58 



SCIENCE. 



[N. S. Vol. IV. No. 81. 



the tributary streams of the Wisconsin. 

 South and west of Mosinee the streams 

 wander about in an area which is largely 

 swamp, and as yet has not been fully divi- 

 ded between them. This swampy plain is 

 crossed between Knowlton and Centralia 

 by the Wisconsin Valley Division of the 

 St. Paul Railroad. This baseleveled plain 

 is easily recognized because over it is 

 spread only the very thin mantle of drift of 

 one of the earlier glacial epochs, the ice 

 of the later Wisconsin epoch not having 

 reached it.* 



From the plain south of Mosinee the eye 

 sweeps to the north over the Wausau area, 

 where the plain is much dissected by the 

 Wisconsin and a number of its larger tribu- 

 tary streams, on the west side the Little 

 Eau Pleine, the Big Eau Pleine, and the 

 Big E:ib rivers, and on the east side the 

 Eau Claire river. Rising from this low- 

 land, about 1,850 feet above sea level, is 

 Big Rib Hill, one of the highest points in 

 the State of Wisconsin. As determined by 

 aneroid measurements by E. R. Buckley 

 and Samuel Weidman, the summit is about 

 450 feet above the ancient baseleveled plain. 

 Another hill, some 12 or 15 miles to the 

 west, of unknown character, rises to a lesser 

 elevation above the plain. The exceed- 

 ingly resistant character of the Big Rib Hill 

 quartzite sufficiently explains the existence 

 of this monadnock. 



From the top of Big Rib Hill itself, about 

 5 miles southwest of Wausau, the im- 

 mediately surrounding country is seen to 

 be dissected by the large tributary rivers of 

 the Wisconsin already referred to. How- 

 ever, to the northward, 12 or 15 miles 

 away, is again seen the horizontal line 

 marking the extension of the baseleveled 

 plain in that direction, and to the southward 

 at an almost equal distance may be seen the 



*Tlie Driftless Area of the Upper Mississippi Val- 

 ley, by T. C. Chamberlin and E. D. Salisbury. Sixth 

 Ann. Eept. U. S. Geol. Surv., 1885, PL XXVII. 



almost perfect plain already described which 

 extends southward from Mosinee. 



Along the Wisconsin river from Wausau 

 to Mosinee, and adjacent to the larger 

 tributary streams, is seen a beautiful system 

 of terraces. The higher glacial flood plain 

 of the Wisconsin and its tributaries is here 

 very extensive. Descending a steep em- 

 bankment 20 or 25 feet high, one reaches a 

 lower terrace, which has very considerable 

 width in proportion to the size of the 

 streams, both in the case of the Wisconsin 

 and in the tributary streams. The present 

 channels and flood plains of the rivers are 

 cut in this intermediate terrace. The sub- 

 ordinate streams are but a few feet below 

 its surface, and as the material is soft sand 

 and gravel they have taken different 

 courses at different times. One may see 

 old channels of these subordinate streams 

 so recently abandoned, that as yet, no 

 forests have sprung up. Older channels 

 have forest growths in different stages of 

 development. On account of not having 

 any topographic map of the area and lack 

 of time, no attempt was made to work out 

 in detail the drainage history of the dis- 

 trict. 



South of Stevens Point and Grand Rapids 

 is the plain of Potsdam sandstone which 

 extends to the Baraboo bluffs, upon the 

 south, and west to the Mississippi river. 

 This is part of the driftless area of Wiscon- 

 sin. At numerous points in its eastern part 

 may be seen various sandstone buttes with 

 flat tops. It has often occurred to me that 

 these all rise to approximately the same 

 elevation, and at various times I have sug- 

 gested to my students that their tops prob- 

 ably represent an ancient baseleveled plain. 

 While not demonstrated by a comparison of 

 levels, I have little doubt that the tops of the 

 Potsdam buttes are a continuation of the 

 plain in the crystalline area to the north. 

 The advanced stage of denudation of the 

 Potsdam is due to its softness. Above the 



