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THE ORIGIN AND PHYSICAL FEATURES           7
CHAPTER II
THE ORIGIN AND PHYSICAL FEATURES OF THE
FOX-WINNEBAGO VALLEY
How THE ROCKS OF THE VALLEY WERE MADE
In very remote ages-to be reckoned in millions of years-a
broad, shallow sea extended into the very heart of the present
continent of North America. North of this sea lay an ancient
land-mass which makes up a large part of what is now Canada,
and from which a shield-shaped portion projected southward.
Part of that shield of ancient rock now forms northern Wisconsin.
During this period, much (if not all) of Wisconsin was covered by
the sea. From the old land-mass at the north, the streams
eroded rock waste and carried it to the ocean, as streams are now
doing. This rock-waste was spread upon the bottom of the
adjacent sea and slowly built up layers of sediment, which in
time became beds of sandstone, made of sand; beds of shale,
made of clay; and beds of limestone, made of limey matter which
settled from the sea-water or accumulated from the skeletons of
corals and other. lime-using creatures. In this way the old shield
became enclosed on three sides by beds of sediments which
accumulated in the sea around it, and which lapped over one
another like shingles. The layers which were deposited first
rest upon the seaward part of the shield, while those next deposited
rest upon the ones laid down first, the third upon the second,
and so on.
THE UPLirr OF THE ROCKS
All of these layers of rock, together with the still older shield
which was well-nigh buried under the sediments, were in a later
period gradually uplifted into dry land. The sea slowly with-
drew and land took its place. This uplift caused a gentle warping
of the land now included in Wisconsin, so that the highest ground
is in the northern part of the state, while the rock layers gently
dip, or slope, toward the south, southeast, and southwest.
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