THE HISTORY OF DEVILS LAKE, WISCONSIN 347 



deposited here, all but 20 feet was eroded away a mile southeast 

 of the Baraboo Narrows, before the deposition of the St. Peter 

 sandstone. At Gibraltar Rock, 8 miles southeast of Devils Lake, 

 there is a thickness of 73 feet of Prairie du Chien dolomite between 

 the St. Peter and Madison sandstones on the southeast slope of 

 the hill and none at all on the southwest slope, the St. Peter sand- 

 stone lying directly on the Madison sandstone on the southwest side. 

 It is believed that the Prairie du Chien dolomite was entirely 

 removed in other places also, before the St. Peter sandstone was 

 deposited, as on the hill a mile south of the Pewits Nest (see 

 Plate II). The St. Peter formation is a massive, medium-grained, 

 quartz formation so similar to the Madison that the two cannot be 

 separated on lithologic grounds. The St. Peter sandstone is 

 thought by some geologists to be of eolian origin, but most of it at 

 least seems to be marine. In thickness the St. Peter varies from a 

 few feet to more than 200 feet within the district. The variation 

 is due to the erosion surface on which it lies, and to post-St. Peter 

 erosion of its surface. 



Platteville limestone, Galena dolomite, and Maquoketa shale 

 are all found at Blue Mounds, 26 miles to the south of Devils Lake, 

 and its seems certain that these formations once covered the Devils 

 Lake district. If so, they were eroded away in some late Paleozoic, 

 Mesozoic, or Cenozoic erosion cycle, leaving no trace of their 

 previous existence. 



The previous existence of rocks of Silurian age in the district 

 is proved by the finding of Niagaran fossils in a gravel deposit on 

 the summit of the quartzite range east of Devils Lake. 1 



Aside from the late Ordovician and mid-Silurian rocks which 

 once undoubtedly covered the region around Devils Lake, it is 

 entirely possible, though not proved, that formations of Devonian 

 and Carboniferous age were deposited also and were subsequently 

 eroded away. Certain it is that thick deposits of rock were laid 

 over the St. Peter sandstone. 



Glacial drift and lacustrine deposits of late Pleistocene age lie 

 unconformably on all the older rocks of the district (Plate II). 



1 R. D. Salisbury, Jour. Geol., Ill, 655-67. 



