352 ARTHUR C. TROWBRIDGE 



Wisconsin glacier is to be read only from the study of the topography 

 and surficial deposits of this and surrounding districts. 



The flat summit areas at various places on the South Range at 

 altitudes between 1,400 and 1,480 feet, lying across the beveled 

 edges of the quartzite beds, can be interpreted only as an ancient 

 plain of degradation now almost destroyed by streams working in a 

 later erosion cycle. The peneplain might be considered to be of 

 pre-Cambrian age but for the fact that there are stream gravels on 

 its surface which are composed of Paleozoic rocks, and contain 

 fragments in which are imbedded Niagaran fossils. Associated 

 with the gravel there are numbers of potholes which are not filled 

 by Paleozoic sediments and which probably are of post-Niagaran 

 age. The idea that the flat is a remnant of the old pre-Cambrian 

 plain buried by Paleozoic formations, resurrected by later erosion, 

 and becoming again the site of deposition as a part of the later pene- 

 plain, is perhaps tenable, although hardly probable, for the extension 

 of this plain has now been traced west into Iowa and south into 

 Illinois, in both of which states it cuts across the beveled edges 

 of the Potsdam, Prairie du Chien, St. Peter, Platteville, Galena, 

 and Niagara formations in order. 



It is not to be understood that this erosion surface had reached 

 a final stage of degradation and was perfectly flat. It is made 

 clear by a study of the Devils Lake district, as of other districts 

 where the plain is known, that it had considerable relief. West 

 of Devils Lake the surface of the plain is 1 ,400 feet A.T., which seems 

 to be the altitude of the portion which was brought to grade. East 

 of the lake the gravels lie on a flattish surface at 1,470 feet, and 

 quartzite at Sauk Point reaches an altitude of about 1,600 feet. 

 Before this surface was dissected, it had a relief of 200 feet in the 

 Devils Lake district, and the gravel occupies a position between the 

 lowest and highest portions of the surface. The gravels probably 

 were not brought from a distance by a long and large stream at 

 grade, but were more likely to have been deposited by a stream 

 tributary to main drainage, the tributary having enough velocity 

 to carry gravel and to cut potholes. The gravels include no 

 material which could not have been derived from local formations. 

 On the other hand, the relief of the surface must have been much 



