THE HISTORY OF DEVILS LAKE, WISCONSIN 351 



But deposition did not end with the Cambrian period. The 

 Prairie du Chien and St. Peter formations, or their time equivalents, 

 must have been deposited in the gap. It is clear that the sediments 

 of the Prairie du Chien stage did not fill the gap, for the base of 

 this formation has an elevation of 1,020 feet in the vicinity, and it 

 would require a thickness of 500 feet to have filled the gap. As the 

 formation is nowhere known to be so thick, it is probable that a sag 

 existed at the site of the present gap, when the Prairie du Chien sea 

 had withdrawn, and it is possible that the gap was again occupied 

 by running water and partly re-excavated before the deposition 

 of the St. Peter sandstone. As the St. Peter sandstone is not 

 found much above 1,300 feet in the district, it seems clear that the 

 gap was not entirely filled with this deposit. 



Although no traces of the Platteville, Galena, and Maquoketa 

 formations are found in Devils Lake gap or in its immediate vicinity, 

 it seems clear that these formations, or their time equivalents, were 

 deposited in the gap, filled it, and buried it, for if the dip of these 

 formations be projected northward from their existing altitude at 

 Blue Mounds and other points to the south, the bottom of the 

 Maquoketa formation would lie 200 or 300 feet above the summit 

 plain on the South Range. Gravels containing Niagaran fossils 

 are found to the very top of the range on the flat summit plain east 

 of the lake, showing that this formation added its thickness to the 

 sediments which buried the filled gap and the ranges. 



If still younger Paleozoic formations were deposited over the 

 district and over the filled gap, they have left no record. So far as 

 the records go, the Paleozoic sea retreated finally at some date after 

 the Niagaran epoch. These seas probably left the district essen- 

 tially flat, with the rough surface of the quartzite buried beneath 

 thick Paleozoic sediments. At this time there was no Devils Lake 

 gap as a topographic feature, but there was a previously existing 

 gap, filled with sediments and buried by formations which also 

 covered the ranges deeply. 



POST-PALEOZOIC — PRE-GLACIAL HISTORY 



The record of the history of the gap following the final with- 

 drawal of the Paleozoic seas and antedating the advance of the 



