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GEOGRAPHY OF SOUTHEASTERN WISCONSIN
during which the snow accumulated and the ice moved south-
ward-and warmer, interglacial periods, during which the ice
melted and the southern margin of the glacier receded toward
the north-at least receded from the section of the continent
including Wisconsin.'
THE ORIGIN OF LAKE MICHIGAN
Lake Michigan and the other Great Lakes did not exist before
the Glacial Period. The basin now occupied by the waters of
Lake Michigan was a former river valley which was later
deepened and broadened by glacial erosion. Before the Glacial
Period a river of considerable size is believed to have flowed
southward through this valley to join the Mississippi, and this
valley offered an easy path of movement for the ice. There is
complete evidence that one of the principal lobes of the great
continental ice sheet occupied it, and that a great deal of glacial
ice traversed the valley from north to south. A smaller lobe
passed southward through the valley in which Green Bay and
Lake Winnebago now lie. Other lobes of the glacier traversed
the basins now occupied by the other Great Lakes, and deepened
them by erosive action. (Figs. 6 and 7.)
Rivers do not erode their channels very much below sea
level, but the bottom of Lake Michigan is, in the deepest place,
nearly 300 feet below. The shape of the depression now occu-
pied by the lake is exactly what we should expect of a basin
deepened by ice erosion. There is good reason for believing
that the glacier, by eroding the rock, deepened the old river
valley to the extent of 500 to 900 feett and thus made the rock
basin in which Lake Michigan is now held.
Another phase of the work of the glacier also helped to make
Lake Michigan. When the glacier melted, it released a large
amount of clay, sand, and stones which it carried, and laid
them down in the form of moraines. The most conspicuous
of these are the terminal moraines which were built up along
* For further discussion of this subject see the following:
Martin, Lawrence, The Physical Geography of Wisconsin, Wis. Geol. & Nat-
Hist. Surv. Bull. XXXVI, pp. 221-254; also consult index.
Alden, W. C. The Quarternary Geology of Southeastern Wisconsin, U. S.
Geol. Survey Professional Paper 106 (1918), Chaps. V-XI.
Alden, W. C. The Delavan Lobe of the Lake Michigan Glacier, U. S. Geol.
Survey Professional Paper 34 (1904).
t For fuller details see Martin, Lawrence, Physical Geography of Wisconsin,
Wis. Geol. & Nat. Hist. Survey, Bull. XXXVI, pp. 222-238.
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