LATER MORAINES OF LAKE MICHIGAN, SAGINAW, AND HUEON-ERIE LOBES. 277 



now, or 750 feet. The northern divide is about 10 miles north of Imlay, and at the same rate 

 of rise — a little over a foot in a mile — the original altitude of that point was, say, 60 feet lower 

 than now, or 740 feet. About LV miles south of North Branch the present altitude on the 

 floor is about 785 feet, and this, measured in the northerly direction of the uplift, is about 4 

 miles farther, so that the floor was originally about 64 feet lower than now, or in round numbers 

 720 feet. 



It is thus evident that the original fall of the Imlay channel was from about 756 feet at 

 Alniont to about 720 feet at North Branch, or 36 feet, whereas the present descent is only 10 

 or 11 feet. By the windings of the channel the distance is 24 miles, giving an original descent 

 of about 1J feet per mile and a present descent of less than 6 inches per mile. 



These figures are based on the altitudes of railroads which cross the channel at Alniont, 

 Imlay, and North Branch and on aneroid measurements for the northern divide. They are 

 not as accurate as could be wished, but are probably not far wrong. 



This part of the Imlay channel is of exceptional interest on account of its relation to the 

 direction of differential uplift. So far as known, none of the other outlet rivers of the glacial 

 lakes nor any of those that ran along the edge of the ice or away from it flowed for any consid- 

 erable distance toward the north; that is, up the slope of the tilted or uplifted land. Where 

 the course of a river was along an isobase, its rate of descent was not changed by the uplift; 

 where it was down the slope of the uplift, its rate of descent was increased; but where its course 

 was up the slope, its rate of descent was diminished and might even be reversed. Where the 

 uplift increased the apparent descent of the channel neither the existence of the uplift nor its 

 general direction can be inferred safely from the channel alone. But where the descent was 

 greatly reduced or reversed this fact and the general direction of the uplift become evident. 

 This part of the Imlay channel is the only one known that affords a test of this effect of land 

 tilting. The channel is strongly developed and shows abundant evidence of scour — certainly 

 more than one would expect from a descent of 10 or 11 feet in 24 miles. From North Branch 

 to Flushing the differential uplift increased the descent of the channel 30 to 35 feet. As it is 

 now about 75 feet it must originally have been 40 to 45 feet in about 45 miles. 



The relation of the Imlay channel and of the temporary small lakes in northern Lapeer 

 Comity to Lake Maumee will be discussed later (p. 348). 



Readvance of the ice. — It was stated above that a moraine which is tentatively correlated 

 with the Fowler moraine (pp. 262, 275) appears to press westward into the Imlay channel, 

 crowding the river against its western bank. That this is a fact seems to be strongly indicated 

 by the manner in which a later moraine presses against this moraine, especially between North 

 Branch and Mill Creek. Between Mil Creek and the Deanville kames the two moraines are 

 pressed into one mass and can not be distinguished as separate inch vi duals. Between the kames 

 and North Branch their crests are a mile apart and are clearly separate. North of Imlay to 

 Mil Creek they are more lightly developed and very distinctly separated. Southeastward from 

 Imlay they are not in contact at all, but separate more and more widely toward the Macomb 

 County line. In southwestern St. Clair and northwestern Macomb counties they show the 

 spacing of 2 to 4 miles that characterizes the same group of moraines in the Saginaw Valley. 



From these facts it seems clear that this part of the Imlay outlet was strongly affected by 

 a readvance of the ice front and narrowly escaped being closed along the fine between North 

 Branch and Mil Creek. The pressure was greatest for about 4 miles between Mil Creek and 

 a small brook which descends to the channel from the Deanville kames. These two morainic 

 ridges both show clearly westward pressure or readvancing movements, the first ridge pressing 

 against the channel and almost closing it and the second ridge pressing severely against and 

 partly overriding the first. Yet they are separated by the usual interval east of Almont. The 

 closing of the channel by a readvance of the ice, so narrowly escaped at the time of both of 

 these moraines, had actually occurred at an earlier stage when a readvance closed the Lum 

 channel. In considering questions relating to oscillations and readvances of the ice front, 

 this group of facts connected with the Lum and Imlay channels and associated moraines is of 

 the greatest importance. 



