THE PRE-POTSDAM PENEPLAIN 295 



gradually grows less, and the Wisconsin valley, and also the side 

 valleys, as already stated, gradually grow shallower, until their 

 floors coincide with the level of the peneplain (compare Plate I, 

 Figs. I and 2). Cappings of erosion remnants of Potsdam sand- 

 stone begin to appear in the slightly dissected peneplain in 

 northern Wood and Portage counties, and become quite numer- 

 ous about Grand Rapids at the border of the continuous sand- 

 stone district. For some distance into the sandstone district 

 south of Grand Rapids, at the rapids of Grand Rapids, Port 

 Edwards, and Nekoosa, the Wisconsin river has exposed crystal- 

 line rock, showing the latter in the river bottom and the over- 

 lying, nearly horizontal Potsdam sandstone outcropping above it 

 in thin patches along the river bank, whence the sandstone 

 extends in low ridgy exposures dotting the low plain of the sur- 

 rounding sandstone district. The dissected peneplain about 

 Wausau thus gradually changes to the undissected peneplain 

 about Grand Rapids, where it is covered with thin sandstone 

 outliers, and for some distance into the sandstone district the 

 pre-Cambrian rocks are seen only in the river bottom, the adja- 

 cent flat-lying land being covered with Potsdam sandstone. 



West of the Wisconsin river are the Yellow and Black rivers, 

 which also lie athwart the pre-Cambrian and Cambrian districts. 

 The Yellow in its upper course flows through the region of thick 

 drift covering a few sandstone outliers. A short distance south 

 of Marshfield the thick drift ceases, and from here as far south 

 as Dexterville, a distance of twenty miles, the Yellow has 

 exposed the pre-Cambrian in the river-bed, showing at numerous 

 places thin cappings of the sandstone lying above it along the 

 river bank. The descent of the pre-Cambrian surface along the 

 Yellow river is approximately between ii and 12 feet per mile. 

 The Black river above Neillsville also lies in the region of thick 

 drift and sandstone outliers, but south of Neillsville to Black River 

 Falls — a distance of twenty-two miles — through the low plain 

 of the thin drift area the Black shows almost continuous crys- 

 talline rocks in the river bottom, with sandstone cappings above 

 them along the bank. The rate of descent of the crystalline 

 area along the Black river from Neillsville, with an elevation of 



