eory has color given to
it by the directions of movement shown by the glacial drift.

The rounded appearance of bowlders was caused by the grinding action of
the ice. These bowlders, when they were first torn from their rocky beds
by the irresistible power of ice pressure, were rough and jagged in
shape, the same as any rock would be, torn from a quarry by a blast.
They have been smoothed and rounded by rubbing against the moving ice
and against each other in the progress of their long journey from their
original homes. Where their home was the geologist can immediately tell
upon examination. It is only necessary then to examine the bowlders of
any particular locality to determine the direction of the ice flow at
that point.

There seem to have existed centers of ice accumulation to the north of
all of the great lakes. And when they had grown to a sufficient height
they joined at their edges, making one grand glacier, the movements of
which were the resultant of the combined pressure exerted by these great
centers of power, so that all of North America north of the line of the
terminal moraine, with the exception of a small area (heretofore noted)
chiefly in Wisconsin, became covered with one vast sheet of ice.

The glacier north of Lake Superior widened out the old river bed by a
process of erosion to its present width.

There may have existed something of a lake in preglacial times, through
which the river ran, but it undoubtedly owes its present width to the
grinding action of the irresistible icebergs and the piling up of debris
on the shores. The river bed was filled up by a glacial drift at the
point of its present outlet until the lake was raised in its level much
higher than that of Lake Michigan. Another glacier plowed down through
Lake Michigan, widening it out to its present dimensions, while the
glacial drift was deposited at what is now the head of the lake, filling
up the old outlet and thus making a great dam. The damming up of these
great water courses was another cause for increasing the width of these
lakes. In a similar way Lake Erie was formed. It is supposed, however,
that this lake is entirely the product of glacial action, as there is no
evidence of an old river bed in its bottom; besides, it is much
shallower than the other lakes. The same action that formed Lake Erie
filled up the old river bed running through the province of Ontario, so
that when the ice receded Lake Erie became the new chan