|THE ANCIENT OUTLET OF LAKE MICHIGAN.|
HARVARD UNIVERSITY, CAMBRIDGE, MASS.
THE reports of several of our State Geological Surveys contain references to former outlets of the Great Lakes, when their waters were for some reason turned from their present lines of discharge. A brief mention of the ancient overflow of Lake Michigan across the flat divide at Chicago and down by the Des plaines and Illinois Rivers to the Mississippi appears as long ago as 1868 in the account of the geology of Cook County, Illinois, by Bannister, in the third volume of the Geological Survey of that State; and of this more below.
A more explicit description of the ancient Maumee-Wabash outlet of Lake Erie was given more than twenty years ago by G. K. Gilbert in the first volume of the Geological Survey of Ohio. The region is very flat, with a faint divide separating the eastern and western slopes; across this divide the old channel is "not less than a mile and a half broad, and has an average depth of twenty feet, with sides and bottom of drift. For twenty-five miles this character continues, and there is no notable fall." To the northeast, the channel opens out on the even floor of an ancient lake, whose shore lines diverge to the outlet. In the southwest the channel touches bed rock at Huntington, and then descends more rapidly. Most of the flat passage from the lake outlet to the sill of rocks is now "occupied by a marsh, over which meanders the Little River, an insignificant stream, whose only claim to the title of river seems to lie in the magnitude of the deserted channel of which it is the sole occupant. At Huntington the Wabash emerges from a narrow cleft of its own carving, and takes possession of the broad trough, to which it was once but a humble tributary." Mr. Gilbert's further account of the peculiar river courses of that district is extremely interesting, and illustrates to perfection how much meaning can be given by intelligent study to a country as flat and apparently monotonous as northwestern Ohio. It is noticeable that the explanation which Mr. Gilbert then suggested for the reversal of the present overflow of Lake Erie was an uplift of the land to the northeast; but Prof. Newberry, director of the State Survey, calls attention in a footnote to the possibility that the overflow resulted from an ice barrier in the valley of the St. Lawrence; and to this conclusion nothing has lent stronger support than Mr. Gilbert's subsequent observations of marked lacustrine shore lines in New York, from which we know that the land was depressed and not raised in the northeast at the time of this and other similar overflows.