LATER MORAINES OP LAKE MICHIGAN, SAGINAW, AND HTJBON-ERIE LOBES. 261 



Lake Chicago west of Grand Rapids, with an altitude of about 640 feet. It therefore descended 

 about 70 feet in. 75 miles. The floor on the divide north of Bannister is 72 feet above Saginaw 

 Bay (652 feet above sea level), and when the channel extended back to the Warren beach in 

 this vicinity its altitude was about 675 feet. The second beach of Lake Chicago has an altitude 

 of about 620 feet, and the descent was therefore about 55 feet in a distance of about 100 miles. 

 If Lake Chicago fell to its third beach while the outlet was flowing the descent would be about 

 75 feet in about 110 miles, for as the lake level fell in Lake Chicago the shore moved farther 

 west from Grand Rapids. 



It is thus seen that the rate of descent of these glacial rivers was generally less than 1 foot 

 per mile, the highest rate being less than 1 \ feet per mile and the least a little more than one- 

 half foot per mile. It is worthy of note that the channel with the lowest gradient was occupied 



longest. 



INTERLOBATE AREA ON THE "THUMB" OF MICHIGAN. 



By Frank B. Taylor. 



GENERAL, FEATURES. 



The moraines bend around the great interlobate angle on the "thumb" from northeast- 

 southwest courses in western Lapeer and south-central Tuscola counties to northwest-southeast 

 courses in western Sanilac County, and to north-south or east of south courses near the forty- 

 third parallel in southeastern Lapeer and northwestern St. Clair counties. The first two or 

 three of the moraines turn more sharply than the later members of the group. They run a little 

 east of north in southwestern Lapeer County and a little east of south in the southeastern part. 

 For these earlier members the bisecting line or axis of the apex of the interlobate angle runs a 

 little west of north. With the later moraines the axis shifts to north and a little east of north. 



The acuteness of this interlobate angle is relatively moderate. In other parts of the region 

 the margins of two lobes at times faced each other directly and were partly in contact, sharply 

 contesting for possession of the narrow strip of dumping ground between them; such conditions 

 produce the most remarkable of the interlobate morainic deposits. On this part of the " thumb," 

 however, the opposing margins were not in contact at the stage of recession under consideration. 

 The first of the slender moraines curves gradually around an intervening open space nearly 20 

 miles wide at the south side of Lapeer County and 12 miles wide at the village of Lapeer, north 

 of which it follows almost a semicircle. The angle grows less and less acute with the later 

 moraines. A few of the moraines appear to be continuous around the reentrant, but others are 

 represented by scattered fragments or are in a more or less tangled condition. 



PRE-WISCONSIN RIDGES. 



Outcrops of pre-Wisconsin till near East Dayton and the numerous reports in well records 

 of hard till beneath the relatively soft Wisconsin drift at various points and especially in elevated 

 places show that the older drift is present in large amount and that it forms the nucleus of 

 some of the prominent ridges and knolls. 



North of Mayville a group of large, irregular drift masses has a relief of 100 to 150 feet. ' 

 The largest mass is in T. 11 N., R. 10 E. (Dayton Township), and runs a mile or two into the 

 eastern part of T. 11 N., R. 11 E. (Fremont Township). As a whole it is roughly triangular in 

 shape, measuring over 3 miles from north to south and about 7 miles from northeast to 

 southwest. To the northeast, beyond a plain one-half to three-quarters of a mile wide, another 

 smaller mass of the same character appears, lying mostly in sec. 31, Kingston Township, but 

 partly also in Koylton, Wells, and Dayton townships. This has its eastern end a mile west of 

 Kingston. South and east of Kingston other masses with very irregular forms rise rather 

 abruptly out of a flat clay plain. The shapes and configurations of these drift masses and the 

 absence in the particular localities of a converging ice movement to produce interlobate ridging 

 suggest pre-Wisconsin age. Confirmation of this interpretation seems to be found in the pres- 

 ence of 75 to 100 feet of hardpan in deep wells located on these ridges and of outcrops of it on 



