lOO ELEVENTH REPORT. 



Xvho hypothecates the existence of "okllancl" in northern Wisconsin and 

 explains the belted distribution of the Paleozoic strata as due mainly to the 

 southward migration of the Paleozoic shoreline thus leaving out of con- 

 sideration the effect of erosion on the distinctly anticlinal structure of the 

 rocks.* 



Broadly considered, the topography of the driftless area may Ije inter- 

 preted as the result of an uplift by which the streams were rejuvenated and 

 were thus enabled to sink their channels rapidly below the uplifted surface. 

 That the uplifted surface was degradational in origin is certain l)ut there 

 is some difference of opinion regarding its physiography. In southwestern 

 Wisconsin it was a peneplain as is indicated l^y evidence already cited. The 

 physiographic history for that part of the area as summarized by Kiimmelt 

 and concurred in by Grant J is: (1) Country almost completely baseleveled. 

 (2) A few monadnocks left on the surface of the peneplain. (3) The region 

 uplifted and the rivers rejuvenated. (4) The rivers have again cut down 

 almost to their grade. The areal extent of this peneplain has not been de- 

 termined. It has been suggested that it was local and develo])ed on the 

 surface of the Galena limestone because of the hardness of this formation. 

 However, the Galena limestone is not peculiarly, resistant to erosion and could 

 not sustain an erosional surface far above the level of the Mississippi which 

 determined the base levSl of erosion for the whole of the region comprised 

 within the limits of the driftless area. 



The description of the topography of southwestern Wisconsin also applies 

 to adjacent portions of Illinois and Iowa and, in a general way, to a very 

 large part of the driftless area to the north including the vicinity of Spring- 

 Valley. While it cannot be proven that the ancient siu'face, the horizon 

 of which is marked l^y the uplands of the driftless area, w^as a peneplain 

 throughout its entire extent, it is proba])le that the topographic cycle had 

 approached the stage of old age and in southwestern Wisconsin had nearecl 

 completion piior to the second pre-Pleistocene marked uplift. Irregularities 

 of surface, which in the earlier stages of the cycle were accentuated by the 

 varying hardnesses of the exposed rocks, had l^een greatly reduced and the 

 degradational plain beveled more or less indiscriminately, at a low angle, 

 the edges of the gently southwestward dipping strata. 



Origin of the Ores. 



Tiu'ning now to the Spring Valley area, we may conceive that prior to the 

 ^\iplift the streams were near grade and their valleys were wide and shallow. 

 The surface of the land approached a plain and was covered with residuum 

 from long ages of rock decay. The water table was near the surface and 

 consequently the belt of weathering was thin, thinner than at any time 

 earlier in the topographic cycle. The underground circulation was sluggish 

 because of the loss of head resulting from the flattening of the surface and 

 lowering of the general level of the land. Because of the low relief and the 

 consequent nearness of the water table to the surface, a relatively large part 

 of the precipitation reached the belt of saturation in which it seeped slowly 

 toward the stream channels where it finally joinetl the overground run off. 



It is well known that in the process of rock weathering ferrous iron is 

 oxidized to ferric oxide which is practically stable under weathering 



*Davis, William Morris. Physical Geography, Ginn and Co., 1899. p. 137. 



tKummel, Henry B. Some Meandering Rive-.s of Wisconsin. Science, New Series, Vol. 1, pp. 

 714-16, 1895. 



JGrant, U. S. Lead and Zinc Deposits of Wisconsin, Bulletin 14, Wisconsin Geological and 

 Natural History Survey, p. 11. 



