GLACIAL LAKE CHICAGO. 357 



CORRELATION. 



The Toleston beach may have been only partly formed by Lake Chicago. It has a level 

 that was closely approximated if not reached by Lake Algonquin. The occurrence of a lake at 

 this level in Algonquin time makes this beach a part of the Algonquin beach of the upper 

 Great Lakes region. 



The Algonquin beach carries in a few places molluscan shells, and this beach of Lake 

 Chicago is in places richly supplied with these shells. In this respect it contrasts with the 

 Calumet and Glenwood beaches, from which molluscan remains have as yet been reported 

 at but one locality, near Bowman ville, north of Chicago. 1 Sea shells found by Aid en and an 

 oyster shell found by the writer may have been brought in by human agencies. Although Lake 

 Algonquin did have two outlets (past Chicago and Port Huron), sufficient reason for its later 

 complete discharge by Port Huron is found in the fact that the latter outlet was through easily 

 eroded drift deposits and the Chicago outlet was over a rock sill. 



BEACHES LOWER THAN THE TOLESTON. 



Numerous ridges in the southern portion of the Lake Michigan basin at levels a little below 

 that of the Toleston beach probably belong, the higher to Lake Algonquin and the lower to 

 Lake Nipissing. Most of them rise 10 to 15 feet above Lake Michigan, or about 595 feet above 

 sea level, but some reach about 600 feet and others not more than 590 feet. The Chicago outlet 

 seems to have ceased to be functional at the time they were developed, for they in a measure 

 choke up or bridge over its head. Furthermore, they are about as low as the bed of the outlet, 

 so that no depth of water would have been possible along the outlet. These ridges are excep- 

 tionally well displayed in the south part of Chicago in the vicinity of the university, and they 

 are even better developed just east of the Illinois-Indiana State line. On a line running north 

 from Gibson station there are by actual count 32 beachlets separated by shallow sags. 



North of Waukegon a cut bank appears at a level corresponding to that of the beaches 

 in the southern end of the lake basin, its base being 12 to 15 feet above Lake Michigan level. 

 Chamberlin 3 made reference to this, and Alden 3 mentions and pictures a fragment of it. On 

 the Michigan shore east of Bass Lake in Mason County a cut bank was noted whose level at 

 base is about 12 feet above Lake Michigan. Slight indications of wave action at a level about 

 12 feet above Lake Michigan are found at several other points in recesses along the Michigan 

 shore. 



SAND DUNES. 



Attention has been directed to sand ridges and low dunes that are apparently connected 

 with the Glenwood beach. They are, however, inconspicuous compared with the dunes that 

 lie along the modern shore. Wherever the beach is sandy, dunes are in process of formation, 

 from the head of the lake in northwestern Indiana along the entire eastern shore of Lake Michi- 

 gan to the Straits of Mackinac. The longest uninterrupted stretches are between the mouths 

 of Kalamazoo and White rivers in Michigan and from the vicinity of St. Joseph, Mich., south- 

 westward past Michigan City, Ind., to the head of the lake. Very prominent dunes occupy 

 much of the interval between Ludington and Manistee, and a prominent dune belt about 15 

 miles long in northwestern Oceana and southwestern Mason counties extends several miles 

 each way from Pentwater. Many of the dunes reach an altitude of 150 feet and in a few places 

 exceed 200 feet. The highest are confined to a belt scarcely a mile in width, but lower ones 

 appear for several miles back of these, in the sandy area between Holland and Muskegon and 

 in that west from Michigan City, Ind. 



Dunes are lacking chiefly at points where the lake is encroaching on morainic ridges, as 

 on those of the Lake Border morainic system in the southern end of the basin. 



1 Baker, F. C, Science, new ser., vol. 31, 1910, p. 715. 



2 Geology of Wisconsin, 1873-1S77, vol. 2, pp. 227-228. 



3 Alden, W. C., Milwaukee folio (No. 110), Geol. Atlas U. S., U. S. Geol. Survey, 1906, p. 9. 



