Topography and Geological Structure. 
47 
areas of Potsdam sandstone. The general altitude of the valley 
is from 250 to 300 feet above Lake Michigan. The surface is 
occasionally rather irregular owing to the varying thickness of 
the drift deposit. This is especially true in the immediate 
vicinity of Madison. 
In the southwestern part of the county, separated from the 
Yahara valley by a belt of hilly country, is the Sugar river val¬ 
ley. The slope is here almost directly south. The surface for¬ 
mations are principally St. Peter’s sandstone, Trenton and Galena 
limestone frequently occurring in limited areas. 
East of the Yahara valley and separated from it by a ridge 
varying from 240 to 400 feet in altitude above Lake Michigan, is 
the third drainage area in that portion of the Pock river basin 
included in Dane county. Here the slope is nearly east with a 
gentle, undulating descent. In the northern part of this region y 
Lower Magnesian and Trenton limestone form the surface strata; 
in the middle area, St. Peter’s sandstone prevails, while Trenton 
limestone forms the greater part of the surface rock in the 
southern part. 
The soil of Dane county is, in general, very fertile, the chief 
exception being found in the sandy lowlands along the Wiscon¬ 
sin river. Since the soil derives its character to a great degree 
from the underlying strata, we might expect that the Yahara 
valley with its Potsdam foundation would also present a sandy 
soil. On the contrary, drift and alluvial additions have ren¬ 
dered it most productive. In the limestone region the char¬ 
acteristic fertile soil is found. 
Although the meteorological observations here quoted were 
made at Madison, it may be assumed that they hold with suffi¬ 
cient accuracy for the entire county. 
The average annual rainfall is about 34.5 inches. The month 
of least average fall is February; the month of greatest fall, 
June. The average number of days in which rain or snow falls 
is 105, so that protracted drouths are very rare. 
The mean annual temperature is about 45°P. The maximum 
summer heat is about 100° and the. minimum of winter is abou 
26° below zero. Continued severe winter weather is the excep¬ 
tion. 
