THE P RE-POTSDAM PENEPLAIN 297 



ward along the beds of the Wisconsin, Yellow, and Black rivers, 

 beneath the Potsdam sandstone, in a plain sloping southward 

 with a pitch the same as that of the peneplain farther north. 



The age of the peneplain. — The deeply dissected peneplain 

 about Wausau thus passes into the slightly dissected peneplain, 

 covered with isolated sandstone remnants, in northern Wood and 

 Portage counties, and at Grand Rapids, Pittsville, and Neillsville 

 it is seen to slip under the Potsdam sandstone and become a 

 buried peneplain (see Plate I, Fig. 3). Hence we must conclude 

 that this peneplain was made in pre-Potsdam time. 



Monadiiocks in the dissected pe?teplai?i. — In our view at Wausau 

 it was noted that certain hills and ridges project above the flat- 

 topped uplands of the pre-Cambrian area. These are the Mosi- 

 nee hills and Hardwood hill, which are conical, and Rib hill, 

 a sharp ridge. The above-named hills consist for the most part 

 of very coarse resistant quartzite, and for this reason they were 

 not worn down during the general degradation of the surround- 

 ing area. These monadnocks, with their pointed crests, are in 

 marked contrast with the flat tops of the uplands forming the 

 peneplain. They are the remnants of a land surface older than 

 the even-crested hills, and are typical monadnocks like Mount 

 Monadnock of New Hampshire, which bears a similar relation to 

 the peneplain of erosion of southern New England. Rib hill, the 

 most prominent of these monadnocks, has a elevation of 1,942 

 feet above the sea and of 500 feet above the surrounding dis- 

 sected peneplain. Its summit reaches over 700 feet above the 

 alluvial plain of the Rib and Wisconsin rivers lying at its base, 

 and it has the distinction of being one of the highest hills in 

 the state. 



Monadjiocks i?i the slightly dissected peneplai?i. — About thirty- 

 five miles southwest of Wausau at Arpin, in central Wood county, 

 is Powers bluff, consisting of fine-grained quartzite and chert, 

 whose elevation is not known, but which apparently reaches 

 about 300 feet above the surrounding area of the slightly dis- 

 sected peneplain. This bluff stands in the midst of isolated thin 

 sandstone remnants lying upon the surrounding crystalline plain, 

 and wrapped about its base are patches of Potsdam conglom- 



