298 5. WEI DM AN 



erate. Powers bluff bears the same relation to the surrounding 

 slightly dissected peneplain that Rib hill bears to the deeply 

 dissected peneplain about Wausau. In adjacent parts of Wood 

 county, and also in Jackson county, there are monadnocks of 

 massive resistant granite, some of which rise from 50 to lOO feet 

 above the general slope of the pre-Cambrian plain. 



From the foregoing it has been concluded that the pre-Cam- 

 brian land was a worn-down country, a peneplain of erosion, before 

 the Potsdam sandstone was deposited upon it, and that later it 

 was elevated, and through the work of erosion is in the process 

 of being uncovered and dissected. The evidence upon which 

 the pre-Potsdam age of the crystalline peneplain is based, is the 

 uniformity in slope of the dissected and buried portions of the 

 pre-Cambrian land. Ordinarily such evidence as this would be 

 deemed conclusive, and it may be so considered in this instance ; 

 but, besides the evidence of the uniformity in slope, which may 

 be considered physiographic, there are other geological evi- 

 dences in the partially uncovered portions of the pre-Cambrian 

 area which also point clearly, it is believed, to the pre-Potsdam 

 age of the peneplain. 



Residual clay at the surface of the pre-Cambria7i. — Lying at the 

 contact of the gently sloping pre-Cambrian and the Potsdam, 

 apparently everywhere except about the pre-Cambrian monad- 

 nocks, is a widespread formation of partly decomposed crystal- 

 line rock and clay. The clay occurs not only at the edge of 

 the sandstone outliers and along the river banks at the margin 

 of the sandstone district, but, as shown by a number of well- 

 borings, it is also at various distances from these now exposed 

 places and beneath a widely variable thickness of sandstone. 

 The clay varies considerably in thickness, but generally has a 

 depth of 10 to 20 feet, though in places it is known to reach the 

 unusual thickness of 40 feet. It occurs in such abundance that 

 it has been used quite extensively for many years for making 

 brick, and is outlined on Dr. E. R. Buckley's map^ of the clay 

 deposits of Wisconsin and shown to be distributed along the 

 boundary of the crystalline and sandstone district. It contains 



"■Bull. VII, Wis. Geol. and Nat. Hist. Surv., Plate I. 



