THE ANCIENT OUTLET OF LAKE MICHIGAN. 217 



THE ANCIENT OUTLET OF LAKE MICHIGAN. 



By Peof. W. M. DAVIS, 



HARVARD UNIVERSITY, CAMBRIUGK, 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 overiiow 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 north- 

 east, 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 posses- 

 sion of the broad trough, to which it was once but a humble tribu- 

 tary." Mr. Gilbert's further account of the peculiar river courses 

 of that district is extremely interesting, and illustrates to perfec- 

 tion 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 sug- 

 gested 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 possi- 

 bility 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 theland was depressed and not raised in the northeast at the 

 time of this and other similar overflows. 



