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More than 3,000 fect of sedimentary strata were accumulated in a
shallow, sinking, continental basin during the Upper Cretaceous and Paleo-
cene epochs. Initial deposition, from a bordering mountainous area that
probably included the Omineca batholith, was everywhere rapid, as attested
by the widespread deposits of coarse sandstones, and at times very rapid
as evidenced by recurrent conglomerate sheets. Sedimentation continued
without recognized interruption into Paleocene time, but by then the
bordering mountains had been greatly reduced in height and the pace of
erosion had slowed accordingly: this allowed time for considerable rock
weathering, and resulted in the deposition of much shale in addition to fine-
and medium-grained sandstones. Voleanoes at some unknown locality
erupted briefly during the Paleocene, as testified by the occurrence of
several thin bands of dacitic tuffs.

An early Tertiary period of orogeny resulted in the disappearance for
all time of large bodies of water from the area. It resulted, too, in the
uplift, folding, and faulting of the Sustut formations, and in the intrusion
of the Kastberg porphyritic rocks.

Ensuing erosion produced an undulating surface, which, perhaps in
Middle Tertiary time, was uplifted more or less contemporaneously with
outpourings of basaltic lavas and related volcanic materials.

Pliocene erosion cut deep valleys in the upland surface and left upon
it a few cap-like remnants of nearly horizontal basaltic lavas.

A great mass of ice nearly or completely covered the arca during
Pleistocene time, and moved towards the east and southeast. At this stage,
and in its initial and final alpine and valley stages, it modified pre-
existing topography both directly by erosive action and indirectly through
subsequent accumulation of glacial and fluvioglacial deposits.

Vulcanism initiated in Middle Tertiary time persisted with interrup-
tions of unknown duration into Recent time, the latest, but prehistoric
phases being represented by a few cone-like deposits of basaltic pumice,
ash, cinders, and other volcanic ejectamenta.

With the melting of the Pleistocene ice the rivers shrank to about their
present volume and commenced to terrace and channel the fluvioglacial
deposits that covered the bottoms of all major valleys. As a result, these
deposits have been incised to depths of 100 feet or more. For short dis-
tances along some of the river channels, and for 13 miles along Sustut
River above the mouth of Bear River, they have been almost completely
removed from the underlying rock. In such places the rivers are now carving
rock canyons.