GENERAL GEOLOGY.


times been remarked that Lake Superior occupies a synclinal trough,
and that, therefore, no appeal to erosive forces is necessary to ac-
count for its origin. But this is a misapprehension. The stratigraph-
ical trough which it occupies was formed in ancient Keweenawan
times, and was subsequently filled - during the Cambrian and
Lower Silurian ages - to some hundreds of feet above the present
lake level, with horizontal sedimentary accumulations. It is, there-
fore, as necessary to account for its excavation as though the
Silurian beds were parts of a wide-spread horizontal series, as in the
case of the other Great Lakes. But in this instance, the south shore
is bordered, throughout almost itsAentire extent, by an elevated rock
terrane. It is only toward the east, near the extremity of the lake,
that there is any doubt as to its continuity.
Now let it be observed again that a river system, supposed to be
competent to excavate the basin of this greatest of known lakes,
should have a broad valley leading out to the sea. The eastern por-
tion of the Peninsula of Michigan presents no evidence of such a
broad, deep valley, but on the contrary, a collation of known out-
crops, shows that no broad valley existed. The glacial forces moved
in such directions as to have further excavated it, had it existed, and
to have kept it open, as in the case of the lake basins on either
hand. Its supposed position was directly in the channel of great
glacial movement, and it is quite incredible that the present topog-
raphy could have been assumed if such a broad valley ever existed.
The case is different with a canon-like valley which might be the
outlet of a somewhat considerable drainage system, which carved
the basin somewhat deeply, as rivers do. But this is quite a differ-
ent hypothesis from one which presupposes the excavation of a
broad basin, 1,000 feet or more in maximum depth. Solvent
agencies were here inoperative, because of the insoluble nature of
the sandstone removed.
We, therefore, arrive at the same conclusion concerning the pre-
glacial work of this region, that has been previously stated in re-
lation to the Lake Michigan basin, viz.: that while it was the site of
a very considerable drainage-erosion, its surface was left, like that
of the Mississippi valley, on the opposite side. of the Archaean nu-
cleus, deeply carved and creased by stream-channels, but not ex-
cavated into the broad deep basin now presented.
It will be remarked that this discussion is confined as closely as
practicable to those phases of the question of the origin of the
Great Lake basins which enter as topographical features into the
study of Wisconsin. The question of the Origin of the Great Lake


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