Landforms of British Columbia 1976 - Department of Geography

Almost universal throughout most of camal British Columbia are the flat or
gently sloping upland surfaces that are characteristic of the plateaus and highlands.
They are remnants of the very widespread late Tertiary erosion surface which was
uplifted and dissected.
During the Tertiary, British Columbia was almost continuously subjected to
subaerial erosion by streams. Some early Tertiay sediments were laid down in iso-
lated lake basins and valleys on a surface of mature relief. Later, extensive Miocene
and early Pliocene* lava plains of low relief were formed, as pla&w basalt filled
some valleys and spread across large areas in the Fraser and Nechako Plateaus.
Thus hy mid-Pliocene the land surface was one of low relief, transecting a wide vari-
ety and age of rocks.
The land was elevated differentially in the Pliocene, stream erosion was rejuve-
nated, and the late Tertiary erosion surface was incised and dissected in varying
degrees. The lava ulains were slizhtlv eroded before the streams were reiuvcnated.
&l to the west, al&q the mom& &ont, incision of the plawxu basalt has dated
the uplift as post early Pliocene. In the course of the uplift the land surface now
represented by the uplands of the Interior Plateau was elevated to 5,000 feet above
sea-level in the Nechako and Fraser Plateaus, to 6,000 feet at the edge of the Chil-
cotin Ranq and in the Quesnel Highland, and to over 7,000 feet in thz Shuswap
Highland. The main valleys were deeply incised as a result. The amount of incision
was progressively less as the valley heads were reached, and in some instances, as on
the west side of the Fraser Plateau, the amount of pwPleistocene incision of the
rivers was slight or had scarcely begun.
Dissection of the upland surface took place contemporaneously with incision
of the streams. Dissection varied widely throughout the Interior Plateau, being
dependent largely upon the depth to which stream incision had progressed and the
elevation to which the late Tertiary surface had been raised. The degree of dissec-
tion was greatest in the highlands along the eastern plateau margin, was less in the
Thompson Plateau, and least in the western side of the Nechako and Fraser Plateaus,
in which large areas of upland to this day remain undissected and scarcely incised.
The amount of dissection of the land is related to the depth of incision of the
s@eams, and this in turn dewmines the dominant landform of a region, Thus pla-
teaus result where the degree of dissection is low to moderate and where consider-
able areas of flat or gently sloping upland remain (see Plates XVIII, XXIVB, and
XXVIA), Highlands represent a %msition between plateaus and mountains and
result where the degree of dissection is fairly high and where the flat and gently
sloo%g upland surfaces are small and at higher elevations (see Plates XVB and
XXVIIIA). Mountains result where the dissection of the old surface is complete or
where only a few small remnants of it are preserved (see PI&es XIXB and XXVIIB).
The plateaus and highlands of the Interior Plateau were formed by the dissec-
tion in the Pliocene of the late Tertiary erosion surface. Most of the Interior Plateau
consists of plateaus, but on the east, where there is a gradual transition to the
Columbia Mountains, a belt of highlands has resulted. On the west, however, where
the transition between plateau and Coast Mountains is abrupt (see Plate XIB), there
is no belt of highlands.
The Tertiary era ended with the land standing comparatively high, and with
a climatic change that brought on a period of refrigeration, the Pleismcene or
Glacial epoch.
In the Pleistocene the Interior Plateau was occupied by an ice-sheet whose
upper surface was in excess of 8,000 feet. There was some erosion of valleys in the
* MatIm”% w. “., “rli”~m~~Y 0, B.C., D@. Of OeoloEY, llqmrt NO. *, ES3.
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