MICHIGAN ACADEMY OF SCIENCE. 99 



Valley, and is essentially of pre-glacial origin. The area in the \dcinity of 

 Spring Valley lies within the margin of the Illinoian drift sheet which trends 

 southward through the western parts of Dunn and Pierce counties a few 

 miles to the east.* However, the drift is here so thin that it does not modify 

 to any great extent the pre-glacial features of the topography. The Eau 

 Galle river, which flows southeastward to the Chippewa, and its tributary 

 creeks are flowing in rather steep sided valleys deeply incised below the 

 uplands. Were the present valleys filled to the average height of the 

 uplands the resulting surface would be a plain. Such a plain probably 

 existed here, in the past^ as part of a greater one which extended over a 

 surrounding broad area and was probably continuous with the southern 

 Wisconsin peneplain, remnants of which are still preserved in the crests 

 of ridges and uplands which rise to an approximately even plane horizon. 

 Above the horizon marked by the inter stream uplands of southwestern 

 AVisconsin, rise the monadnocks represented by the Blue, Platte, and Sin- 

 siniwa Mounds, and below it are the valleys in some of which rivers are still 

 flowing in the meandering courses developed on the ancient peneplain. f 



The erosional history of the driftless area of the Tapper Mississippi Valley 

 is recorded in the features of the present topography, which is also an index 

 to the pre-glacial erosional history of an area of much broader Ijut undeter- 

 minable extent, including the vicinity of Spring Valiey, now covered in part 

 ])y deposits of glacial drift completely surrounding the driftless area. J 

 As the formation of the ore deposits described above was an incident 

 in the physiographic de^'elopment of this region, the physical history will 

 be briefly summarized. 



A glance at the geological map of Wisconsin shows a concentric banded 

 surface distribution of the successive rock strata around the southern edge 

 of the northern Wisconsin highland of Pre-Cambrian rocks. This distribu- 

 tion is due in part perhaps to each of three factors; (1) the highland of northern 

 Wisconsin may have existed as a land mass in the sea in which the various 

 Paleozoic sediments were deposited; (2) these sediments were later involved 

 in earth movements Avhich produced a low broad anticline pitching gently 

 southward with axis trending in a general north-south direction through 

 the central part of Wisconsin. (3) Erosion has been operative since the 

 appearance of the rocks above the sea. The crest of the fold has been more 

 deeply denuded than the limbs because of its relatively higher altitude, 

 which is seen by a glance at the lines of drainage, and the surface distribution 

 of the rocks is practically that which would be produced theoretically l:»y 

 the descent of a plane horizontal erosional surface on the low gently south- 

 ward pitching anticline. It is not possible to estimate the relative values 

 of these three factors in ])ringing about the present surface distribution of 

 the successive rock strata. Indeed, it is not certainly known that the northern 

 Wisconsin highland formed a land area in the Paleozoic sea. The occurrence 

 of Cambrian strata on the south shore of Lake Superior indicates a possible 

 former connection across the intervening highland with the Cambrian sand- 

 stone of central Wisconsin. The highland may have been covered with 

 rocks of still younger age.§ This possibility is apparently ignored by Davis 



♦Chamberlain, T. C, and Salisbury, R. D. The Driftless Area of the Upper Mississippi Valley. 

 United States Geological Survey, Sixth Annual Report. See map opp. page 259. 



tKummel, Henry B., Some Meandering Rivers of Wisconsin. Science, New Series Vol. 1, pp. 

 714-10. 1895. 



tChamberlain, T. C, and Salisbury, R. D. The Driftle.ss Area of the upper Mississippi Valley. 

 United States Cleological Survey, Sixth Annual Report, pp. 206-322. 



§Weidman, Samuel. Geologv of North Central Wisconsin, Bulletin 16. Wisconsin Geological and 

 Natural History Survey, pp. 600-601, 1907. 



