TOPOGRAPHY. 105 



tioned. Its average elevation above Lake Micliigan varies from about 

 400 feet, in the south and central portions, to 200 feet, at the north, 

 while isolated points considerably exceed these figures. From this 

 crest the surface slopes eastward to Lake Michigan, and onward under 

 its waters down to and below the sea level. With an unessential ex- 

 ception, this slope is everywhere underlaid by the Niagara limestone, 

 to the dip of which, and the softness of the underlying shale, the 

 ridge owes its origin. Indeed, the ridge is simply the projecting edge 

 of the inclined ]S"iagara strata. 'At the south, where this eastward 

 slope enters the state from Illinois, it is about forty miles wide. It 

 retains this amplitude for upwards of 100 miles, extending in an 

 almost due north course, beyond which it curves more rapidly to the 

 eastward, and gradually narrows till the limiting waters of Lake Mich- 

 igan and Green Bay on either side meet and mingle throiigh Porte 

 des Morts. But in fact it does not end here. As a submerged ridge 

 it extends onward to Michigan, its crest appearing as a line of islands, 

 stretching across to the peninsula east of Big Bay de IToquet, which 

 presents similar features due to the same cause. 



Without destroying the truth of what has been said of this east- 

 ward sloping rock-ridge, the region presents a secondary topographi- 

 cal feature of no insignificant importance. 



It consists of a line of immense drift hills, superimposed upon 

 the terrane just described, and known as the Potash Kettle, or Pots 

 and Kettles Range. As the term " Potash " has now ho special sig- 

 nificance, it will be dropped from this report. At the south these 

 drift hills rest directly upon the summit of the rock-ridge, their 

 added height reaching an altitude of from 400 to 800 feet above Lake 

 Michigan. To the northward, however, the drift moraine has a some- 

 what more easterly trend than its indurated companion, and creeps 

 down the eastern slope of the latter, until it ends in Kewaunee county, 

 midway between the shore of Lake Michigan and the rock-crest that 

 overlooks Green Bay. This modifies, somewhat, the simplicity of 

 contour that would otherwise be presented, but owing to the great ir- 

 regularity of the drift accumulation, the streams find their way across 

 it, and the drainage system of Lake Michigan is not essentially afiect- 

 ed by it. In Walworth county a branch from this drift ridge extends 

 westward, crossing the Rock river, beyond which it curves to the 

 northward, and passes beyond the limits of my district. The eifect 

 of this upon the features of the Eock river valley has already been 

 noted. 



These then are the salient topographicaZ featwes of the district, 

 the great Lake Michigan basin, the Green Bay valley, with its two- 



