632 GEOLOGY OF CENTRAL WISCONSIN. 



lias supposed that these owe their origin to the melting of ice masses 

 included within the moraine materials, and this may possibly be true 

 with regard to the more regularly circular kettles. 



The thickness of the great glacier we can only conjecture. It is 

 easy to see, however, that it was at least a thousand feet, for it was 

 able to accommodate itself to variations in altitude of many hundred 

 feet. Morainic drift occurs on the summit of the Baraboo ranges 

 over 900 feet above Lake Michigan, and on the immediately adjacent 

 low ground 700 feet below. 



(3) The Driftless Region of Wisconsin owes its existence, not to 

 superior altitude, hut to the fact that the glaciers were deflected 

 from it hy the influence of the valleys of Green Bay and Lake 

 S^ipenor. Some writers ^ have thrown out the idea that the driftless 

 area is one of present great altitude compared with the i-egions around 

 it, and that by virtue of this altitude during the Glacial period it 

 caused a splitting of the general ice sheet, itself escaping glaciation. 

 This idea may have arisen from the fact that in the southern part of 

 the area the district known as the "Lead Kegion" has a considerable 

 elevation; but the facts heretofore given have shown that in reality 

 the driftless area is for the most part lower than the drift-covered 

 country immediately around; the greatest development, for instance, 

 of the western lateral moraine of the glacier of the Green Bay valley, 

 having been on the very crown of the watershed between the Lake 

 Michigan and Mississippi river slopes, whilst the driftless region is 

 altogether on the last uamed slope. Moreover, to the north, towards 

 Lake Superior, and to the west, in Minnesota, the whole country 

 covered with drift materials lies at a much greater altitude. J. D. 

 Whitney, in his report on the lead region of Wisconsin, favors the 

 idea that the driftless district stood, during the glacial times, at a 

 much greater relative altitude than now, and so escaped glaciation. 

 But it is evident that in order that this could have been the case, 

 either (1) a break or bend in the strata must have taken place along 

 the line of junction between driftless and drift-bearing regions; or 

 else (2) the driftless region has since received a relatively vastly 

 greater amount of denudation than the drift-bearing. That no break 

 or bend ever took place along the line indicated is abundantly proven 

 by the present perfect continuity of the strata on both sides of the 

 line, the whole region of Central Wisconsin being in fact one in 

 which faults of any kind are things absolutely unknown. That no 

 sensible denudation has taken place in Wisconsin since the Glacial 

 times, in either drift bearing or driftless areas, is as well proven by the 

 ' See Geological Survey of Ohio, Vol. IT. 



