THE PRE-POTSDAM PENEPLAIN 293 



hence the region must have been, not a plain, but a peneplain of 

 erosion. 



In retrospect, the pre-Cambrian area was once a mountainous 

 region. Subsequently the mountainous area was worn down by 

 erosion to a peneplain, and must necessarily have been near sea- 

 level. At a much later period the peneplain was uplifted to its 

 present elevation, and again subjected to a period of erosion 

 which is continued into the present time. Out of this ancient 

 plain of erosion the present valleys about Wausau are seen to be 

 in process of construction, and hence the region may be 

 described as a dissected peneplain. 



Present slope of the pe?ieplai?i. — Since the topography of a con- 

 siderable portion of this region has recently been mapped, the 

 elevation of a part of the peneplain is definitely known. At 

 Wausau the elevation of the dissected peneplain represented by 

 the summits of the even-crested uplands is approximately 1,420 

 feet above the sea. About twenty miles north of Wausau, in 

 the vicinity of Merrill, it is between 1,550 and 1,600 feet above 

 sea-level. North of Merrill the thick drift of the Wisconsin 

 Epoch is present, and the exact elevation of the pre-Cambrian 

 is not known ; but the slope upward to the north, so far as 

 known, is about the same as it is between Wausau and Merrill. 

 Going south of Wausati about twenty miles to the vicinity of the 

 boundary of Marathon and Portage counties, the slope gradually 

 descends to an elevation of about 1,200 feet, and twenty miles 

 still farther south, in the vicinity of Grand Rapids, at the border 

 of the Potsdam sandstone, the plain is 1,000 feet above the sea. 

 The descent of the peneplain from seven miles north of Merrill 

 to Grand Rapids, a distance of about sixty miles, where eleva- 

 tions are definitely known, is between 550 and 600 feet. 



Valleys in the dissected peneplain. — The Wisconsin river from 

 Merrill to Grand Rapids has carved a U-shaped valley with steep 

 sides and variable width in the dissected peneplain. North of 

 Merrill in the region of thick drift the valley is either post- 

 glacial or much modified by glacial deposition. That part of 

 the valley between Merrill and Wausau in the area of old thin 

 drift has a depth of 200 to 300 feet (see Plate I, Fig. i). The 



