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Throughout the whole 800 miles of Laurentian plateau lying within the basin
of Mackenzie river the higher points show only slight differences in their altitudes
above sea-level. For example, the maximum altitudes at Cree and Wollaston
lakes in the southeast end of the basin are given by Tyrrell and Dowling as about
1,650 feet. On the north shore of Athabaska lake they are 1,490 feet, on Great
Slave lake 1,520 feet, on the divide between Great Slave and Great Bear lakes
about 1,700 feet, and on the east of Great Bear lake between 1,300 and 1,400 feet
above sea-level. Taking the lower levels of the plateau, however, there is a much.
greater difference in altitude between the plateau at the southeast end and that
at Great. Bear lake. The difference here is about 1,200 feet, giving “an average
slope to the whole region of about 1-5 feet to the mile in a northwesterly direction.

The Laurentian Plateau portion of the Mackenzie basin is essentially a lake
country, and its surface is covered with thousands of lakes of all sizes, ranging
from mere ponds to lakes hundreds of square miles in extent. ‘So numerous
are these lakes and so rocky and irregular the country between them that the
only method of travel used by the natives or travellers in summer in this region
is by canoe. By portaging from one lake to the other it is possible to travel
by canoe in almost any direction required, although the only routes in regular
use are those leading from the trading posts to the hunting grounds or fishing
resorts of the Indians.

The lakes are almost invariably shallow rock basins with smooth rocky
shores and comparatively few beaches of sand, gravel, or boulders. They are
usually very irregular in outline and their shape and alignment have been
determined partly by the structure and composition of the rocks in which they
lie and partly by the direction of movement of the glacial ice-sheet. Long con-
tinued decomposition of the bedrock of the plateau region and subsequent
removal of the decomposed surface by continental glaciation are the chief causes
in the production of a pitted; mammillated surface favourable to the accumula-
tion of water as lakes. ,

Owing to the peculiar character of the surface of the plateau and the
relatively short time that has elapsed since this character was developed, the
rivers flowing through the plateau have no well-defined valleys nor have they
evenly graded profiles. They are characterized rather by a succession of level
stretches of river or lakes separated from each other by falls or strong rapids.
Frequently they are merely a succession of lakes joined to each other by narrow,
gorge-like openings at which falls or rapids occur which interrupt navigation.
Their courses are very erratic and are frequently dependent on chance irre-
gularities of the bedrock floor. In consequence, none of them in the plateau
is navigable without interruption for any great distance, and in travelling on
them portages have frequently to be made. They carry practically no material
in suspension, for there is very little loose material on the surface of the plateau
to transport and what they do pick up is quickly deposited again in the lakes
through which they flow.

All the rivers of the plateau region drain to one or the other of the three
great depressions, Athabaska, Great Slave, or Great Bear lakes, and owing
to their peculiar characteristics, namely, lakes and waterfalls, they are excellent
streams for the development of water powers.

The Laurentian plateau in the Mackenzie basin has as a rule little or no
mantle of soil or other loose material covering its bedrock. (Plate II A), and

15850—2