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GEOLOGY OF CENTRAL WISCONSIN.
CHAPTER V.
THE QUATERNARY DEPOSITS.
TTIE GLACIAL DRIFT.'
The first and most striking fact that presents itself to the investi-
gator of the drift phenomena of Wisconsin is the existence of an
extensive driftless region, the remainder of the state at the same
timte displaying an altogether extraordinary development of the drift
materials. In the driftless region, which occupies 12,000 square
miles of the southwestern part of Wisconsin, or nearly one fourth
the entire area of the state, the drift is not merely insignificant, but
absolutely wanting.   Except in the valleys of the largest streams,
like the Wisconsin and Mississippi, not a single erratic bowlder, nor
even a rounded stone, is to be seen throughout the district; whilst the
exception named is not really an exception, the timall gravel deposits
that occur on these streams having evidently been brought by the
rivers themselves, during their former greatly expanded condition,
from those portions of their courses that lie within the drift-bearing
regions.
The outline of the driftless area is for the most part quite
sharply defined, both by a more or less sudden cessation of the drift
materials, and by a change in the topography as the line is crossed
from one side to the other. This is more especially true of the east-
ern boundary, close to which are often found heavy morainic heaps,
with numerous bowlders of a large size, and oln the different sides of
which the topographical effects of purely subaerial erosion with-
out drift, and those of partial glacial erosion. with drift, are strongly
contrasted. The northern b)oundary, oln the contrary, is largely in a
level country, the drift naterials increasing quite gradually in quan-
tity as it is left behind to the southward.
' It is not possible to give, in the space available, m ore than a brief
summary of the
large amount of material with regard to the glacial drift, that has been
obtained during
the survey of the Central Wisconsin district, -with the main conclusions
reached.
These, it is believed, when taken together with the results of the work of
other meam-
bers of the corps, will be found of considerable interest.
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