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10        GEOGRAPHY OF SOUTHEASTERN WIWCONSIN
the margin of the glacier when the ice front stood in one place
for a long time, while the ice and its load of rock waste kept
pushing forward and melting along this near  stationary front.
Under such a condition great heaps of glacial iriftt were piled
up in the form of mounds and hills (Fig. 4). Several ranges
of these hills extend north and south in the counties with which
this bulletin deals (Fig. 5). In fact, most of the hills and
valleys in these counties are due to the unevenness if the glacial
deposits. If the glacial drift were all removed from this re-
gion, we should have a very different landscape; for the hills,
lakes, swamps. and prairies with which we are sow familiar
would be gone. There is scarcely a feature of tVe surface of
the ground in southeastern Wisconsin which i not due to
glaciers-mostly to glacial deposition.  Fig. t shows how
these mioraines extend parallel to the shore of Lake Michi-
gan.  The extensive deposits of drift prevent the waters of
Lake Michigan from draining southward. By xneans of the
Chicago Drainage Canal, some water from Lake Michigan is
now conducted southward into the Illinois River and thence
into the Mississippi.
LAKE MICHIGAN IN THE PAST
When the glacier advanced from the north, an arctic climate
gripped this region, and the streams changed to ice and ceased
to flow. Yet, along the southern margin of the glacier, melting
occurred in summer, and streams flowed away toward the
south. The river that flowed southward in a valley where
Lake Michigan now is, carried away the water from the Lake
Michigan lobe of the glacier (Fig. 6). When the cold climate
became milder, and the gradual melting of the ice caused the
front of the various lobes to retreat, so-called "marginal lakes"
were formed. Such a lake was formed at the southern end of
the Lake Michigan basin; it is known as Lake Chicago (Fig. 6).
This was the beginning of Lake Michigan; as the ice melted
back farther and farther, the lake increased in size until it be-
came even larger than at present. During this period of en-
largement, the lake had different outlets at different times.
When the outlet was at Chicago, the lake surface stood at a
somewhat higher level than the present surface of Lake Miclhi-
,gan, and low lands along the present margins of the lake were
t Glacial drift is the term applied to all kinds of rock waste deposited
by
glaciers when they melt. It is made up of clay, sand, gravel, bowlders, etc.