351 



channels which is sliown in Fig, 4 (100, 101). Spencer suggested that Lake 

 Michigan had a preglacial outlet to the south or southwest (93). 



Upham (125, 128) took exception to Spencer's interpretation of the 

 direction of the Laurentian preglacial drainage, and offered the theory that 

 "A great trunk stream flowing south along the bed of Lake Michigan drew 

 its chief tributaries on one side from the basins of Lakes Huron, Erie and 

 Ontario, and the other side from the basin of Lake Superior." He held 

 that during the latter half of the Cretaceous period nearly all the drainage 

 area which now forms Minnesota and the drainage basin of the Missouri 

 liver was depressed and covered by the sea, while the contiguous area 

 forming the Great Lakes region was dry land and continued so up to the 

 coming of the Ice Age. The divide separating this area from the basins 

 draining to the Atlantic, extended "along the Allegheny mountain belt and 

 directly onward northeasterly to the Adirondacks, turning thence north- 

 westerly across the Ontario highlands ... to the present height of 

 land north of Lake Superior." Spencer's preglacial stream system was. 

 therefore, probably limited to the headwater streams now represented 

 by the Lake Cliamplain basin and the Saguenay and Ottawa rivers. 



Lately Grabau (4.3) has interpreted the preglacial drainage of the 

 Great Lakes region in a manner different from Spencer and Upham. His 

 theory briefly stated is this : The old surface of the pre-Cambrian rocks 

 was worn away by long continued erosion and there were laid down upon 

 them horizontally, but nnconformably, the newer beds of Ordovician and 

 Silurian rock. Then followed an uplift greater in the north, tilting the 

 new beds southward with a dip of about 2.5 feet per mile. Following the 

 uplift was a period of erosion, wherein the region "suffered an enormous 

 amount of denudation, liaving been brought to the condition of a low nearly 

 level tract or peneplain a little above sea level." Then the surface was sul)- 

 merged and beds of Devonian limestone, shales, and sandstones were laid 

 down over it. The sea bottom became dry land and another cycle of erosion 

 began. The uplifted beds formed a "broad essentially monotonous" coastal 

 plain sloping gently southward. Consequent streams flowed southward down 

 the slope. The great master streams developed were the Saginaw, Dundas 

 and Genesee rivers, and probably some of the Finger lake valleys. As ero- 

 sion proceeded, the sloping harder beds endured and cuestas were formed, 

 having tlieir steeper slopes to the nortii. Along the foot of the escarp- 

 juents the subsequent stre:in>s flowed to the master streams. The Buffalo. 



