258 



which flowed across the present state of Michigan. Withdrawal of 

 the ice-sheet uncovered an opening in the Mohawk Valley through 

 which was drained Lake Warren, formed by the coalescing of the 

 lakes in the Huron, Erie, Ontario, and Saginaw basins. Contempo- 

 raneous with this new outlet was the abandonment of the Grand 

 River outlet into Lake Chicago. As the ice withdrew further, the 

 lakes in the Michigan and Huron basins coalesced through the Straits 

 of Mackinac, and the dismembemient of Lake Warren followed. 

 With the uncovering of the Superior basin the lakes of that region 

 together with those of the Michigan and Huron basins formed Lake 

 Algonquin, which at first had a discharge through Port Huron and, 

 at times of high water, through the Chicago outlet also. It seems 

 possible that there may have been, in addition, an outlet to Lake Iro- 

 quois through the Trent Valley in Ontario. The land in the north- 

 east began to rise when relieved of the weight of the glacier, and 

 both Chicago and Port Huron outlets were in use until the Port Hu- 

 ron outlet was lowered, when this received all the drainage. 



The next step was the opening of a pass near North Bay, Ontario, 

 which resulted in what are tenned the Nipissing Great Lakes. These 

 were at a low stage and discharged through the northeastern outlet. 

 Warping of the land there, however, finally brought the water up to 

 the Port Huron level, and when the land in the northeast continued 

 to rise the Port Huron outlet was resumed. From that time to the 

 present, such changes in level as have occurred, are due to the wid- 

 ening and deepening of the Port Huron channel and to the fluctua- 

 tions incident to variations in rainfall. Detailed accounts of the his- 

 tory of the lakes since the glacial epoch can be found in nearly any 

 w^ork dealing with the geology or physiography of the Upper Lakes 

 region. The three following have been consulted especially : 



GoLDTHWAiT, J. W. The Abandoned Shore-Lines of Eastern 

 Wisconsin. Wisconsin Geological and Natural History Sur- 

 vey, Bulletin 17:2-9. 1907. 



GoLDTHWAiT, J. W. The Records of the Extinct Lakes. Illinois 

 State Geological Survey, Bulletin 7:54-68. 1908. 



LEvere^tt, Fr.\nk. Outline of the History of the Great Lakes. 

 Twelfth Report of the Michigan Academy of Science, pp. 19- 

 42. 1910. 



The Beach area itself consists merely of sand-bars which were 

 formed during the Tolleston stage, at which time the water was cut- 

 ting into the Calumet ridge. The sudden drop in level which ended 

 the Tolleston stage left these sand-bars emerged. Fonnerly this ter- 



