PREGEAGIAE VALEEVS OF THE MISSISSIPPI. 759 
north of this line only slight irregularities of bottom appear. 
But the north end of the basin is very irregular in depth. 
The low altitudes of the rock floor in northeastern Illinois 
and northwestern Indiana seem readily accounted for by a dif- 
ferential crust movement without the aid of glacial erosion. 
But in producing the variations in altitude displayed by the 
floor of the Lake Michigan basin glacial erosion was probably 
an important agency. With fuller light concerning the floors of 
the great valleys parallel with the lake basin there will probably 
be developed criteria for estimating, more accurately than is now 
possible, the effect of each agency which has been influential in 
the shaping of the lake basin. 
The Ohio Valley—Concerning this great eastern tributary of 
the Mississippi we have space for but a few remarks. 
The lower portion of the Ohio valley has received, as yet, 
- very little attention and we have data concerning the altitude of 
the rock floor at butone point below Lawrenceburg, Indiana, 
viz, Shawneetown, Illinois, where it is 65 feet below the present 
stream. At Lawrenceburg, Indiana, and Cincinnati, Ohio, the rock 
floor is but 50 to 75 feet below low water, or but little below the 
scour of the present stream. Above these points, so far as 
ascertained, the rock floor is usually within 30 to 60 feet of the 
present stream bed until we reach the upper Allegheny, when it 
drops down rapidly and leads through buried channels to the 
Lake Erie basin. The northern tributaries of the Ohio, being 
as a rule deeply filled with drift, are now flowing at considerable 
heights above the rock floor. The present streams show a much 
more rapid descent than the rock floors, even where the rock 
floors slope toward the Ohio. In the headwater portions of 
these south-flowing tributaries, the rock floor has been found in 
several instances to slope northward. This fact, together with 
the occurrence of deeply filled channels, leading across the 
present watershed, is thought to indicate that the drainage basin 
of the Ohio has been greatly enlarged, through the influence 
of the glacial invasions and deposits within the state of Ohio, as 
well as in Pennsylvania and western New York. 
The depth of preglacial erosion in the upper Ohio region is a 
