GEOLOGY. 235 



Great Lakes, whore, as sliown by the first-named .ceoloj^ist in particu- 

 lar, tliere lias been so decided tiltin<;' of tlie land since tlic retreat of 

 the latest ice-sheet that some of the terraces and shore lines carved out 

 by Lake Ontario when its northern confine was the receding^ ice-sheet, 

 now incline southward as much 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 affluents of Lake 

 Michigan and the consequent conversion of their mouths into swanii)s 

 and lakes as shown by Wooldridge.* 



The inferences of Shaler in New England and (Hlbert in New York 

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

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

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

 the divide between the Mississippi drainage an<l that of ILidson's Bay, 

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

 cumulated in swamps, ponds, and lakes along its front. Tiie 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 found outlet over the portage between Lakes Traverse and Big 

 Stone, and tbence through the IMinnesota and Mississippi Rivers to the 

 Gulf; and, although it has now utterly disappeared from the fice of 

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

 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 ofthe wheat 

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

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

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

 war<l about G 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 sheet,f 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 7<]ngland by Dana and 

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

 Michigan by Chamberlain, and mon^ recently by Woolbridge. Leading 

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



* Am. Geologist, 1888, vol. i, pp. 14:5-146. 



t Final Report of the Geology of Minnesota, 1888, vol. n, pp. .MT, r)27 ; Bull. U. S. 

 Geol. Siiiv. No. S9, 1887. 



\ Ball. 11. S. Geol. Siirv. No. 48, 1888, p. 67. 



