GLACIAL AND POSTGLACIAL HISTORY OF GREAT LAKES REGION. 327 



laminated and partly pebbly and massive, which was described by Chamberlin. 1 Later study 

 of this deposit by Alden, 2 under the direction of Chamberlin, shows that the larger part of this 

 deposit, the massive pebbly clay, is a glacial till which was laid down during a readvance of 

 the glacier in the Lake Michigan basin as far south as Milwaukee and of the Green Bay lobe in 

 the Green Bay-Lake Winnebago trough to a point south of Fond du Lac, Wis. The ice also 

 crowded westward in the Fox and Wolf river valleys. The red silt composing the laminated 

 clay and the matrix of the massive pebbly clay is thought to have come from the Lake Superior 

 region, being brought into the Green Bay and Lake Michigan basins by the opening of a south- 

 ward outlet southeast of Marquette. The first opening of this outlet must have been at or near 

 the climax of the ice retreat, immediately before the readvance to the first red till moraine. 

 The phenomena indicate a readvance over a relatively wide interval, and it seems certain that 

 if a lower outlet had been opened by the retreat it was closed again by the readvance, and that 

 the level of the glacial waters in the western hah of the Lake Superior basin was raised again 

 to the level of some earlier and higher outlet. In fact, Mr. Leverett's studies have led him to 

 the tentative interpretation that at the time of this readvance the ice completely occupied the 

 western Superior basin, so that all the beaches of Lake Duluth are later. 



On the east side, the retreating ice front finally reached the straits of Mackinac, where an 

 opening allowed the waters of Lake Chicago to unite with those of the Lake Huron basin. 

 Whether this merging of the waters in the Lake Michigan and Lake Huron basins occurred 

 before or after the opening of the Trent Valley outlet in Ontario is not certainly known. 



LAKES IN THE GREEN BAY BASIN. 



Very little has been written concerning the glacial lakes in the Green Bay basin, and their 

 extent has been as yet only partly worked out. Upham 3 published a paper in 1903 suggesting 

 Jean Nicolet as the name for a lake that discharged from the Fox River drainage past Portage, 

 Wis., to the Wisconsin Valley. Upham's paper was based on a brief visit to the outlet, and 

 the existence of the lake was inferred partly from the presence of the outlet channel and 

 partly from Chamberlin's description of the red clays in the Green Bay basin as lacustrine. 4 

 Alden' s studies 2 have shown that the red clay was largely worked over and formed into 

 morainal ridges and till sheets by a readvance of the ice, so that the limits of the red 

 clay in the Green Bay basin mark a glacial instead of a lake border. Alden's studies 

 have also shown that the lake history in this basin is somewhat different from that set forth by 

 Upham. A lake, which discharged from the district south of Lake Winnebago southward past 

 Horicon into Rock River, persisted until the ice which formed the moraines at the head of 

 Lake Winnebago had receded far enough northward to open a passage westward from Oshkosh 

 to the headwater part of Fox River. Then the discharge was shifted past Portage to the 

 Wisconsin Valley. Later, when the melting of the ice cleared the Green Bay peninsula, the 

 waters lowered to the Lake Winnebago level and to a lake in the Green Bay basin by discharging 

 eastward into Lake Chicago. Similar events accompanied the preceding recession of the ice 

 front and also, in reverse order, the readvance of the ice which formed the red till moraines. 



GLACIAL LAKES IN THE LAKE SUPERIOR BASIN. 



EAELY LAKES. 



From the first small lakes at the extreme western end of the Lake Superior basin the glacial 

 waters expanded as they did in the Lake Erie and Lake Michigan basins. In- the earlier stages, 

 while the lakes were small, slight changes in outlet and level occurred, the early stages having 

 southward outlets directly or indirectly to the St. Croix Valley. 



'Geology of Wisconsin, vol. 2, 1877, pp. 221-228. 

 2 Alden, W. C personal communication, 

 a Am. Geologist, vol. 32, 1903, pp. 106-115, 330-331. 

 * Geology of Wisconsin, vol. 2, 1877, pp. 221-22S: Geologic Atlas of Wisconsin, PI. II, 1881. 



