THE PRE-POTSDAM PENEPLAIN OF THE PRE-CAM- 

 BRIAN OF NORTH-CENTRAL WISCONSIN. 



The region of central and northern Wisconsin consists princi- 

 pally of various igneous and sedimentary rocks of pre-Cambrian 

 age surrounded and overlapped on the southeast, south, and 

 southwest by nearly horizontal beds of Paleozoic sedimentaries 

 having a slight dip downward and away from the central core of 

 the pre-Cambrian. The highest portion of the pre-Cambrian 

 region is the divide between the waters of Lake Michigan and 

 the Mississippi river on the south and of Lake Superior on the 

 north. This divide lies from fifteen to thirty miles south of 

 Lake Superior and trends in a direction slightly north of east 

 into northern Michigan. The surface of the pre-Cambrian is 

 gently sloping, with a more gentle inclination to the south than 

 to the north. 



The present paper describes the physiography of the south- 

 ward-sloping portion of the pre-Cambrian area and particularly 

 that part in central Wisconsin' in Taylor, Lincoln, Marathon, 

 Portage, Wood, Clark, and Jackson counties which is adjacent 

 to the Paleozoic area. A part of this section of the state is 

 either free from drift or very sparingly covered with drift of one 

 or more of the earlier glacial epochs, and is thus better adapted 

 than other portions of the Lake Superior region for a study of 

 the older non-glacial features of the land surface. The physi- 

 ography of this area, including also the post-Cambrian of the 

 southern part of the state, has been referred to by Professor 

 Van Hise^ in a brief description of a base-level in central Wis- 

 consin, a reference to which is again made in a later part of this 

 paper. 



Surface feature of the pre-Cambrian. — If one should climb the 

 hill immediately northwest of Wausau and look eastward across 



'See Plate I, Atlas of the Geology of Wisconsin, i88l, or Plate I, Bull. IV, Wis. 

 Geol. and Nat. Hist. Surv. 



'^Science, Vol. IV(i896), pp. 57-9. 



289 



