78

material prevails. The boundary line between the region of erosion and that of
deposition follows more or less closely the boundary between the Precambrian
and Paleozoic rocks.

The region of erosion is characterized by bare rock surfaces, rounded and
glaciated by the passage of the ice-sheet over them. Deposited material is found
in the hollows of the surface and consists mainly of boulder clay. Moraines,
drumlins, eskers, sand-plains, and some curious drift hills called “ispatinows’*
by J. B. Tyrrell occur here and there, but are not notably abundant.

In the central and western portions of the basin, where deposition is more
evident than erosion, the surface is in general so heavily drift-covered that the
older, underlying rocks are nearly everywhere concealed. The glacial deposits
have not been uniformly distributed but are thickest in old, pre-glacial depressions,
whereas on the ridges they become greatly attenuated. Inequalities have been
reduced and a rolling topography developed.

Sections of the glacial deposits of the central portion of the basin show in
many cases boulder clays usually underlain by stratified sands and gravels and
frequently overlain by a second series of sands and gravels. Such sections occur
frequently in the basins of Athabaska and Peace rivers and in the valley of the
Mackenzie. The stratified sands and gravels, both below and above the boulder
clay, are, according to McConnell,? evidently lacustrine in origin and were
doubtless deposited in greatly expanded lakes near the ice front, both before the
advance and immediately after the retreat of the ice-sheet, whereas the boulder
clay represents the ground moraine of the ice-sheet itself. Thicknesses of these
beds vary considerably. Sections of the boulder clay occasionally show a thick-
ness of 250 feet and the sands and gravels over and beneath the boulder clay are
in some cases fully 150 feet thick? Kindle states that glacial drift 400 feet
thick is exposed at Wrigley. Moraines are apparently fairly prevalent in
this portion of the basin. McConnell‘ describes a prominent morainic ridge
southwest of the west end of Great Slave lake as consisting of a “medley of steep-
sided, interlacing hills and ridges, similar in appearance to those found on the
Grand Céteau de Missouri of the plains” and evidently of like origin.

On the final retreat of the ice-sheet from the basin of Mackenzie river,
lakes occupied not only the present lake basins but other large tracts of country
which have since been filled up or drained. Filling of some of these lakes
by the streams flowing into them is still in progress. Both the Peace and
Athabaska rivers in the flood season carry a great deal of sediment and this
is being deposited as fine sand, clay, or silt in their deltas, gradually filling
up the western end of Athabaska lake. A former southern arm of. Great Slave,
which at one time extended up the valley of Slave river as far as Fort Smith,
has already been filled with the same sort of material from Slave river. The
Mackenzie itself carries some sediment in flood time and is building a great delta,
the emerged portion of which is about 80 miles across its seaward side and
100 miles deep. The submerged portion extends in a broadening front far out
into the Arctic ocean.

The rivers of the Mackenzie basin are very largely post-Glacial and have
cut deep valleys into the Glacial and underlying Cretaceous shales and sand-
stones. In certain instances, as in the case of Little Buffalo, Hay, and Beaver

1 Geol. Surv., Can., Ann. Rept., vol. VIII, 1895. p. 23 D.

2Geol. Surv.. Can., Ann. Rept., vol. V, 1889-90-91. p. 61 D.

3 McConnell, R. G., Geol. Surv., Can., Ann. Rept., vol. IV, 1888-89, p. 26 D.
“Op. -cit., 1p. 25 D.