Bancroft, Genevieve; Madison Public Schools (Wis.); Instructional Materials Center / Glacial geology of Wisconsin
Thwaites, F. T.
The two creeks forest bed, pp. 21-22
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
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