496 W. Upham — Quaternary Changes of Levels. 



according to Dr. G. M. Dawson's observations, it covered mountain 

 summits 5000 to 7200 feet above the sea. 1 



Wbile thus heavily ice-laden, nearly the whole glaciated area sank 

 below its present level, but for the most part only to a slight amount 

 in comparison with its previous elevation. Beginning at a line 

 drawn north-eastward through New York, Boston, and Nova Scotia, 

 the extent of the submergence of the land by the sea at the time of 

 recession of the ice-sheet, as shown by fossiliferous marine deposits 

 overlying the till, increases from 150 and 225 or 230 feet on the 

 coast of New Hampshire and Maine to 520 feet at Montreal ; 300 

 to 500 feet on the country south-west of James Bay ; and about 1500 

 feet, according to Dr. Robert Bell, at Nachvak on the eastern coast 

 of Labrador. In British Columbia, including Vancouver Island and 

 the Queen Charlotte Islands, Dr. Dawson finds evidence of sub- 

 mergence to the amount of 200 or 300 feet while the glacial con- 

 ditions still endured. During Postglacial time the Atlantic and 

 Pacific coasts have been again uplifted, attaining generally a some- 

 what greater height than now, the most recent movements being 

 mostly subsidence. But in the basin of Hudson Bay, and probably 

 also in Labrador and northward, the uplift from the glacial depres- 

 sion is still in progress. 2 



In the interior of the continent, the northward ascent of the 

 beaches of the glacial Lake Agassiz shows that the differential 

 uplift attending the departure of the ice-sheet amounted to about 

 one foot per mile, increasing from south to north or north-north-east, 

 in the Red River Valley and the basin of Lake Winnipeg, for 400 

 miles from Lake Traverse to the north end of Duck Mountain. 3 



On account of the same upward movement, the beach formed 

 by Lake Ontario when it was dammed on the north-east by the 

 receding ice upon the area of the Adirondacks, the Thousand Islands, 

 and the Ottawa basin, is found by Gilbert to have an ascent of 

 three feet per mile north-eastward about the west end of this lake 

 and along its south side, with increase to four or five feet per mile 

 about its east end. The Postglacial tilting of the old beaches of 

 Lake Erie, and similar changes that have been studied by Chamberlin 

 in Wisconsin between Lakes Michigan and Superior, seem to present 

 less regularity, probably because the restoration of equilibrium of 

 the earth's crust is combined with independent crustal movements 

 and strains, whereby a large part of the glacial depression of the 

 basins of these lakes is still retained. The beaches and deltas of 

 a small glacial lake in the valley of the Contoocook River, New 

 Hampshire, have a northward rise of about five feet to the mile 



1 Geol. Mag. Dec. III. Vol. YI. 1889, pp. 350-2. 



2 For a more detailed review of these Postglacial oscillations, and of Quaternary 

 movements of uplift and subsidence in other parts of the world, both in glaciated 

 and unglaciated regions, see Prof. G. F.Wright's " Ice Age in North America," 

 1889, Appendix by Warren Upham, pp. 573-595. 



3 Geol. Mag. Dec. II. Vol. X. 1883, pp. 427-8 ; Dec. III. Vol. IV. 1887, 

 pp. 344-8 ; Vol. VI- 1889, p. 37. Geologyof Minnesota, vols. i. and ii. Bulletin 39, 

 U.S. Geol. Survey. Geol. and Nat. Hist. Survey of Canada, Annual Report, vol. iv. 

 for 1888-89, Part E. 



