THE PRE-POTSDAM PENEPLAIN 311 



trict south of the peneplain is only two feet per mile; and hence 

 there is a distinct unconformity in the slope of the two districts. 

 The dissection of the peneplain about Wausau is considerable, 

 the valleys having a depth of 200 to 300 feet. Going south- 

 ward toward the sandstone district, the dissection gradually grows 

 less, and the valley bottoms in the pre-Cambrian gradually rise 

 nearer and nearer to the level of the peneplain. At Grand 

 Rapids, at the border of the sandstone district, the Wisconsin val- 

 ley bottom is even with the peneplain, showing the sandstone 

 along the river banks and the beveled pre-Cambrian in the river 

 bottom. At Grand Rapids the slope of the peneplain to the north 

 is seen to be uniform, not with the more gentle slope of the sand- 

 stone district to the south, but with the less gentle slope of the 

 pre-Cambrian surface in the Wisconsin river bottom. Thus it is 

 shown that the deeply dissected peneplain about Wausau changes 

 to the slightly dissected peneplain covered with isolated sand- 

 stone remnants immediately north of Grand Rapids, and at 

 Grand Rapids and also at Pittsville on the Yellow river and 

 Neillsville on the Black river the peneplain is seen to dip under 

 the Potsdam sandstone and become a buried peneplain. It is 

 therefore concluded that the peneplain of the pre-Cambrian area 

 is older than the sandstone ; that is, it is of pre-Potsdam age. 



A further evidence of the pre-Potsdam age of the peneplain 

 is believed to be shown by the occurrence of thick deposits of 

 clay lying beneath the Potsdam sandstone. The clay is not of 

 sedimentary origin, but is decomposed pre-Cambrian rock, and 

 generally has a thickness of 10 to 20 feet, sometimes attaining 

 a thickness of 40 feet. The residual character of the clay and 

 its widespread distribution beneath a variable thickness of sand- 

 stone, it is believed, point to its origin before the deposition of 

 the overlying Potsdam sandstone, at a time when the pre-Cam- 

 brian land was so flat-lying, that is, so near base-level, that 

 conditions were favorable for the deep weathering and decom- 

 position of the pre-Cambrian surface without removal by erosion. 

 The absence of conglomerate at the base of the Potsdam, with 

 the exception of the conglomerate beds about the monadnocks, 

 which are in their simple character in marked contrast with the 



