THE PRE-POTSDAM PENEPLAIN 303 



stone. In the various wells also that penetrate the sandstone to 

 the crystalline rock the clay formation generally occurs and 

 conglomerate is rarely or never found. On the other hand, 

 about the monadnocks in the peneplain, such as Powers bluff, 

 which stand well within the region of outlying sandstone rem- 

 nants, and about the low mounds of ferruginous schist in the 

 vicinity of Black River Falls, there is a variable thickness of 

 conglomerate, as would be expected. The conglomerate about 

 the monadnocks generally, if not always, contains pebbles 

 representing simply the quartz rock or massive granite forming 

 the monadnocks which it lies against, and in this respect the 

 Potsdam conglomerate is in marked contrast with the Huronian 

 conglomerates of the region with their generally complex variety 

 of pebbles. 



Relative distribution of the pre-Potsdam clay and Potsdam 

 conglomerate. — The distribution of the clay upon the gently 

 sloping pre-Cambrian, as shown along the pre-Cambrian rock 

 rapids in the border of the sandstone district, and also about 

 and beneath the erosion remnants of sandstone upon the slightly 

 dissected peneplain, has already been described. The occur- 

 rence of the simple conglomerate about the resistant rocks of 

 the monadnocks is very obviously because sufificiently steep 

 slopes existed at these places for the continuation of- erosion 

 long after the surrounding softer rocks had been reduced to a 

 grade too low for erosion. The development of the conglomerate 

 was very probably due to the monadnocks serving as barriers 

 for the accumulation of coarse sediments by the waves of the 

 Potsdam sea, in combination with stream action down the gorges 

 and ravines of the monadnocks. 



Thus the widespread occurrence of the blanket of residual 

 clay on the gently sloping crystalline area, indicating the deep 

 weathering of the pre-Cambrian land before the deposition of 

 the Potsdam sandstone, as well as the distribution of Potsdam 

 conglomerate oMy about the isolated monadnocks, is c6mpletely 

 in harmony with the geographic evidence of the peneplain char- 

 acter of the pre-Cambrian land, and may therefore be considered 

 as additional proof of the pre-Potsdam age of the peneplain. 



