260 PLEISTOCENE OF INDIANA AND MICHIGAN. 



bars and a few bowldery patches projecting, thus turning much of the floor into arable land, 

 which is, however, subject to overflow. 



What has been said above concerning the Grand River channel refers only to its earlier 

 and relatively small volume drainage and to the early work of the Imlay outlet river. This 

 covers only the early part of the development of the great channel. For a relatively long time 

 after these two stages the channel was occupied by the outlet river of the Huron-Erie glacial 

 lakes, and it was then that the principal work was done in deepening and widening it to its 

 present large dimensions. This part of its development and history is discussed in connection 

 with glacial Lake Saginaw. (See pp. 360-361.) 



GRADIENTS OF THE GLACIAL RIVERS. 



After the retreat of the ice sheet the northern part of the region of the Great Lakes was 

 affected by differential uplifts or warpings of the land. These movements are recorded in a 

 wonderful way by the tilting of the old glacial-lake shore lines, which, although horizontal 

 when they were made, are no longer so but rise gently toward the north or northeast. The 

 uplifts and their effects on the old shore lines are discussed later (pp. 430, 461, 502-518). So far 

 as known the southern part of the lake region was not affected by the uplif ts, the old shore lines 

 in that part still remaining horizontal. The dividing line between the affected and the unaffected 

 areas is therefore not a nodal line, as it has sometimes been called, but. a "binge" line, the lands 

 only on the north side having been affected. 



The isobase of zero or the "hinge" line of the early deformation (see p. 342), which affected 

 Lakes Maumee, Arkona, and Whittlesey, passes about west-northwest 4 or 5 miles north of Bir- 

 mingham, Mich., from the middle of Lake St. Clair. The beaches of the Saginaw basin show 

 that it extends in the same direction through that area. South of this line is the "area of 

 horizontality," in which the beaches show substantially no deformation. All the drainage 

 lines mentioned, excepting a little of the Imlay channel in eastern Genesee County, he in this 

 unaffected area. The gradients of the channels in this area therefore do not now differ from 

 those which they had when they were made. 



The head of the Holly channel in Hadley Township, Lapeer County, has an altitude of 

 about 940 feet. About 2 miles west of Fowlerville, beyond which its identity is uncertain, it 

 has an altitude of 880 feet. It therefore descends about 60 feet in a distance of about 45 miles. 



The head of the Lookingglass channel was at the outlet of glacial Lake Davison, 3 miles 

 north of Linden, where the altitude is about 850 feet. At its western end, where it entered the 

 Grand River channel 2 miles northwest of Collins, its altitude is about 740 feet; the descent is 

 110 feet in about 70 miles. When the course of this river shifted to the route along the 

 St. Johns moraine and down Stony Creek, the altitude at its mouth 2 miles east of Lyons was 

 about the same as before, but the distance had been reduced to about 60 miles. 



The highest Maumee beach in Goodland Township, Lapeer County, has an altitude of about 

 S55 feet. The outlet river entered Genesee County at an altitude of about 800 feet, was at about 

 790 feet at the outlet of Kersley Lake near Duffield, entered the Grand River channel south of 

 Maple Rapids at about 710 feet, and at Ionia was at approximately 700 feet. It probablyincluded 

 a glacial lake in Lapeer County in addition to the Kersley glacial lake in Genesee County. This 

 gives a total descent of about 155 feet in 120 miles. But the head of the outlet was a little 

 over 30 miles north of the hinge line in the direction of greatest uplift and the altitude there 

 is about 55 feet higher than the highest Maumee beach in the area of horizontality (p. 342), so 

 that about 55 feet must be subtracted from the £>resent descent of the channel to find the 

 actual descent at the time the river was flowing, and this would leave about 100 feet for the 

 total descent in 120 miles. 



At the time of the middle Maumee beach the altitude at the head of the channel was about 

 25 feet lower, or 830 feet above sea level, and the river then entered Lake Saginaw at Flushing, 

 where the lake level was about 710 feet. But Flushing is about 12 miles north of the hinge line, 

 and allowing 45 feet for uplift gives a descent of 75 feet in about 52 miles. 



The Grand River channel was at first about 75 miles long, extending from the Arkona 

 beach near Maple Rapids, with an altitude of about 710 feet (aneroid), to the upper beach of 



