16 BULLETIN 142, U. S. DEPARTMENT OF AGRICULTURE. 
they are by no means numerous in northwestern Ohio and northern 
Indiana. In eastern Wisconsin there are some clifflike outcrops of 
limestone where the deposits of drift are relatively thin and do not 
entirely cover the irregularities of the preglacial topography. They 
are most numerous to the east of the basin of Green Bay, from the 
center of the Door Peninsula southwestward beyond Fond du Lac. 
Ledges of limestone and sandstone are exposed also along the west- 
ern margin of the glaciated area of southern Wisconsin, from the 
vicinity of Portage to Beloit, Wis. These mark the thin outer edge 
of the till sheet. 
Within the main glaciated regions there are numerous small areas 
where the till sheet is only a few feet in thickness, and in some of 
these localities erosion has so far removed the surface covering that 
only the surface soils are derived directly from the till, while the 
deeper subsoils are formed from materials resulting from the partial 
weathering of the underlying rock. The total extent of these areas 
which are occupied by rock outcrop or by a thin veneer of glacial 
material over the local rock is so small that they are relatively unim- 
portant. 
It is probable that, taking the region as a whole, the depths of the 
different till sheets average as much as 150 feet and that the area in 
which the till covering is 100 feet or more in depth greatly exceeds 
that in which the average thickness is less than 100 feet. The depth 
of the later Wisconsin till alone varies from a few feet to more than 
150 feet. 
There is one other common characteristic of the majority of the 
areas in which the soils of the Miami series are extensively devel- 
oped. Owing to the irregularities of surface configuration the re- 
gion is one within which numerous large and small areas of ponded 
and obstructed drainage exist. In the regions of greatest variation 
in relief, such as in southern Michigan and eastern Wisconsin, there 
are numerous large and small lakes and many depressed areas which 
are either in a swampy condition or have remained poorly drained 
until within the time of human occupation. Even within the gently 
rolling to undulating region of western Ohio and central Indiana 
the hollows and extremely level areas were poorly drained in their 
natural condition. In all such localities there has been an accumu- 
lation of partially decayed organic matter which gives a distinctly 
black or very dark brown color to the surface soils. For this reason 
the surface of the broad region chiefly occupied by the lighter col- 
ored soils of the Miami series is frequently and repeatedly inter- 
rupted by large and small areas of these darker soils which have 
been correlated mainly with the soils of the Clyde series. Where 
such areas are of too small extent to be separated on the scale used 
m^. 
