ll narrower and somewhat
interrupted, but preserves its main physical features to the Arctic
Ocean about the mouth of the Mackenzie. This interior plain of the
continent represents the area of the ancient sea by which it was
occupied in Mesozoic times, with a more ancient margin towards the
north-west against the Archean, where undisturbed limestones and other
rocks of the Silurian and Devonian rest upon the downward slope of the
Laurentian Shield. Most of the plains are underlain by Cretaceous and
early Tertiary shales and sandstones lying nearly unaltered and
undisturbed where they were deposited, although now raised far above
sea-level, particularly along the border of the Rocky Mountains where
they were thrust up into foot-hills when the range itself was raised.
These strata have been subjected to great denudation, but owing to their
comparatively soft character this has been, in the main, nearly uniform,
and has produced no very bold features of relief, Coal and lignitic coal
are the principal economic minerals met with in this central plain,
though natural gas occurs and is put to use near Medicine Hat, and "tar
sands" along the north-eastern edge of the Cretaceous indicate the
presence of petroleum. Its chief value lies in its vast tracts of
fertile soil, now rapidly filling up with settlers from all parts of the
world, and the grassy uplands in the foot-hill region affording
perennial pasturage for the cattle, horses and sheep of the rancher.
Though the region is spoken of as a plain there are really great
differences of level between the highest parts in south-western Alberta,
4500 ft. above the sea, and the lowest in the region of Lake Winnipeg,
where the prairie is at an elevation of only 800 ft. The very flat and
rich prairie near Winnipeg is the former bed of the glacial Lake
Agassiz; but most of the prairie to the west is of a gently rolling
character and there are two rather abrupt breaks in the plain, the most
westerly one receiving the name of the Missouri Coteau. The first step
represents a rise to 1600 ft., and the second to 3000 ft. on an average.
In so flat a country any elevation of a few hundred feet is remarkable
and is called a mountain, so that Manitoba has its Duck and Riding
mountains. More important than the hills are the narrow and often rather
deep river valleys cut below the general level, exposing the soft rocks
of the Cretaceous and in many places seams of lignite. When not too deep
the