-82 IOWA STUDIES IN NATURAL HISTORY 



zoic eras. There is, however, no indication in this that some 

 relatively recent peneplain, such an one completed in late 

 Tertiary, could not to-day be represented by remnants, 



(3) Believing- that sediments must have been derived 

 from the surface of the Driftless Area in the formation of 

 the Dodgeville plain, and not certainly finding such sedi- 

 ments in the Devonian and Carboniferous rock adjacent to 

 the Area, Martin objects to the peneplain theory. Clearly 

 he is laboring under a misconception as to the age of the 

 plain. Whether or not it is an old peneplain, its surface is 

 much younger than the Paleozoic. All of the Paleozoic for- 

 mations, including the Devonian and Carboniferous are 

 known to be bevelled by the Dodgeville plain south and west 

 of the Driftless Area. Most of those who have previously 

 interpreted it as a peneplain have assigned it to the Creta- 

 ceous and the writer will later in the paper present evidence 

 for the late Tertiary age of the plain. Most likely then the 

 sand, silt, and clay derived in the formation of the Dodge- 

 ville plain were carried westward into the Cretaceous sea, 

 or most likely southward into the Tertiary embayment. 

 Indeed, both the Cretaceous and Tertiary systems contain 

 materials which must have been derived from erosion in 

 the Driftless and adjacent areas during these periods, and 

 there is evidence that peneplanation was in progress. Thus 

 by reference of the Dodgeville plain to its proper geological 

 period Martin's point of objection to the peneplain theory 

 is converted into an additional argument in favor of that 

 theory. 



(4) It is true, as pointed out by Martin, that the Devo- 

 nian and Carboniferous rocks lie on surfaces of less relief 

 than that of the Driftless Area, and that this does not show 

 that the peneplain of the Driftless Area is projected to lie 

 beneath these sediments. However, it is not to be con- 

 sidered in any way as an objection to the peneplain theory. 

 The plain is clearly much younger than any Paleozoic sys- 

 tem. If the Cretaceous or Tertiary sediments could be 

 proven to lie on a projection of the Dodgeville plain, strong- 

 evidence would be offered that it is a peneplain. The Creta- 

 ceous rocks of Minnesota are bevelled by the plain and 



