ceded
is

A small area of Tertiary sediments occurs on Peel river in the vicinity of
Wind and Bonnet Plume rivers and reaches from 20 to 30 miles up the valleys of
these rivers. The rocks consist of sandstones and clays carrying beds of lignite.
These have an unconformable relation to the underlying strata, and have been
gently folded into a number of anticlines and synclines.*

Pleistocene and Recent

With the exception of the higher points of the Rocky and Mackenzie moun-
tains on its western side, the whole basin of Mackenzie river was overridden by
elacial ice during a part at least of the Pleistocene period. Ice appears to have
entered the basin from two sides, namely, from. the Keewatin centre on the east
which is believed to have occupied a position on the northwestern side of Hudson
bay, and from the Cordilleran ice-sheet on the west, the centre of which lay to the
west of the Rocky mountains in the northern part of the province of British
Columbia. At the same time tongues of ice flowed down the valleys on the eastern
side of Mackenzie mountains, rising from a confluent ice-sheet which lay on the
western slope of those mountains and formed the northern extension of the Cor-
dilleran ice-sheet.2 Keele states that “the valley of the Mackenzie river was
occupied by an ice-sheet of considerable thickness which pushed up the valley of
the Gravel river before the ice from the Cordilleran glacier began to pour down.”

Ice from the western sources entered the basin of Mackenzie river in-the
form of valley glaciers which flowed down the valleys now occupied by the western
tributaries of the river. Keele has estimated that a thickness of 2,000 feet of ice
oecupied the valley of Gravel river in its lower part, and on Wind river, a tribu-
tary of the Peel, Camsell found that the maximum thickness of the ice in that
valley must have been about 1,000 feet. Ice tongues also occupied the valleys of
the Athabaska, Pine, Peace, and Liard rivers in the mountains and foothills
portions of their courses and probably spread out as piedmont glaciers on entering
the more level country of the plains.

On the eastern side, ice from the Keewatin centre invaded the Mackenzie
basin as a continental ice-sheet, its encroachments gradually extending farther
and farther westward until it joined with the ice from the western sources. From
the evidence of striations and transported material the direction of movement of
the Keewatin ice-sheet is shown to have been southwestward in the southern part
of the basin and westward and even northwestward in the northern part. South
of Athabaska lake, for example, the general course of the ice was south-southwest;
immediately north of Athabaska lake it was southwest; and on Great Slave lake
it was almost west. Farther north, in the valley of Mackenzie river, about latitude
66 degrees north, McConnell® noted evidence of a more northerly flow of an ice-
sheet approximately 1,500 feet thick, which extended down the valley of the river
and thence out to sea. Kindle found deep glacial grooves trending east and west
in the limestone at an elevation of about 1,500 feet on mount Charles.

The results of glacial action in the basin of Mackenzie river are everywhere
shown by the presence of various kinds of deposited material as well as by the
eroded surfaces. Over a large part of the basin the.effects of erosion are predom-
inant, and over the remainder of the basin deposition of eroded and transported

* 4:Camsell, C., Geol. Surv., Can., Ann. Rept., vol. XVI, p. 41 OC.

2Keele, J.. “A reconnaissance across the Mackenzie mountains,” Geok Surv., Can.,

4910.
3 Geol. Surv., Can., Ann. Rept., vol. IV, 1888-89, p. 27 D.

15850—6

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