46 Cheney and True—Flora of Madison and Vicinity. 
square miles. In shape, it is almost a perfect rectangle, the- 
northwest corner being cut off by the Wisconsin river. From 
north to south, its extent is thirty miles; frcm east to west,, 
forty-two miles. 
Except an area of 120 square miles in the northwestern part,, 
which is drained by the Wisconsin river, Dane county lies in 
the Rock river basin. The water-shed between these two basins 
crosses the county in a southwesterly direction, cutting the 
north and the west lines fifteen miles and thirteen miles, re¬ 
spectively, from their intersections with the Wisconsin. There- 
is maintained, therefore, a valley of approximately regular- 
width. This ridge has an altitude of from 500 to 600 feet 
above Lake Michigan and a width sometimes equal to a township. 
Both sides are irregularly eroded, the northern side, espec¬ 
ially, being fringed out into long, abrupt headlands. The top 
is occupied by a prairie belt. Except in the middle third of its 
course, in the towns of Middleton and Cross Plains, its surface- 
stratum is Lower Magnesian limestone. Here St. Peter’s sand¬ 
stone and Trenton limestone appear. Toward the Wisconsin 
river the level is broken by outlying patches of high country.. 
The southern slope from this water-shed is much more gradual,, 
more nearly conforming to the slope of the underlying strata. 
Although included in the Rock river basin, the remaining part 
of the county is drained in three different directions by a num¬ 
ber of small streams. Chief among these is the Yahara river. 
The valley occupied by it and the chain of lakes which it con¬ 
nects, forms the central topographical feature of the county. 
This river, entering from the north, flows south to the middle of 
the county and empties into the largest and most northern of 
the lakes. From here a general southeast direction is followed 
to the point of exit from the county. At varying intervals 
along its lower course, the river widens to fill cross valleys of 
various sizes, thus forming the well-known series of lakes. 
Along this valley as far south as the lowest of these lakes, Pots¬ 
dam sandstone is the surface stratum, bordered at the sides by 
Mendota limestone and Madison sandstone (Lower Magnesian). 
Along the remainder of its course in this county, the surface- 
formations are in general Lower Magnesian limestone with limited. 
