EROSIOXAL HISTORY OF DRIFTLESS AREA 83 



therefore do not lie on its projection. Salisbury^ has pre- 

 sented evidence that the plain does slope down beneath the 

 Tertiary deposits of the lower Mississippi valley and that 

 those deposits lie on a plain similar to and continuous with 

 the Dodgeville plain in the Driftless Area. This point af- 

 fords strong evidence in favor of the peneplain theory. 



(5) Martin concludes his objections to the peneplain 

 theory by stating that there are no wedge-shaped bodies of 

 non-resistant rock overlying the south-dipping resistant 

 layers, as there should have been when the Dodgeville pene- 

 plain was undissected. He agrees, however, that these 

 wedges could have been removed by the rejuvenated 

 streams. Their absence, therefore, is no objection to the 

 peneplain theory, but is in harmony with the cuesta, as 

 well as with the peneplain theory. 



As a matter of fact, but apparently unknown to Martin, 

 there are just such wedges of St. Peter sandstone north of 

 the Platteville-Galena cuesta in Wisconsin, and Maquoketa 

 shale north of the Niagara cuesta in Iowa. Fig. 5 affords 

 an illustration of the St. Peter wedges. Other illustrations 

 are found in the south part of the Richland Center quad- 

 rangle, where ever-increasing thicknesses of St. Peter cap 

 the north-south divides to the foot of the Platteville cuesta, 

 where the full thickness of the St. Peter is represented. In 

 Iowa the south rim of the valley of Turkey- river, south of 

 Osterdock, is underlain by a few feet of Maquoketa shale 

 which dips south with the Galena dolomite below. Along 

 a road which follows a flat-topped divide southward, the 

 ^Maquoketa gradually thickens until its full thickness is 

 found at the foot of the Niagara escarpment. Martin says : 

 "They (the wedge-shaped bodies) would furnish excellent 

 evidence of previous baselevelling, but no such remnants 

 are known to exist." Now that such lenses of non-resistant 

 material have been discovered, this point is transferred 

 from the unfavorable to the favorable column for the pene- 

 plain theory. 



In conclusion it may be said that the summit areas in 



1. SrJIsbui-y. K- D- Bull. Gecl. Soc. Am.. Vol. 3, pp. 1S2-1S6, -Joui-. Geol., Vol. HI. pp. 

 655-667. 



