136

was final and complete. In the south, on the site of southern Alberta and
Saskatchewan, it was otherwise. There the deltas retreated to admit of
two incursions of the sea, in both Pakowki and Bearpaw time, before the
final advance of the delta plains in the late Upper Cretaceous (Russell,
1939). As in southern Alberta and Saskatchewan, the alluvial plains may
have persisted into Paleocene time.

The advance of the alluvial plains in northeastern British Columbia is
recorded by the non-marine strata of the Wapiti group and by the non-
marine beds on Petitot River, correlated with it. Possibly this plain
extended to the west and to the Trench, where beds of the Sifton formation
may be of similar age.

It is not impossible that seas may have persisted into late Upper
Cretaceous time far to the east, but if so their deposits have been removed
by erosion (Jeletzky, 1950).

SOURCE OF CRETACEOUS SEDIMENTS

Since 1887, all geologists interested in palxogeography have agreed
with Tyrrell that the bulk of the Cretaceous sediments of the western
interior of Canada have come from the west and beyond the Rocky Moun-
tains. It is only natural, therefore, to look for the source of the Cretaceous
sediments of northeastern British Columbia to the west of the Trench, in
the Omineca and Cassiar Mountains and adjacent areas.

Too little effort has been made in previous studies to co-ordinate the
history of folding, elevation, and other geological events gained by a study
of the source area itself with the history inferred from the nature of the
sediments in the area of deposition. Diastrophism in the one should be
reflected in the sediments of the other.

After folding and intrusion of the Omineca and Cassiar batholiths at
the end of Jurassic or beginning of Lower Cretaceous time (Roots, 1948;
Armstrong, 1949) mountains rose on the site of the present Omineca and
Cassiar Mountains. Their denudation would produce a large amount of
sediment, some of which would be of sand grade, particularly if the moun-
tains formed by folding and uplift were high. The erosion was sufficient
to expose the top of the batholith, but not sufficient to erode it deeply nor
to remove all of the Jurassic strata, even some that may have been deposited
at a late stage before the uplift. Armstrong (1949) believes that the sedi-
ment so derived may have been a main source of the Lower Cretaceous
rocks. It may well be that the coarse sediments of the Bullhead group
have come from these old mountains west of the Trench.

Some features of sedimentation in Bullhead time require explanation.
Although these explanations, with our current knowledge, cannot be com-
plete, they are worth considering.

(1) The question can be asked, why did marine conditions prevail in
earlier Bullhead and non-marine conditions in later Bullhead time. As one
hypothesis, it may be assumed that although a large quantity of coarse
sediment was being delivered to the sea at both times, subsidence of the sea