PALEOZOIC ROCKS FOUND IN DEEP WELLS 551 



across Illinois to the outcrops near Prairie du Chien and La Crosse, 

 Wisconsin, as well as directly northwest to Sauk City, Wisconsin. 



EAU CLAIRE EGRMATION 



Distribution. — The Eau Claire formation underlies a large part 

 of the lowlands of central Wisconsin. It forms flat-topped terraces 

 and rolling country of low relief, only occasionally making a steep 

 escarpment like that of the Dresbach. The formation outcrops 

 along Mississippi River as far south as La Crosse. The thin 

 bedded, cross bedded rock in the Dells of the Wisconsin is believed 

 to belong to the Eau Claire. 



Character. — The Eau Claire formation consists of sandstone and 

 shale whose relative proportions differ greatly according to locality. 

 In northeastern Wisconsin both outcrops and well records show 

 clearly that the entire formation is sandstone of medium to fine 

 grain, indistinguishable except by its thin bedding from either the 

 overlying or underlying formations. In western Wisconsin, where 

 the Eau Claire was first studied, the upper and lower parts are fiUed 

 with thin seams and small lenses of greenish or bluish gray shale 

 and most of the sandstone is very fine grained. There is some 

 glauconite but not so much as in the younger Eranconia; linguloid 

 shells are locally prominent. The formation with increasing 

 percentage of shale has been followed south in well records into 

 northern Illinois. The section in that region bears so close a 

 resemblance to that near Chicago that there can be no reasonable 

 doubt that the shales below the white sandstone (Dresbach) of the 

 east part of Illinois are the Eau Claire. A three-part subdivision 

 is persistent; west of Chicago a fine-grained calcareous sandstone 

 separates two members of gray and red calcareous shale or marl 

 with some dolomite beds. On the whole, however, the Eau Claire 

 is marked by extreme variability in character. Scarcely any two 

 wells record exactly the same succession in detail; beds of gray 

 marl, red marl, fine, medium, or rarely coarse-grained sandstone 

 of pink, gray, or white colors alternate with very calcareous sand- 

 stones and in places pure dolomites, but in almost every case some 

 vestige of the tripartate subdivision can be made out. Dolomite 

 beds are commonest near the bottom and the top of the formation. 



