330 PLEISTOCENE OF INDIANA AND MICHIGAN. 



northern limits of the upper Algonquin beach have not yet been satisfactorily determined, and 

 the amount and differential effects of the elevation there are not accurately known. 



The middle group of Algonquin beaches comprises the Battlefield beach of Mackinac Island 

 and two or three fainter strands not far below it. These, like those above them, converge to the 

 hinge line. 



The third group comprises the Fort Brady beaches, generally several in number and of mod- 

 erate strength. These beaches he close above the Nipissing beach and appear to converge with 

 the other beaches to the hinge fine. 



The Kirkfield outlet was closed rather late in the life of Lake Algonquin. The idea held 

 formerly that the postglacial uplifts of the land in the Great Lakes region were gradual and that 

 they were evenly distributed through time is an error. The uplifting movements were evidently 

 spasmodic, relatively sudden, and rapid. Almost all the deformation has occurred since the 

 later part of the life of Lake Algonquin. 



Port Huron-Chicago stage. — When the Kirkfield outlet was abandoned it seems probable 

 that the overflow went first in greater part to Chicago, leaving only a relatively small part to flow 

 south at Port Huron. But the Chicago outlet rested on a rock sill and held firm, and the Port 

 Huron outlet deepened with relative rapidity, so that before long the lake level had fallen 10 feet 

 and the Port Huron outlet had taken almost all the overflow away from Chicago. The Toleston 

 Beach was formed at the time of this large-volume discharge at Chicago ; any earlier beach of 

 Lake Chicago controlled by the same sill must have been overwhelmed and worked over entirely 

 by the Algonquin. On this hypothesis Toleston is in reality only a local name for the part of 

 the Algonquin beach developed in the southern part of the Lake Michigan basin. 



From the fact that the Kirkfield outlet appears to have carried the whole discharge of the 

 upper three lake basins and that it was uplifted and abandoned rather late in the fife of Lake 

 Algonquin, the conclusion follows that while this outlet was active, the Chicago and Port Huron 

 outlets were both left dry. This means that during this time the shores of Lake Algonquin 

 throughout all the region south of the isobase of Kirkfield stood at a level at least a little below 

 these two abandoned outlets. Hence, the upper Algonquin beach south of the isobase of Kirk- 

 field was not made during the principal activity of that outlet, but during the Algonquin transi- 

 tion stage of the lake, when a considerable part of the overflow had left Kirkfield and gone to 

 Chicago and Port Huron. The upper Algonquin beach was therefore a transition beach, made 

 principally during the gradual lowering of the lake for 10 feet or more while the overflow was at 

 Chicago and at Port Huron, with Port Huron gradually cutting down and gaining in volume. 

 The character of the Algonquin beach agrees remarkably well with this conception of Lake 

 Algonquin's history. 



It was during the third or Port Huron-Chicago stage of Lake Algonquin that the larger part 

 of the remarkable uplift of the Great Lakes region occurred. It began when the Kirkfield outlet 

 was active, but soon raised Kirkfield higher than Port Huron and Chicago and shifted the outflow 

 to these two places. The great uplift caused a remarkable northward splitting and divergence 

 of the beaches blow the highest Algonquin. As a consequence the beaches of this stage are 

 many in number and show a large vertical divergence northward. They seem to fall readily into 

 three groups, the upper or main Algonquin, the Battlefield, and the Fort Brady. The main 

 uplift began during the second or Kirkfield stage, but much the greater part occurred during 

 the third or Port Huron-Chicago stage, and the most rapid upward movement was during the 

 making of the Battlefield group of beaches. At Sault Ste. Marie the vertical interval between 

 the highest Algonquin and the Nipissing beaches is 365 feet, whereas in the area of horizontality 

 (at Port Huron and Chicago) the interval between these same beaches is 10 or 12 feet. 



Transitional stage. — At its very end Lake Algonquin appears to have been held up by a rela- 

 tively small glacial barrier at some point in the Ottawa Valley east of Mattawa. When this last 

 dam broke out or shrank back northward the waters rushed eastward from Lake Algonquin 

 through the Mattawa Valley to temporary glacial lakes in the Ottawa, Petawawa, and Madawaska 

 valleys, and thence to the Champlain Sea, and came to a settled level in the upper lake basins 

 only when the eastward-flowing outlet had been established on the col at North Bay. 



