256 THE GLACIAL LAKE AGASSIZ. 



From tlie western part of the basin of Lake Superior a glacial lake 

 outflowed to the Mississippi at the lowest point of the present watershed 

 between the Bois Bruld and St. Croix rivers, in northwestern Wisconsin. 

 The bed of the old outlet is 1,070 feet above the sea, or 468 feet above 

 Lake Superior, and it is bordered by bluffs of drift about 75 feet high, 

 showing that when the course of outflow began here the West Superior 

 glacial lake was approximately 550 feet above the present lake level. The 

 divide in this former watercourse is a SAvamp, extending several miles in 

 a valley eroded 75 to 100 feet below the adjoining countiy, the distance 

 between its bluffs being mostly about 1 mile, but in the naiTowest place 

 only about 1,000 feet. The highest part of the swamp at the divide is 

 1,070 feet above the sea, but it has jDrobably been filled 20 or 25 feet since 

 the lake forsook this mouth, which was thus lowered by erosion to 450 feet, 

 approximately, above the present Lake Superior. Springs in the swamp, 

 outflowing partly to the Bois Brulci and partly to the St. Croix, are nearly 

 60 feet above the long and narrow Upper St. Croix Lake, which is 1,011 

 feet above the sea. This and the similar but larger Lake St. Croix (low 

 and high water 667 to 687 feet above the sea, with maximum depth of 25 

 feet at the stage of low water), through which the river of this name flows 

 near its mouth, lie in portions of the glacial river course which have now 

 become dammed at the upper lake by the gravel and sand deposits of trib- 

 utaries, and at the lower lake by those of the Mississippi, raising the level 

 of the mouth of the St. Croix since the depaiture of the ice reduced the 

 river to its present size.^ 



Silts referable to the Western Superior glacial lake are found near 

 Superior and Duluth, and its delta deposits and shore-lines are traceable 

 here and there along the northwestern shore of Lake Superior in Minne- 

 sota, but it may well be doubted whether they extend into Canadian terri- 

 tory. Before the ice-sheet had retreated so far as to uncover the region 

 about Port Arthur, its departure from Wisconsin and the greater part of 

 Miclugan had probably permitted Lake Superior to become confluent with 

 Lake Michigan, thus forming the glacial Lake Warren, with outlet by Chi- 

 cago to the Des Plaines, Illinois, and Mississippi rivers. Like the beaches 



' Geology of Minnesota, Vol. II, pp. 642, 643. 



