17

separated by a wide longitudinal valley and flanked on either side by high
plateaus. The eastern range has a width opposite Fort McPherson of 7 miles and
its higher peaks are estimated to reach an altitude of 4,000 feet above the sea.
They rise more or less easily from the valley of Peel river, but present a steep face
about 3,000 feet in height to the delta of the Mackenzie. The western range is
much narrower and at Rat river does not exceed 4 miles in width, but spreads
out to the south. The valley of Peel river, which skirts the eastern base, is fully
1,200 feet lower than Bell river on the western side and the drainage of the
mountains is mostly towards Peel river. :

The summits of this northern division are usually rounded in outline and
show few sharp peaks, a feature which may be due either to mature erosion or to
lack of close folding of the strata. The pass at the head of Rat river has an
elevation of about 1,100 feet above the sea, and this is about the upper limit of
tree growth in this part of the range. ;

The Great Central Plain

This physiographic province occupies the central portion of the Mackenzis
basin and includes all the region between the Laurentian plateau on the east and
the Cordilleran region on the west. It extends throughout the whole length cf
the Mackenzie basin and has a length of about 1,300 miles. Its width ranges
from a maximum, of 420 miles along a line drawn across it through Fort Ver-
milion, to about 200 mNes in latitude 63 degrees.

The Great Central plain of the Mackenzie basin is not as might be supposed |

a vast area of level country tilted with a uniform grade to the north, but its
surface features are somewhat diversified. It has a general northward slope and
along the valley of the main streams flowing through the plain the slope is
generally uniform; but the Great Central plain is really made up of a north-
easterly sloping plateau at the south, bounded by an escarpment or series of
escarpments facing on to a lowland which extends northward to the Arctic.
South of the Mackenzie basin in southern Alberta and Saskatchewan, this
plateau is referred to .by Dowling’ as the Cretaceous plateau since its limits
are coterminous with the boundary of the Cretaceous rock and its escarpment is
built of Cretaceous’ strata. In the Mackenzie basin, however, the name is not
appropriate since the escarpment of the plateau consists more often of Devonian
rocks. It is proposed, therefore, to call this plateau of the Great Central plain
the Alberta plateau, as its northern limits almost coincide with the boundary
of that province. A-smaller plateau occupies the upper part of the basin of

Peel river and this has been called the Peel plateau”. The lowland portion of |

the Great Central plain is known as the Mackenzie lowland.®

Both the plateaus and the lowland have their surfaces broken by a number |
of hills or higher plateaus which rise from 1,000 to 3,000 feet above the level °

of the surrounding country.

The Alberta Plateau. This portion of the Great Central plain covers
virtually the whole region south and southwest of Great Slave lake... It corre-
sponds to the second and third prairie steppes in the Great Plains region south

1 Physical geography of Canada: Thirteenth report of the Geoagraphic Board of
Canada, 1914. See map. '

2 Camsell, C., Geol. Surv., Can., vol XVI, pt. CC, p. 23.

3 Dowling, D. B. op. cit. —