GEOLOGY. 235 



Great Lakes, where, as shown by the first-named geologist in particu- 

 hir, there lias been so decided tilting of the land since the retreat of 

 the hitest ice-sheet that some of the terraces and shore liues carved out 

 bj*Lake Ontario when its northern confine was the receding icesheet, 

 now incline southward as mnch as 5 feet per mile in western New York. 

 Southward tilting in the western part of the Great Lake region is also 

 indicated by the backing of water in the southern afiiueuts of Lake 

 Michigan and the consequent conversion of their mouths into swamps 

 and lakes as shown by Wooldridge.* 



The inferences of Shaler in New England and Gilbert in New York 

 as to the southward tilting of the land are in line with a notable inves- 

 tigation of the ancient terraces and beaches of the extinct Lake Agassiz 

 by Upham. As the last ice-sheet of the Pleistocene withdrew beyond 

 the divide between the Mississippi drainage and that of Hudson's Bay, 

 the waters formed by its melting were dammed by the divide and so ac- 

 cumulated in swamps, i3onds, and lakes along its front. The largest 

 of the lakes occupied the valley now drained by the Red River of the 

 North. It was a veritable mediterranean sea, albeit of fresh water, and 

 confined on the north by walls of ice alone; for at the period of its 

 maximum size it was fully 600 miles long and 200 miles in maximum 

 breadth. Pending the final melting of the northern mer de glace this 

 lake fonnd outlet over the i)ortage between Lakes Traverse and Big 

 Stone, and thence through the Minnesota and Mississippi Rivers to the 

 Gulf; and, although it has now utterly disappeared from the f<ice of 

 the earth, it has left an unmistakable record of its existence and its 

 extent in the terraces and beach lines already traced by Upham over 

 many hundreds of miles, and in the vast beds of lacustral sediments 

 which make the valley of the Red River the paradise of the wheat 

 grower.! Now the old shore lines of this extinct lake (named in 

 honor of the illustrious Swiss naturalist by Upham) are no longer hori- 

 zontal as when fashioned by the wind-swept waters, but incline south- 

 ward about 6 inches per mile on an average. This departure from hori- 

 zontality in the beach lines has indeed been ascribed (in part at least) 

 by Upham to deformation of the surface of the lake by the gravitational 

 .attraction of the contiguous ice sheet; but since it has been shown by 

 Woodward that this cause is alone incompetent under probable assump- 

 tions as to volume of the ice sheot,:j: most geologists who concern them- 

 selves with such questions have settled down to the conviction that 

 there is here another example of that southward tilting of the area of 

 Pleistocene glaciation already noted in New England by Dana and 

 others, in New York by Gilbert and his contemporaries, and about Lake 

 Michigan by Chamberlain, and morerecently by Woolbridge. Leading 

 students of the general subject of terrestrial deformation are indeed dis- 



* Am. Geologist, 1888, vol. i, pp. 143-146. ' 



t Final Report of the Geology of Minnesota, 1888, vol. ii, pp. 517, W7 ; Bull. U. S. 

 Geol. Siirv. No. 39, 1887. 



t Bull. U. S. Geol. Surv. No. 48, 1888, p. 07. 



