

 formations which underlie it, A plain of marine erosion 

 should be bordered by higher land and separated from this 

 land by a shoreline. It should bevel the edges of rock for- 

 mations ; its surface should contain marine deposits young- 

 er than the formations which the plain bevels and any rem- 

 nants which stand above it should be isle-like. A structural 

 plain would be located on a resistant formation and would 

 be parallel with the dip of that formation. If upland tiats 

 are remnarjts of a true peneplain, the surface represented 

 by them when reconstructed should not, except in unusual 

 cases, be paiallel with rock structure, should be more or 

 less uneven, have dendritic erosion remnants above it, and 

 have fluvial deposits on its surface. There might be cases 

 in which it would be impossible to determine the correct 

 one of these four origins of upland plains, but if they be 

 studied carefully enough and over sufficiently wide areas, 

 correct interpretation should be possible. 



Recently, Martin- has expressed the opinion that features 

 cf the topography of the Driftless Area of Wisconsin, which 

 have been most generally interpreted as even-crested sum- 

 mit areas representing an old peneplain, can better be ex- 

 plained by assuming that the topography is due to the un- 

 equal erosion in a single cycle of series of unequally resist- 

 ant rock formations having a slight monoclinal dip. After 

 defining a cuesfa as "an upland with a short steep descent. 



1. Trowbridpre. A. C. Jour. Geol., Vol. 21. pp. 731-738. 



2. Martin, Lawrence, Bull. No. 36, Wis. Geol. and Nat'l Hist. Surv.. pp. 63-70. 



