134

of the alluvial plain and restrict it to a narrow area to the west or southwest
on the site of the Rocky Mountains and leave a wide belt of fringing marine
sand to the east or northeast.

UPPER CRETACEOUS GEOGRAPHIC OUTLINES
General Statement

The geographic lineaments varied less in Upper Cretaceous than in
Lower Cretaceous time; that is, there were fewer styles of outlines. The
marginal alluvial plains or deltas advanced and retreated, mainly in an
east-west direction and with a more or less north-south or northwest-
southeast direction of the delta front. There appears also to have been a
tendency for the deltas to advance a little farther eastward in the northern
than in the southern part of the Canadian interior.

All seas now extended south into the United States interior and
probably to the Gulf of Mexico. Many, perhaps all, extended to the
Arctic, so that the continent was divided into two by a great interior sea
stretching from the Arctic to the Gulf. Palzogeographic maps of later
Upper Cretaceous time have been prepared by Russell (1939).

Marginal Alluvial Plain in the Northwest

In very early Upper Cretaceous, probably Cenomanian, time, a great
marginal alluvial plain or delta occupied a large part of what is now north-
eastern British Columbia and northwestern Alberta, an area that had pre-
viously been flooded by the Fort St. John seas (McLearn, 1932). It is
recorded by sediments of the Dunvegan and Fort Nelson formations. It
extended southeast along the Foothills almost to Athabasca River, north-
west at least as far as the Liard, and eastward at least as far as Dunvegan,
but probably not to Athabasca River, as inferred by McLearn (1922) in an
earlier paper. Its western extension is not known, but may have been to or
even beyond the site of the Trench.

Over most of this area, the sediments record a marginal alluvial or
delta plain with scattered coal Swamps, at times inundated by the sea.
If the Fort Nelson formation is a northern extension of the Dunvegan, it
records an environment closer to the Piedmont slopes of that time than the
sediments ot the Dunvegan formation.

Except where it flooded the Dunvegan delta and there left traces of its
fauna, little is known of the sea of this time. It may be recorded by shales
low in the Blackstone formation in the central Canadian Foothills (See
Chapter III). It may also be recorded by beds near the base of the speckled
shale zone, with Inoceramus athabaskensis, on Lower Athabasca River (See
Chapter III), and possibly by beds near the top of the Ashville formation
on the Manitoba escarpment (McLearn, 1937; Wickenden, 1945). How-
ever, it must have inundated parts of the southern Canadian interior, tor
it certainly was connected with early Cretaceous seas in Utah and Colorado;
it was the first of the Cretaceous seas of the Canadian interior to connect
with seas ot the southern interior ot the continent (McLearn, 1932, 1937).
The northern distribution of this sea is not known. No record ot it has yet
been found in Mackenzie River Valley.