THE PRE-POTSDAM PENEPLAIN 307 



quartzite bluffs, whose highest points reach an elevation of 1,600 

 feet. Surrounding the Baraboo quartzite are great thicknesses 

 of the Potsdam conglomerate. The Necedah quartzite probably 

 attained an approximate elevation of 280 feet and the Baraboo 

 quartzite ranges an elevation of 1,200 to 1,400 feet above the 

 surrounding pre-Cambrian plain. These two elevations of the 

 pre-Cambrian surface consist of hard resistant rock and bear a 

 similar relation, it is believed, to the surrounding buried pre- 

 Cambrian that the Powers bluff and Rib hill quartzite monad- 

 nocks bear to the slightly dissected and deeply dissected portions 

 of the peneplain farther north. 



Southeast of the pre-Cambrian area and also southwest there 

 is a much steeper slope to the pre-Cambrian surface, as indicated 

 by the artesian wells at Oshkosh and La Crosse. To the south- 

 east are the pre-Cambrian outliers of rhyolite and granite along 

 the Fox river, which are believed to have been monadnocks in 

 the pre-Cambrian plain. 



The uplifting and consequent folding of the region which has 

 produced the present slope of the uncovered pre-Cambrian in 

 central Wisconsin, as well as its buried portion farther south, is 

 outside the scope of the present paper. Briefly stated, however, 

 as generally accepted,^ there is a broad anticlinal extending 

 southward from central Wisconsin into Illinois, with a corre- 

 sponding synclinal depression extending under Michigan on the 

 east and a similar one under Iowa on the west. The uniform 

 slope of the buried pre-Cambrian at Kilbourn City and Madison 

 and the uncovered peneplain about Wausau is thus along 

 the anticlinal. It is believed that the buried pre-Cambrian to 

 the east at Oshkosh and to the west at La Crosse also had a 

 common peneplain slope, in Potsdam time, with the now exposed 

 portion of the pre-Cambrian, and that the greater rate of descent 

 in these directions at the present time is to be explained as the 

 corresponding synclinals formed in the pre-Cambrian since the 

 beginning of Potsdam time. 



Contemporaneous erosio?i of the pre-Cambrian a?id post-Catnbrian 

 rocks. — The main features of the pre-Cambrian [)eneplain area 



' T. C. Chamberlin, Geol. of Wis., Vol. IV, p. 424. 



