THE PRE-POTSDAM PENEPLAIN 305 



In Great Britain it was formerly believed that only sea-waves 

 could accomplish the work of reducing land areas to the near 

 level of the sea. But the presence of the residual clay forma- 

 tions beneath the sandstone, indicating the deep weathering and 

 decomposition of the pre-Cambrian surface before the Potsdam 

 formation was deposited upon it, and the occurrence of simple 

 conglomerate about the isolated monadnocks obviously indicate 

 that the region must have been flat-lying near to sea-level a long 

 time before the encroachment of the Potsdam sea. Hence the 

 sea-waves could have had little or nothing to do with the level- 

 ing of the pre-Cambrian to a peneplain, for the degradation was 

 accomplished mainly by sub-aerial erosion long before the sea 

 was present. 



The time of the co?istnictio?i of the peneplai?i a?id the deep weathering 

 of Its surface. — The period of the uplift of the pre-Cambrian hori- 

 zon and the construction of the peneplain of central Wisconsin, 

 and the deep weathering and decomposition of the surface, 

 must have occupied a long period, even as time is reckoned in 

 geological chronology. This period is certainly somewhere 

 between the age of the youngest rocks of the peneplain, the 

 Upper Huronian, and the age of the sandstone overlying them, 

 the Upper Cambrian. It seems very probable that pre-Cambrian 

 rocks younger than the Upper Huronian, the Keweenawan, may 

 also be included in this old peneplain on the south slope of the 

 crystalline district, for they are abundant in the adjoining pre- 

 Cambrian area on the north slope along the shore of Lake 

 Superior. It seems, therefore, that this period of erosion repre- 

 sents the whole of the Lower and Middle Cambrian periods, and 

 that it may also have reached back some distance into pre- 

 Cambrian time. The physical geography of the pre-Cambrian 

 land at the close of pre-Cambrian time and at the beginning of 

 Cambrian time here depicted for central Wisconsin is strikingly 

 favorable for the conditions governing the concentration of much 

 of the abundant iron ores^ in other parts of the pre-Cambrian area 

 of the Lake Superior region. These ore deposits are viewed by 



■"The Iron-Ore Deposits of the Lake Superior Region." Twenty-first Ann. 

 Rept. U. S. Geol. Stirv., 1901, Part III, p. 330. 



