THE LOWER SILURIAN ROCKS. 599 



Immediately east of the Catfish valley the country lies higher, but soon sinks 

 again, descending with the eastward descent of the strata, this part of the county run- 

 ing from 240 to 400 feet in altitude. Here we find a gently undulating surface, the 

 ridges having a flowing contour, and all topographical features showing the linear di- 

 rection induced by the glacial movement. Numerous narrow and linear marsh strips 

 are found on the lowest portions, whilst prairies of some size occur on the highest, be- 

 ing for the most part underlaid by limestone. 



On the west side of the Catfish valley is a high and hilly belt of country from 400 to 

 600 feet in altitude, which extends southward from Middleton, along the adjoining 

 parts of Verona, Fitchburg, Oregon and Montrose. Crossing the divide in the Sugar 

 river valley, we find ourselves in an entirely different looking country one where all 

 irregularities are due solely to subaerial erosion, where the ridges are high and bold, and 

 the branch valleys ramifying, narrow, and steep-sided. The two main branches of 

 Sugar river separate on the southern line of the town of Montrose, one setting back in 

 a more westerly direction than the other. Both have numerous branch-streams, each 

 of which has its steep-sided flat- bottomed ravine. Here the ridges rise to 500 or 600 feet 

 in altitude, and are nearly always occupied by fertile prairie, whilst the valley bottoms 

 stand at 300 to 400, are wooded with a growth of small oaks, and show, rarely, narrow 

 strips of marsh. 



As to fertility of soil, Dane ranks as one of the best counties in the state. The 

 prairies, found for the most part on the higher ground, owe their especial fertility, usu- 

 ally, to the underlying limestone, but the low ground of the Catfish valley, though often 

 on the upper sandy layers of the Potsdam series, has everywhere an excellent soil, which 

 it owes to alluvial depositions, or to the drift materials. A poor soil is seen only on the 

 low grounds adjoining the Wisconsin river, where the sand comes from, the Potsdam 

 sandstone. The St. Peters sandstone rarely affects the soil over any considerable area. 

 East of the drift limit, it is buried beneath drift materials, whilst west of the same line 

 it appears only on the steep sides of ravines. The prevailing timber of Dane county is 

 email oak, occurring in patches or groves, constituting what are known as "oak open- 

 ings." 



The Dane county list of geological formations includes nearly the whole Wisconsin 

 series. The Cincinnati and Niagara, however, occur only on the Blue Mounds, and in 

 that portion of the county that is not included in the Central Wisconsin district. 



The Arcncean does not come to the surface in Dane county, but the Artesian borings 

 at Madison reach it at some 800 feet below the surface, and 480 feet below the level of 

 Lake Michigan, at winch point a dark-grey felsitic rock is -struck. Into this one of the 

 wells penetrates for 187 feet, reaching a point 667 feet below Lake Michigan, and 82 

 below the level of the sea. The Potsdam sandstone comes to the surface along the val- 

 ley of the Wisconsin, and along the bottoms of a number of smaller tributary valleys in 

 Dane, Roxbury, Berry and Cross Plains. It is also at the surface over a considerable 

 area at the head of the Catfish valley, and in the bottoms of branch valleys, in Spring- 

 field, Westport, Windsor, Burke, etc. ; but in all this area only the uppermost layers of 

 the formation are at surface. The Mendota and Madison beds are the surface rocks over 

 a large portion of the Catfish valley, reaching from the south side of Lake Monona to 

 the south side of Lake Kegonsa. These layers are at surface along some of the valley- 

 bottoms of northern Middleton, southern Springfield, and adjoining towns, as also on 

 the flanks of the higher ground and outliers that border the valley of the Wisconsin. 

 The total thickness of the two layers in Dane county is about 70 feet. The Lower Mag- 

 nesian limestone forms the upper part of all the dividing ridge of the north part of the 

 county. It fo ms, also, the flanks of the high ground on both sides of the Catfish valley, 

 whose bottotb *t becomes in the region south of Lake Kegonsa. It comes up again un- 

 derneath tno o,v marshy ground that borders Waterloo creek in York, Bristol, Sun 



