86 :

is the result mainly of simple deposition of sediments on a comparatively even
surface. The Devonian strata show slight undulations in the sections exposed on
Athabaska river, but the trend of the undulations has not been determined and it
is doubtful whether there is any uniformity of trend. On Peace river and the
Slave, where these rocks overlie beds of gypsum of Silurian age, there is con-
siderable local disturbance. The beds immediately overlying the gypsum are
fractured and brecciated, and are thrown into a series of anticlines and synclines
sometimes with dips as high as 70 degrees. The disturbance is believed to be
due not to mountain-building forces but to the alteration of beds of anhydrite to
gypsum and the consequent expansion as a result of that alteration. A gentle
anticline crosses Great Slave lake from Pine point to Nintsi (Windy) point.
In the lower part of Mackenzie river the Devonian rocks of the Great Central
plain have been considerably influenced by the disturbances which elevated the
Cordillera, and anticlinal structure is common. The folding and possible faulting
that are expressed in such features as Franklin mountain, Bear mountain, and
the ridges near the mouth of Carcajou river are related to the Cordilleran dis-
turbances and their trend conforms in a general way to the trend of Mackenzie
mountains. :

The Cretaceous and younger rocks of the Great Central plain in the southern
part of the basin are apparently undisturbed and in the sections exposed on
Peace and Athabaska rivers the dip of the strata, except in the foothills, does not.
exceed a few feet per mile. The hills of Cretaceous rocks which rise out of the
country between the Peace and Athabaska rivers, for example, Birch mountain,
Buffalo Head hills, Pelican mountain, and Martin mountain, are believed by
McConnell* to be due to erosion rather than to upwarping.

In the northern part of the basin parts of the Cretaceous of the Great Central
plain have been involved to some extent, as has been the Devonian, in the Cordil-
leran disturbances, and the beds are arched into low anticlines striking north
and south and broken by faults. The greater part of the beds here, as in the
south, lie, however, in an almost horizontal attitude.

Structure is naturally more evident, if not more pronounced, in the Cordillera
than in either the central plain or the Laurentian Plateau portions of the basin.
The Rocky mountains, which extend northward along the western edge of the
basin as far as the Liard river, are built up of sedimentary rocks that have been
elevated by pressure exerted from the west into a series of parallel ridges which
strike about north 30 degrees west. These ridges are made up of closely folded and
faulted beds sometimes thrust one over the other, and dipping at high angles
generally towards the west. Nearer the central plain in the foothills, the beds
are not so closely compressed and the pressure has produced more open folds with
lower dips, but with, however, the same general strike.

The Rocky mountains die out in the latitude of Liard river and that stream
flows around their northern extremity, cutting through only a few of the more
persistent ridges. The beds are here not so compressed and the structure of the
ridges is anticlinal.

In Mackenzie mountains “ the structure is characterized by folding, generally
on a broad scale, which has thrown the strata into a series of anticlines and
synclines; but the folding is sometimes close, and in certain cases the folds appear’
to be overturned and overthrust.”* The general strike of the beds in these moun-
tains as far north as latitude 65 degrees north is slightly west of north, but at

1Geol. Surv., Can., Ann. Rept., vol. V, pt. I, p. 44 D.

* Keele, J.. “A reconnaissance across the Mackenzie mountains,” Geol. Surv., Can:
1910.