20 PROCEEDINGS OF BROOKLYN MEETING. 



leveling so far as to cover the area of lake Winnipeg. As Hind and Dawson have 

 well pointed out, it was by the erosion of the eastern portion of these beds, after 

 the great western expanse of the plains had received nearly its present form, that 

 this steep escarpment was produced.* At the time of uplifting of the plains, near 

 the beginning of the Quaternary era, this great baseleveled region appears to 

 have stretched from the Rocky mountains to the Archean hills on the eastern border 

 of the area of the later glacial lake Agassiz. The east margin of the soft Creta- 

 ceous strata was then anew subjected to rapid erosion, with the result that it was 

 almost wholly worn away to the floor of Archean gneiss and granite and the Paleo- 

 zoic limestones upon a width of 100 miles or more and to a depth westward of 300 

 to 1,000 feet, as shown by the height of the Pembina mountain and Manitoba 

 escarpment. 



In Minnesota and North Dakota the flat Red river valley plain, averaging 50 

 miles wide, with a depth of 200 to 500 feet below the country on each side, and 

 extending more than 200 miles from south to north, opening into the Manitoba 

 lake area, appears also to have been eroded at the same time. The conspicuous 

 Pembina mountain escarpment of Cretaceous shales, overspread by drift, on the 

 west side of this valley, deep wells penetrating through the drift to Cretaceous 

 beds and older strata along the low valley plain, and the topographic features of 

 the land rising eastward from it, with nearly the same rate of ascent as on the west, 

 lead to the belief that the eastern, like the western, border of this wide valley is 

 formed by an escarpment of Cretaceous shales beneath the drift. The baseleveled 

 plain of the Tertiary era has been broadly and deeply channeled during a later 

 time of high continental uplift. 



Relationship of the later Baseleveling to the Ice Age. 



Flowing so great distances before reaching the sea, the rivers of both these cycles 

 of baseleveling may have denuded their areas of drainage, during the first cycle very 

 completely and during the second partially, to broad plains, while yet the altitude 

 of the Manitoba lake region equaled or exceeded that of the present time. Lake 

 Winnipeg is 710 feet and lake Manitoba 809 feet above the sea. Newly uplifted as 

 a high plateau during the early portion of the Quaternary era, this north part of 

 the continent, rising probably somewhat faster in the Arctic region than farther 

 south, may have continued to present favorable conditions for the baseleveling of 

 the Red river valley and the district of the great Manitoba lakes until tlie mean 

 altitude of the area which became covered by the North American ice-sheet and 

 its drift was 3,000 to 5,000 feet higher than now, as indicated by the fjords and 

 submarine valleys of our northern Atlantic, Arctic and northern Pacific coasts. 

 The culmination of this uplift appears to have brought such cold and snowy climate 

 that a vast sheet of snow and ice was gradually accumulated, under whose weight 

 the land finally sank mostly somewhat below its present height, causing the ice- 

 sheet to be melted away, with deposition of its glacial and modified drift. 



This paper is published in full in the American Geologist, vol. xiv, 

 October, 1894, pp. 235-246. 



* H- Y. Hind, Report of the Assiniboine and Saskatchewan Exploring Expedition, Toronto, 1859, 

 pp. 168, 160 ; Narrative of the Canadian Exploring Expeditions, London, 1860, vol. ii, pp. 48, 55, 265. 

 G. M. Dawson, Geology and Resources of the Forty-ninth Parallel, 1875, pp. 253, 254. 



