PALEOZOIC ROCKS FOUND IN DEEP WELLS 547 



most of northeastern Wisconsin. The maximum thickness of the 

 Jordan is about 75 feet; its lower Hmit is determined by the finer 

 grain and more dolomitic character of the underlying Trempealeau. 



TREMPEALEAU rORMATION 



Distribution. — The Trempealeau formation has been called the 

 St. Lawrence formation, and in older literature was correlated with 

 the Mendota of the Madison region. The definition of this forma- 

 tion has varied from merely the thin "Black Earth" or original 

 St. Lawrence dolomite bed, to all the strata between the base of 

 the Jordan and the top of the Dresbach. In view of this conflict 

 Ulrich has recently proposed a change in name.'^ 



The Trempealeau formation caps narrow terraces just beneath 

 the steep slopes of the Jordan outcrop. It is known throughout 

 western Wisconsin and northwestern Illinois, but has not been 

 definitely distinguished southeast of a northeast-southwest line 

 through Madison. 



Character. — The Trempealeau formation is divided by Ulrich 

 into the following members from base up: {a) sandy dolomitic 

 shales of local distribution, {h) St. Lawrence or Black Earth dolo- 

 mite, a rock almost exactly like the Mendota, (c) Lodi yellow 

 and purple sandy thin bedded dolomite, locally called "shale," 

 and {d) Norwalk fine grained dolomitic sandstone. Of these the 

 last is most conspicuous in western Wisconsin; along the Wis- 

 consin River the Lodi "shales" predominate and farther south 

 under cover there is less sand and {h) seems to make up the bulk 

 of the formation. The yellow color does not persist in depth but 

 is replaced by gray; this is not true of the purple tints. The base 

 of the formation is marked by a greensand conglomerate; glauconite 

 is only sparingly present at higher horizons. 



The red and purple dolomites, for the most part quite sandy, 

 that underHe the cherty gray Oneota in northeastern Wisconsin 

 have been correlated by the writer with the Trempealeau formation; 

 they are in few places separated from the younger dolomite by a 

 sandstone. In western Wisconsin, however, the Trempealeau is 

 overlain by the Jordan sandstone. 



^ At the date of writing the name Trempealeau has not been approved by the 

 Board of Geologic Names of the U.S. Geological Survey. 



