532 GEOLOGY OF CENTRAL WISCONSIN. 



Archaean rocks becoming more and more restricted to the stream bed, 

 until they finally disappear. 

 The boundary between the Potsdam and Lower Magnesian areas 



is much more easily traceable, it being possible in the driftless dis- 

 trict to map it with almost any degree of accuracy, the only limit be- 

 ing the amount of lime spent in following its windings. In drift- 

 covered regions this degree of accuracy is not attainable, but a dis- 

 tinct break in the topography generally suffices to give the line very 

 closely. It has already been said that in the Central Wisconsin dis- 

 trict there occur well-marked beds of passage between the Lower 

 Magnesian and the Potsdam, whose surface distribution has been 

 separately mapped. These layers, however, only occasionally have a 

 wide surface spread, appearing generally on the steep flanks of the 

 higher ground occupied by the Lower Magnesian, and thus forming 

 on the map a narrow strip along the outer edge of the Lower Magne- 

 sian area. The limits of the Lower Magnesian, and of the beds of 

 passage, are so close together that, in a general description like the 

 present, they may be regarded as one. On the east we find this 

 boundary without the limits of the Central Wisconsin district until 

 the northern line of Columbia county is reached. This county it 

 crosses in an irregular line, curving from northeast to southwest, and 

 marked by a prominent and deeply indented escarpment. North and 

 west of this line the country shows everywhere the Potsdam as the 

 surface formation, except on the summits of the numerous outliers 

 which flank the escarpment. At the southwest corner of Columbia 

 county, the Wisconsin enters upon the territory of the Lower Mag- 

 nesian, through which it cuts, however, deeply into the underlying 

 sandstone, so that along the valley bottom we have a broad strip of 

 the latter formation at surface, and along the numerous tributary 

 streams on each side, strips of greater or less width. In the Four 

 Lake country, about the city of Madison, the upper layers of the 

 Lower sandstone are again brought to the surface by a different sys- 

 tem of erosion, that of one of the main branch streams of Rock river. 

 The valley surface is never, however, more than 30 to 50 feet below 

 the summit of the sandstone (the Mendota base), and south of Lake 

 Monona the southerly dip carries even the uppermost beds below the 

 valley bottom. In Sauk county, north of the Wisconsin, the boundary 

 of the main Potsdam area follows the west side of the town of Honey 

 Creek, then bending around the western end of the Baraboo quartzite 

 ranges, in the towns of Westfield and Honey Creek, crosses Eeedsburg, 

 Ironton, La Yalle and Woodland, in a northwesterly direction along 

 the west side of the valley of the Baraboo river to the very south- 



