PHYVSIOGRAPHIC EXTENSION OF UNITED STATES 241 
THE WESTERN LAKE SECTION OF THE CENTRAL LOWLAND PROVINCE 
East of the Great Plains province lies another plains region 
that is distinctly lower than the first. Various names have been 
applied to it, but none seems better adapted to the region as a whole 
or more inclusive of the several topographically diverse subdivisions 
than the name used herewith—Central Lowlands. It is subdivided 
into several sections, but only the two that have Canadian exten- 
sions will be discussed in this paper. ‘The first of these is the 
Western Lake section. ; 
This section is irregular in shape and embraces a large part of 
the northern interior of the United States and a smaller part of 
south-central Canada. Its western boundary is the Tertiary 
erosion searp (Missouri coteau) previously mentioned. The 
eastern boundary is also marked by a prominent topographic break 
throughout most of its length, and where a topographic separation 
is not easily made from the province next to the east there are good 
reasons for drawing the line empirically. The eastern boundary 
begins near Stillwater, Minnesota, and runs northwestward, 
skirting the Superior Highlands to Rainy Lake. From this point 
it continues north and west close to the eastern shore of Lake 
Winnipeg to a point near longitude 102°W. and latitude 55°N., 
where it closes with the western boundary. This line with minor 
exceptions is essentially the contact of pre-Cambrian rocks of the 
Laurentian Plateau with the lacustrine sediments of glacial Lake 
Agassiz. 
An optional line might be run along the most prominent western 
beach of Lake Agassiz and continued along a series of water-worn 
cliffs of Cretaceous rock overlooking a region of Silurian bedrock 
and the alluvial sediments of the former lake. Collectively these 
cliffs are known as the Manitoba escarpment.t Eastward flowing 
streams have cut numerous deep valleys in this escarpment and 
have left the intervening remnants standing as isolated hills, the 
most prominent of which are the Pembina, Riding, Duck, and 
Porcupine mountains.?, Beyond Porcupine Mountain there are 
no adequate data for drawing the line, and it might therefore be 
~C. A. Young, op. cit., p. 108. 
2D. B. Dowling, Geol. Survey Canada, Guide Book, No. 8, Part I, p. 80. 
