EROSION AL HISTORY OF DRIFTLESS AREA 65 



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coiioin it lies on Huronian quart- 

 zite and in the Sparta district on 

 the Prairie du Chien and St. Peter 

 formations. Military Ridge and 

 its spurs r.re capped by Galena 

 dolomite, as are also the upland 

 divides around Waukon in Iowa. 

 South of Turkey river in Iowa, and 

 south of Lancaster in Wisconsin 

 and Illinois, the plain lies on Nia- 

 gara dolomite. In southeastern 

 Minnesota it lies for the most part 

 on the Platteville formation but it 

 places cuts across to St. Peter. In 

 a general way the Dodgeville plain 

 bevels the south-dipping forma- 

 tions, lying on progressively young- 

 er beds from north to south. Either 

 because of a decrease in the slope 

 of the plain north of the Iowa line, 

 because of a change in the direc- 

 tion of slope, or because the plain 

 has been very slightly warped since 

 its formation, its remnants are not 

 greatly higher in Minnesota than 

 in Iowa, and its stratigraphic posi- 

 tion is somewhat but not much 

 lower in Minnesota than farther 

 south. It also bevels the crest of 

 the arch. No remnants of the plain 

 have been observed north of the 

 axis and therefore it cannot be 

 definitely stated that the north- 

 dipping beds are also bevelled by 

 it. although such is probably the 

 case. It is noticeable, however, 

 that only the more resistant forma- 

 tions are found capping the high- 

 est divides where these divides are 

 flat-topped and accordant in their 



