THE PRE-POTSDAM PENEPLAIN 309 



raneous. The dissection of the pre-Cambrian peneplain about 

 Wausau, the uncovering of the buried peneplain about Grand 

 Rapids, and the degradation of the sandstone district, it is 

 believed, have thus gone on hand-in-hand under the same period 

 of sub-aerial erosion, and all phases of this work of erosion may 

 be seen as one travels from the dissected pre-Cambrian pene- 

 plain to the broad river plain of the sandstone district. 



Former exte7mon of the Potsdam. — How far to the north over 

 the pre-Cambrian land the Potsdam sandstone was once dis- 

 tributed is not now known. Isolated remnants of the sandstone 

 have been found by the writer within nine miles southeast of 

 Wausau and four miles southwest of Medford, which are points 

 much nearer the center of the pre-Cambrian area of Wisconsin 

 than it was formerly supposed the sandstone occurred.' These 

 sandstone remnants far within the general pre-Cambrian area are 

 hard, resistant, consolidated rocks, which would seem to indicate 

 that the beds now remaining were once covered over with a con- 

 siderable thickness of sandstone in order to be thus consolidated. 

 It seems not unlikely, therefore, that a great part of the ancient 

 peneplain of the pre-Cambrian land, perhaps the whole of it, was 

 once covered with the sandstone formation and that the sand- 

 stone has since been removed by erosion. 



When Van Hise^ wrote his brief sketch on the base-level 

 features of this region, it was his belief that the sloping sky-line 

 of the dissected peneplain of the district about Wausau was con- 

 tinuous with the tops of certain sandstone buttes in the Potsdam 

 district, and probably with the limestone uplands of southern 

 Wisconsin, and that, though the pre-Cambrian land may have 

 been degraded to a low slope before the deposition of the Pots- 

 dam formation, it had later been degraded, perhaps in Cretace- 

 ous time, to form a continuous peneplain across the Paleozoic and 

 pre-Cambrian formations. It is the thesis of the present paper 

 that the clear-cut peneplain of the pre-Cambrian region is buried 

 beneath the surrounding Potsdam sandstone, and therefore lies 

 at the base of the sandstone buttes in the sandstone district and 



'Plate I, Atlas of the Geology of Wisconsin, 188 1. 

 "^ Science, Vol. IV (1896), pp. 57-9. 



