CHAPTER II. 



PHYSICAL FEATURES. 



By Frank Levekett. 



The region embraced in this monograph includes' the deep basins of Lakes Michigan and 

 Huron as well as the country between them and its southward continuation to the glacial 

 boundary. These basins and their bordering lowlands controlled the courses of glacial move- 

 ment to a marked degree, while the highlands between the basins checked the flow of the ice 

 and gave it a lobate border. 



TOPOGRAPHY. 

 ALTITUDE. 

 MICHIGAN. 



The greater part of the bed of Lake Michigan lies between 300 feet above and 300 feet below 

 sea level, a narrow strip, trending north and south about in the direction of the axial movement 

 of the ice from latitude 44° 20' to latitude 44° 40', reaching the latter depth. 



In the Huron basin, where only a few square miles are down to sea level, the deep portion 

 trends northwest and southeast nearly at a right angle to the general direction of ice movement 

 from the Labrador center; it hes, however, more nearly in line with the probable course of ice 

 movement from the high land north of Lake Superior at an early stage of glaciation. Saginaw 

 Bay, the large southwest arm of Lake Huron, although in the line of axial movement from 

 the Labrador center, is a very shallow body of water, its bed being more than 500 feet above 

 sea level. The strong flow of ice through the Saginaw basin was probably induced by topo- 

 graphic conditions on the eastern side of Lake Huron. Ice moving into the Lake Huron basin 

 from Georgian Bay would naturally have passed on directly into the Saginaw basin. 



The altitude of the rock surface in Michigan is greatest in a strip leading from the " thumb," 

 or Huron-Saginaw upland, southwest toward the corners of Michigan, Indiana, and Ohio. The 

 culminating points, situated in Jackson and Hillsdale counties, exceed 1,100 feet above sea 

 level, and the drift knolls there reach nearly 1,300 feet. This high tract formed the belt of 

 separation between the Saginaw and Huron-Erie ice lobes. The rock surface maintains a high 

 altitude southward only to the southern part of Hillsdale County, Mich., but a prominent belt 

 of very thick drift, rising above the 1,000-foot contour, continues south westward for some dis- 

 tance into northeastern Indiana. It was heaped up between the Saginaw and Huron-Erie lobes. 



On the northwest border of the Saginaw basin is an elevated tract, the Michigan-Saginaw 

 upland, which embraces the highest points of the drift surface in the southern peninsula. As 

 shown by the contours of the topographic map ' of Michigan (PL I) about one-fourth of the 

 area north of latitude 44° has an altitude between 1,100 and 1,200 feet, and one-fourth has an 

 altitude of 1,200 to 1,700 feet. The descent from 1,100 to 900 feet is abrupt, giving the area 

 between 1,100 and 1,200 feet a shelf-like or table-land appearance. This shelf, however, is not 

 due to the configuration of the underlying rock, for so far as known the rock in this region 

 nowhere rises 900 feet above sea level; it is a great accumulation of drift, averaging perhaps 400 

 feet in depth, which was banked up between the converging ice lobes from the Michigan, the 

 northern Huron, and the Saginaw basins.. 



1 First published in Water-Supply Paper U. S. Geol. Survey No. 182. 



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