THE NORTHWARD EXTENSION OF THE PHYSIO- 
GRAPHIC DIVISIONS OF THE UNITED STATES 
W. N. THAYER 
Consulting Geologist, Cincinnati, Ohio 
PART II 
THE GREAT PLAINS PROVINCE 
East of the Rocky Mountain System lies a plateau area that 
slopes gently away from the mountains. Custom has firmly 
attached to the part of this region lying within the United States 
the name of “‘Great Plains.” In the present paper this name will be 
used to include the Canadian and Alaskan sections of the province. 
The Great Plains extend uninterruptedly from the Pecos and 
Rio Grande rivers northward through the United States'and Canada 
to the shores of Great Bear Lake.? At this place the Mesozoic and 
Tertiary rocks of the Great Plains Plateau are cut off by a westward 
extension of the pre-Cambrian rocks of the great Laurentian 
Plateau and the province becomes restricted to a narrow belt. 
However, it widens again near the boundary between the Northwest 
Territories and Alaska, and gradually develops into the Anatuvuk 
Plateau of Alaska which has a width of about 150 miles. This 
section of the province terminates near the one hundred and 
sixty-third meridian, where its mountainous southern boundary 
closes with the Arctic shore line. As shown on the accompanying 
map the northern part of the province bends to the west in con- 
formity with the general trend of the neighboring Cordillera. 
The western boundary of the province throughout its entire 
extent is the Rocky Mountain System. These two divisions are 
sharply distinguished topographically. On the east the province 
tN. M. Fenneman, Aun. Assoc. Am. Geog., IV, pl. 1. 
2C. A. Young, Geol. Survey Canada, Pub. 1085, p. 107. 
3 A. H. Brooks, U.S. Geol. Survey, Prof. Paper 45, pp. 46, 47. 
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