University of Wisconsin Digital Collections
Link to University of Wisconsin Digital Collections
Link to University of Wisconsin Digital Collections
The State of Wisconsin Collection

Page View

Bancroft, Genevieve; Madison Public Schools (Wis.); Instructional Materials Center / Glacial geology of Wisconsin

Thwaites, F. T.
The two creeks forest bed,   pp. 21-22


Page 21

 
THE TWO CREEKS FOREST BED 
                                 F. T. Thwaites 
       It is less than a century since geologists were all convinced that
much of 
  the northern United States was once buried under immense masses of moving
  ice, glaciers. At first it was generally believed that only one ice invasion
had 
  occurred, bringing conditions like those of Greenland and Antarctica. But
the 
  discovery of buried soils and vegetation at length demonstrated that the
Ice Age 
  was not a simple, isolated event but was instead divided into a number
of glacial 
  maxima separated by intervals during which the climate of that particular
locality 
  moderated to something like the present. Among such discoveries the vicinity
of 
  Two Creeks, Wisconsin is famous. Although first described by J. W. Goldthwaite
  in 1907, it was not until the studies of L. R. Wilson from 1930 to 1936
that the 
  significance of the occurrence was well understood. 
       The lowest, and hence oldest, deposit which is exposed by storm waves
in 
 the bank of Lake Michigan is one made directly by the melting of glacial
ice, 
 a till. At this time the ice moved almost due south across this locality
as shown 
 by scratches on the limestone at Valders. That till is gray and contains
limestone 
 pebbles from the local rock along with boulders brought from far to the
north. 
 Overlying the gray till is a very tough clay from 7 to 20 feet in thickness,
re- 
 placed locally by silt and sand. The clays are banded red and gray, the
former 
 the finer material which must have settled from a glacial lake when it was
frozen 
 during the winter. Geologists term these "annual rings of the earth"
verves, but 
 despite many earnest attempts it proved impossible to connect the years
they 
 register with those of our modern calendar, for such bands are not being
formed 
 in the lakes of today. The lake in which the clays were deposited is called
Early 
 Glacial Lake Chicago and its level was maintained by the melting ice to
the 
 north. When the ice margin had melted back far enough to free the Straits
of 
 Mackinac, the water level fell below its present stage. Then trees and other
 vegetation grew on the silts and clays, although not yet found where the
soil was 
 the older till as it was on ridges in the landscape of that day. Stumps
and roots 
 of these spruce trees are often exposed in the lake bank of today. Peat
shows 
 that the land was wet and marshy. Some of the trees were 142 years old before
 the return of the glacier and the making of later Lake Chicago killed them.
 A number of different kinds of mosses, water mollusks, mites, fungi, and
wood- 
 boring beetles have been found. The outer growth rings of the trees are
narrow 
 suggesting a deterioration of climate as the ice front again advanced. Silt
was 
 then deposited around the trees but the logs were reasonably sound when
the 
 trees were pushed over by ice advancing toward the southwest. 
      In many places the silt and clay in which the forest once grew were
greatly 
disturbed by the ice and little trace is left of the Forest Bed. South of
the old 
wharf, however, the deposits were little disturbed. The ice margin at last
reached Milwaukee and on melting left a till which contains much red iron
oxide. 
21 


Go up to Top of Page