We are not permitted, however, to assume that there have been no changes of level since the deposition of these beaches surrounding the ancient glacial Lake Erie-Ontario. On the contrary, there appears to have been a considerable elevation towards the east and northeast in post-glacial times. The highest ridge south of Lake Erie, which at Fort Wayne is about 780 feet high, is now about 795 feet in Lorain County. The second of the ridges above-mentioned, which is about 740 feet above tide at Cleveland, Ohio, rises to 870 feet where the last traces of it have been discovered at Hamburg, N. Y. The third ridge, which is 673 feet at Cleveland, has risen to the height of 860 feet at Crittenden, about one hundred miles to the east of Buffalo, N. Y. A similar eastern increase of elevation is discoverable in the main ridge surrounding Lake Ontario. What Professor Spencer calls the Iroquois beach, which is 363 feet above tide at Hamilton, Ontario, has risen to a height of 484 feet near Syracuse, N. Y.; while farther to the northeast, in the vicinity of Watertown, it is upwards of 800 feet above tide. There is also a similar northward increase of elevation in the beaches surrounding the higher lands of Ontario eastward of Lake Huron and Georgian Bay. All this indicates that at the close of the Glacial period there was a subsidence of several hundred feet in the area of greatest ice-accumulation lying to the east and north of the Great Lake region. The formation of these ridges occurred during that period of subsidence. The re-elevation which followed the disappearance of the ice of course carried with it these ridges, and brought them to their present position.[CN] [Footnote CN: See Spencer, in Bulletin of the Geological Society of America, vol. ii, pp. 465-476.] In returning to consider more particularly the remarkable gorge joining the Minnesota with the Red River of the North, we are brought to the largest of the glacial lakes of this class, and to the typical place in America in which to study the temporary changes of drainage produced by the ice itself daring the periods both of its advance and of its retreat. [Illustration: Fig. 58.--Map showing the stages of recession of the ice in Minnesota as described in the text (Upham).] By turning to our general map of the glaciated region of the United States,[CO] one can readily see the relation of the valley between Lake Traverse and Big Stone Lake to an area marked as the bed of what is called Lake Agassiz. During the Glacial period Brown's Valley, the depression joining these two lakes, was the outlet of an immense body of water to the north, whose natural drainage was towards Hudson Bay or the Arctic Ocean, but which was cut off, by the advancing ice, from access to the ocean-level in that direction, and was compelled to seek an exit to the south. [Footnote CO: See page 66.]