74 



ULRICH: major causes of OSCIIvIvATIONS 



continuous exposures of Cambrian rocks in the bluffs and valley 

 walls along Wisconsin River. 



Beginning at Boscobel and going upstream, the Franconia, in 

 typical development, was found to hold its own for a distance 

 of about 20 miles, when it began slowly to lose thickness by 

 overlap. The succeeding 15 miles, which brought us to the town 

 of Lone Rock, sufficed to pinch the formation out entirely. 

 Beyond Lone Rock, for a distance of about 10 miles, in which we 

 passed through the town of Spring Green, the Franconia is ab- 

 sent, the top of the underlying Dresbach sandstone has risen 

 considerably above the river level and is immediately followed by 

 characteristically fossiliferous shales and limestone of St. Law- 

 rence age. (See figure 3.) 



SOUTH or BARABOO PRE- CAM BRIAN RANGES 



WEST 

 Mississipp' .■^:vet 



Green Vil !age 



Madison and 

 Devils Lake 



SHAKOPEE DOLOMITE 



SHAKOPCE. OOLjOMITE. 



ONEOTA DOU3M1TE 



_|npnAN SANDS TONg^ 

 "ST LAWRENCE' 



Fig. 3. — Section across southern Wisconsin, showing sequence of Upper 

 Cambrian (St. Croixan) and Ozarkian formations, the apparently similar strati- 

 graphic positions of the Franconia and Mazomanie formations, and the absence of 

 both on the summit of the pre-Cambrian anticline. 



Just east of Spring Green the closed contact between the Dres- 

 bach and St. Lawrence opens again to receive the wedge of mag- 

 nesian sandstone whose age was the quest of the undertaking. 

 Where first exposed in the bluffs east of Spring Green the Mazo- 

 manie sandstone, as the new formation is called, is about 10 

 feet thick. Four miles east of the town it has thickened to 80 

 feet, and at Fairy Bluff it reaches 100 feet. Wherever it rises 

 to considerable heights above the valley bottoms in Dane, Sauk, 

 and Columbia counties it forms cliffs, which is not at all true 

 of the typical Franconia. 



