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bottom was too rapid to permit delta building during deposition of the

lower Bullhead, whereas later, slower subsidence made delta building
possible.

(2) The disappearance of the Bullhead group to the north also requires
explanation. It is to be assumed that uplift west of the Trench was less,
for example, in the old pre-Cassiar Mountains in the north than in the old
pre-Omineca Mountains in the south.

The sediments of the Fort St. John group are mostly fine grained.
Sand entered into the deposition of the Commotion and Gates formations,
and sand passed into mud and non-marine into marine conditions from
south to north. Some of this sediment may have come from the west in a
late phase of the denudation of the ancient pre-Omineca and pre-Cassiar
Mountains. Some, however, may have come from the southwest or south,
from a northern extension ot the pre-Selkirk Mountains. McLearn (1932,
1935) has proposed that much of the late Lower Cretaceous sediment came
from the pre-Selkirk Mountains and spread out northeastward onto the site
of the Plains.

The main reason, generally accepted since Tyrrell’s original observa-
tion, for the exclusion of the site of the Rocky Mountains as a source of
Cretaceous sediment is its inability to supply feldspar to the sands and
igneous boulders to the conglomerates. It must be admitted, however, that
not much is known of the petrography of either the sandstones or con-
glomerates of the Cretaceous of northeastern British Columbia.

The source of the Upper Cretaceous and Paleocene sediments of north-
eastern British Columbia is a greater problem than that ot the Lower Cre-
taceous sediments in the same region. In particular, large quantities of
coarse sediments are required to fill in the sea and build out the early
Upper Cretaceous Fort Nelson-Dunvegan and late Upper Cretaceous and
Paleocene Wapiti marginal alluvial plains. It has been noted that eleva-
tion and erosion of the late Jurassic of early Lower Cretaceous pre-Omineca
and pre-Cassiar Mountains may well have furnished the coarse sediments
of the Bullhead group. The evidence is not so strong that these old moun-
tains were the chief source of the Upper Cretaceous sediments. It is true
that some sediment may have come from this source, because some increase
is shown in size of erain of sediment from east to west. An examination,
however, of the Aiken Lake, McConnell, and adjoining map-sheets will
show that the Sustut group covers a large area west of the Trench, and that
only a comparatively narrow strip of terrain is left between the easternmost
outcrops of the Sustut group and the T rench to serve as an elevated and
source area. The question is: would the denudation of an area this wide,
even if highly uplifted, supply all of the sediment required to build up the
Upper Cretaceous formations that accumulated on the site of the Rocky
Mountains, Foothills, and Plains of northeastern British Columbia and
northwestern Alberta. Moreover, if the Sifton formation in the Trench is
a part of the Sustut group (Roots, 1948) and were once continuous with it,
an hypothesis that cannot be entirely rejected, the site of the Omineca

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