242 W. N. THAYER 
placed arbitrarily at the contact of Cretaceous rocks on the west 
with Silurian rocks on the east, which is essentially the condition 
existing all along this boundary in Canada, if we neglect the vertical 
separation of the two systems of rocks at the escarpment. As thus 
drawn the line curves to the west and closes with the western 
boundary just north of the fifty-fourth parallel. This line has not 
been used on the accompanying map because it is desired to 
correlate this work as closely as possible with the most recent work 
on the physiographic divisions of the United States." 
The surface of this region consists largely of a monotonous 
flat plain covered by the deposits of glacial Lake Agassiz, which at 
its maximum extended from the latitude of the southern boundary 
of North Dakota to the northern boundary of Manitoba. The 
Winnipeg Lake System is a remnant of this great lake. Morainic 
ridges along the edges of till sheets, intermorainic tracts of rolling 
till plain, and level lacustrine tracts marking the sites of smaller 
glacial lakes are also prominent features. The topography may 
be described in summary as pre-eminently glacial, and the effects 
of the ice sheets are evident to even the most casual observer.’ 
Similar topographic conditions are found in the Canadian 
division. Here also the surface is gently undulating and every- 
where covered by a veneer of drift which either obscures or accen- 
tuates an earlier relief. In southwestern Saskatchewan is the level 
floor of glacial Lake Saskatchewan, a large and probably short- 
lived contemporary of the smaller glacial lakes of the United States, | 
such as Souris and Dakota. The valley of the Qu’Appelle, which 
is one of the several deep valleys cut during postglacial erosion 
intervals, marks the former course of the South Saskatchewan at 
the time when the northward flow of that stream was blocked by 
ice. It is analogous in several ways to the Missouri River.’ 
Though the present relief of this section is pre-eminently of 
glacial origin, it is really a product of both glaciation and pre- 
glacial erosion, and to a lesser extent of postglacial erosion. This 
region as well as the Great Plains to the west was probably pene- 
plained, or at least subdued, in late Tertiary time. It was later 
N. M. Fenneman, Ann. Assoc. Am. Geog., VI, pl. t. 
2 J. E. Todd, U.S. Geol. Survey, Bull. 144. 3D. B. Dowling, op. cit., p. 80. 
