188 The American Geologist. March, i89i 
HISTORY OF LAKE AGASSIZ. 
By YVakuex Opham, Somerville, Mass. 
From Part E. of the annual report of the Gool. and Nat. Hist. Sur., Canada, 1888-89. 
I. 
During the recession of the ice-sheets of both the earlier and 
later epochs of glaciation, drainage from the ice-border in many 
places flowed in channels from which the streams became turned 
by the slopes of the land into more northern courses when this 
was permitted by the farther retreat of the ice. Where the slope 
is southward, free drainage from the melting ice took place along 
the present valleys, and these were partially filled with modified 
drift, remnants of which form terraces and plains on each side of 
the present streams. But on areas that sloped more or less di- 
rectly toward the receding ice-border, the streams of that time 
eroded channels which were abandoned when lower outlets were 
uncovered. Because of the large supply of water from the glacial 
melting, some of these river-courses became conspicuous topo- 
graphic features, as noted by Dawson* McConnell,f and TyrrellJ 
in various parts of the region between lake Agassiz and the Rocky 
mountains. On a slope near]}- parallel with the retiring ice-border, 
the deserted river-courses were seldom the outlets of lakes of con- 
siderable size : but where a large area was inclined toward the ice- 
sheet, it was covered by an expanse of fresh water, formed by the 
streams that flowed down from the melting ice surface and over- 
flowing across what is now a line of water-shed between great drain- 
age basins, until the continued recession of the ice allowed the 
lake to be discharged b}' the natural slope of the land. Lake 
Agassiz was the largest of these glacial lakes. Others existed in 
the basins of the James, Souris, and Saskatchewan rivers, of which 
the two last named outflowed eastward into lake Agassiz. The 
basins of the great Laurentian lakes, which are being studied by 
Mr. (I. K. Gilbert of the United States geological survey, were 
also filled at this time to higher levels than now, determined by 
the elevations of the outlets through which they then flowed south- 
* Report cm the Geology and Resources of the region in the vicinity 
of the Forty-ninth Parallel, pp. 263-265; Geological Survey of Canada. 
Report of Progress for 1882-83-S4. p. i:>o C. 
I Geological Survey of Canada, Annual Report, vol. i. for 1SS5. pp. 21 
and 74 C. 
t Do., Annual Report, vol. ii, for 1886. pp. 43, 45 E, and 145, 146 E. 
