drift carried
down by the rivers has somewhat raised its level, but hardly to the
extent indicated by the old river beds. The question naturally arises,
Where did all the dirt come from to fill up these great river beds and
change the whole topography of the northern half of the continent? Dr.
Wright estimates that there is not less than 1,000,000 square miles of
territory in North America covered with glacial debris to an average
depth of 50 feet. Of course, the depth varies in different places from a
few inches to several hundred feet. Of the carrying power of these great
glaciers we will speak more fully in a future chapter. In preglacial
times the watershed of the Mississippi and of the great rivers east of
the Alleghany Mountains, the Susquehanna and Hudson, extended probably
farther north than it does to-day. The larger portion of the drainage
area that now finds an outlet through the River St. Lawrence at one time
undoubtedly drained off through the Mississippi Valley into the Gulf and
the Valley of the Mohawk into that of the Hudson.

It is supposed by those who have made this branch of geology a study
that prior to the glacial period a river flowed down through Lake
Superior, which connected with Lake Michigan at a point near its present
outlet at Sault Ste. Marie, the channel of the river passing down
through what is now the bottom of Lake Michigan, which had an outlet at
the head of the lake near Chicago and flowed off into the Mississippi
River. All of the lake bottoms of this great chain, with the exception
of Lake Erie, are now below sea-level. The reason for this exception
will appear further on. Before the ice age there was supposed to be no
connection between Lake Michigan and Lake Huron, as there is now,
through the Straits of Mackinac.

Another preglacial river had its rise in the region of Lake Huron and
flowed through an old river bed extending from the Georgian Bay in a
southeasterly direction through the province of Ontario, and emptied
into the present Lake Ontario. From Lake Ontario there is an old river
bed running through the Valley of the Mohawk which empties into the
Hudson at Troy. Neither of these two rivers, having their sources in the
north, found an outlet through the present St. Lawrence River. During
the time of the glacial period there is evidence that there was more
than one center of snow and ice accumulation and each of these great
centers probably had several subcenters. This th