15

there. Because of this maintenance of the local base level, the river has
developed a comparatively low gradient within the Foothills. Not only has
this bedrock dam raised the base level within the Foothills, but it has also
trapped a considerable amount of the sand and gravel ‘fill’ on the bottom
of the valley and to a considerable depth. This is a fairly uniform material,
on which the river can maintain a fairly even gradient. Were the ‘fill’
washed out of the valley the conditions would be very different; the river
would flow on the bedrock floor of the valley, inequalities of the rocks
in resistance to erosion would produce irregularities in gradient, and
probably frequent rapids would result.” (See also Beach and Spivak, 19438A.)

ROCKY MOUNTAINS

The Rocky Mountains north of Peace River are still largely unex-
plored, but H. S. Bostock has studied their physical features in great
detail from air photographs, and has described them as follows (1948, p. 6).

“The Rocky Mountains stand between the Foothills and Rocky
Mountain Trench, which marks their west boundary. They form a
remarkably continuous wall of mountains traversed by only one stream,
Peace River. On the east their boundary with the Foothills is well de-
fined, as it follows the first chain of ranges of upfaulted, massive, Paleozoic
limestone and quartzite, whose resistance to erosion has given the mountains
their particularly rocky and rugged character [See Plate II B]. For the
greater part, the mountains consist of ridges with a northwesterly aline-
ment nearly parallel with that of the entire Rocky Mountain area. Their
thick competent strata have broken and warped to form larger units than
those comprised of Foothills strata. The ridges are separated by deep
valleys cut along zones weakened by folds to erosion, fractures, and the
presence of relatively soft strata. They are mainly formed of Paleozoic
sediments, but include some of Late Precambrian age.

“The Rocky Mountains are approximately 50 miles wide from the
Forty-ninth Parallel northward to Yellowhead Pass. Beyond this, they
gradually narrow until they reach a minimum width of less than 25 miles
at Peace River. Still farther north their west boundary continues north-
westerly along the trench, but their east boundary swings to nearly north
and, in consequence, they broaden in this direction to reach their greatest
width of more than 85 miles.

“The Rocky Mountains form the continental divide from the Forty-
ninth Parallel to about 150 miles north of Yellowhead Pass. Farther north
the divide lies west of the Rockies, which are drained on that side by
tributaries of Peace and Liard Rivers.

“South of Peace River, the Rocky Mountains are divisible into three
parts. In the southern and central parts they are rugged, and the greater
peaks, many more than 10,000 feet high, including Mount Robson (12,972
feet) the highest known peak of the Eastern system, form the continental
divide. In the northern part, south of Peace River, the mountains narrow,
their elevations decrease, no really great peaks are present, and they are
characteristically less rugged.

“North of Peace River, where the Rocky Mountains are least known,
they comprise a border zone surrounding a central area. The central area,
130 miles long and 30 miles wide, extends from about 210 miles southeast