28 TWELFTH REPORT. 



which for any given line is at a right angle to the isobase, varies on 

 different lines from abont 10" to nearly 30^ east of north. It appears 

 that the entire basin lies within the region of uplift, for there is a de- 

 crease in altitude clear to the sonthwest end of the lake. 



It will be observed that the head of the outlet is only 1,013 feet 

 while the lowest part of the upper shore is 1.066 feet, or a difference 

 of 53 feet. It is scarcely to be sujiposed that the outlet carried a depth 

 of 50 feet of water at its head. Instead it seems probable that it was 

 deepened 40 to 50 feet after the lake waters were turned into it. In 

 fact the head of the outlet is low enough to afford disclmrge for the 

 lake after it had dropped to the level of its second beach. 



Lake Duluth represents a somewhat long stage in the glacial lake 

 history. Its duration is measurable either in terms of ice recession or 

 of work by the lake and its outlet. The recession of the ice border 

 s-eems to have been more than 100 miles in the axis of the lobe and per- 

 haps an even greater distance in the region north of Lake Superior. The 

 time involved in such a recession must be expressed in hundreds if not 

 in thousands of years. The Avork accomplished in the outlet has not 

 been fuly ascertained for studies have been restricted to the immediate 

 head of the outlet. At the head of the outlet there ai^pears to have been 

 a deepening of 40 to 50 feet in a channel nearly a mile in average 

 width cut in sandy drift. The amount of work in this outlet seems to be 

 less than in outlets from glacial lakes in the Michigan and Erie basins. 

 This may be partly because of its briefer duration and partly because of 

 its smaller volume. The beaches of Lake Duluth are strong and easily 

 traced in all situations and indicate long duration as clearly as the outlet 

 channel. 



The occurrence of certain beaches in the western Superior basin that 

 are too low to open into the St. Croix outlet and too high to correlate 

 with the highest beach having an eastward outlet, the Algonquin beach, 

 seems to demand an outlet somewhere on the north side of the basin. 

 The ice down to the time of this snp];osed northward outlet probably 

 covered the region around Lake Xipigon north of Lake Superior. A 

 further recession would have let the waters into the Nipigon basin, 

 and thence perhaps northward and westward into the basin of the great 

 Glacial Lake Agassiz. This region, however, awaits investigation to 

 settle the course or coiu-ses of discharge for the waters. With further 

 withdrawal of ice on the southern part of the Lake Superior basin the 

 waters became connected with those in the Lake Michigan and Lake 

 Huron basins to form the great Lake Algonquin, discussed below. 



GREEN BAY BASIN. 



That there was a glacial lake in the Green Bay basin with an outlet 

 into the' Wisconsin River near Portage, Wisconsin, is knoAvn from the 

 well defined outlet channel, discussed by Upham in 1903^ and by tJie 

 presence of shore features at corresponding altitude at ])oints along 

 the borders of the Green Bay basin (Fig. 3). W. C. Alden of the 

 United States Geological Survey has found a well defined shore east 

 of Lake Winnebago at about 800 feet above sea level, or only 10 to 12 



^American Geologist, Vol. 32, 1903, pp. 105-115, 330 331. 



