138

Mountains may have been the site of aggradation, not denudation, during
late Cretaceous or Paleocene time.

If future research should eliminate the site of the Omineca and Cassiar
Mountains as the area of source of the Upper Cretaceous sediments in north-
eastern British Columbia, other regions of uplift and erosion at this time
must be sought. Far to the west or southwest, beyond the outcrops of the
Sustut group, is a long way to seek for the sediments of the Dunvegan, Fort
Nelson, and other Upper Cretaceous formations of northeastern British
Columbia. Other possible regions of Upper Cretaceous uplift can be con-
sidered—north or northwest of the Cassiar Mountains, and north of Liard
River, for example, in the Liard Plain, Hyland Plateau, and Logan Moun-
tains, or even farther to the north or northwest. It is probable that when
the origin of the sediments of the Sustut group is known, that of the Upper
Cretaceous beds of northeastern British Columbia, now outcropping in the
Foothills and Plains, will also be known.

ROCKY MOUNTAIN REVOLUTION

The great mass of sediment that had accumulated in the Rocky Moun-
tain geosyncline was finally folded, faulted, and elevated to form the Rocky
Mountains. It is difficult to date this revolution accurately in northeastern
British Columbia. In the central and southern Foothills, where beds of
Paleocene age have been accurately identified, the folding of the Paleocene
with the Cretaceous strata and the lack of an angular uncontormity between
them indicate that the Rocky Mountain orogeny did not affect the beds on
the site of the Foothills until post-Paleocene time. No direct evidence of
the date of the earliest movements in the Rocky Mountains can be obtained,
because of the removal by erosion of all Upper Cretaceous deposits from
there. Russell and Wickenden (1933) have shown that the first gravels
derived from the erosion of the Rockies appeared on the Plains of south-
western Saskatchewan in Upper Eocene time.

No igneous intrusions were associated with this revolution in north-
eastern British Columbia, east of the Trench, nothing even comparable
with the Ice River complex in the Field area of the central Canadian
Rockies. Little volcanic material is interbedded with the Cretaceous sedi-
ments. The Crowsnest volcanic formation of southwestern Alberta and
occasional bentonite beds are the only possible manifestations of igneous
activity during deposition of the Cretaceous in the western interior of
Canada (Williams, 1947).

TERTIARY
DENUDATION

Since their first uplift, the Rocky Mountains of northeastern British
Columbia have been deeply eroded. From the site of these mountains all
Cretaceous and Jurassic and, indeed, most of the Triassic sediments have
been removed, and most ot the Cretaceous has also been removed from the
Foothills. On the Plains, however, strata as late as Upper Cretaceous and
Paleocene have been retained.