PALEOZOIC ROCKS FOUND IN DEEP WELLS 



541 



and elsewhere they are a very difficult horizon to penetrate with the 

 drill. The caved material mixes with cuttings from lower horizons 

 making some records very hard to interpret. The record is typical 

 of the St. Peter where the basal beds are thick (Table II). The 

 thickness of the St. Peter sandstone and underlying shales and 

 conglomerate is known to reach at least 325 feet and may possibly 

 exceed 600 feet in IlHnois; in Wisconsin the maximum thus far 



TABLE II 



Partial Log of Waterworks Well, Shtjllsbxjrg, Wisconsin 



Depth 

 (Feet) 



Sandstone, coarse, gray, very calcareous; pyrite 



Sandstone, medium to fine, light gray 



Sandstone, medium to coarse, light gray 



Sandstone, medium, light yellowish gray 



Sandstone, medium, light pink 



Sandstone, medium, light yellowish gray 



Sandstone, medium to coarse, reddish gray; some red shale at 



top 



Sandstone, medium, light yellowish gray 



Sandstone, coarse, red; shale, red 



Shale, dark red with greenish gray spots and pebbles of white 



chert 



Conglomerate: pebbles white chert, matrix red shale 



Shale, red with greenish gray spots and a few pebbles of white 



chert 



Conglomerate like above 



Shale, red and green mixed, pebbles of white chert and some sand 

 Sandstone, fine, gray, very calcareous; chert and red shale, caves 



badly, probably conglomerate 



Sandstone, fine, gray, pink and green mixed; and red shale, 



caves badly, probably conglomerate 



Total thickness 



recorded in 332 feet, at ShuUsburg. Its average thickness is very 

 much less. 



Suh-St. Peter unconformity. — Study of well records leaves no 

 doubt that there is an unconformity of great magnitude at the base 

 of the St. Peter. The shales at this horizon are virtually all non- 

 calcareous and appear to be more or less deoxidized residium from 

 the underlying dolomites. The chert beds and conglomerates 

 represent assortment of residual deposits by water. The rehef 

 of the surface of the underlying dolomites is over 300 feet; places 

 with no St. Peter sandstone occur within a few miles of locahties 



