THE CLAYS OF WISCONSIN.

CHAPTER IX.
CLAYS OF THE DRIFTLESS AREA.
The Driftless Area comprises that portion of the state
which has not been modified by the ice sheets of the glacial
period. It comprises the southeastern portion of the state
as outlined on the general map, plate I. None of the
rivers of this region with the exception of a small part of
the Wisconsin drain that portion of the state comprising
the two, areas previously described. The northern and
northeastern parts of the area have been slightly modified
with overwash from the glaciers but other than this it is a
region of typical erosion topography in which the surface
deposits are either the result of rock disintegration in situ
or deposition from  streams and air. The underlying
rocks consist of sandstone and limestone. The northern
part of the area, including Adams, Juneau, Monroe, Jack-
son, La Crosse Trempealeau and Buffalo counties, is
largely underlain with Potsdam sandstone. The southern
two-thirds of the area is mainly underlain with limestone.
The clay deposits of this region are largely residual having
been formed through the decomposition of the underlying
limestone and sandstone.
In the transported deposits large quantities of quartz
which has been derived from the decomposition of the asso-
ciated sandstone is frequently mingled with the clay result-
ing from the decomposition of the limestone. Through
the decomposition of the limestone most of the calcium
and magnesium salts are taken into, solution and removed
by percolating waters  leaving a residuum of quartz, iron
oxide, and kaolin.

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