Landforms of British Columbia 1976 - Department of Geography
Landforms of British Columbia 1976 - Department of Geography
Landforms of British Columbia 1976 - Department of Geography
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Almost universal throughout most <strong>of</strong> camal <strong>British</strong> <strong>Columbia</strong> are the flat or<br />
gently sloping upland surfaces that are characteristic <strong>of</strong> the plateaus and highlands.<br />
They are remnants <strong>of</strong> the very widespread late Tertiary erosion surface which was<br />
uplifted and dissected.<br />
During the Tertiary, <strong>British</strong> <strong>Columbia</strong> was almost continuously subjected to<br />
subaerial erosion by streams. Some early Tertiay sediments were laid down in iso-<br />
lated lake basins and valleys on a surface <strong>of</strong> mature relief. Later, extensive Miocene<br />
and early Pliocene* lava plains <strong>of</strong> low relief were formed, as pla&w basalt filled<br />
some valleys and spread across large areas in the Fraser and Nechako Plateaus.<br />
Thus hy mid-Pliocene the land surface was one <strong>of</strong> low relief, transecting a wide vari-<br />
ety and age <strong>of</strong> rocks.<br />
The land was elevated differentially in the Pliocene, stream erosion was rejuve-<br />
nated, and the late Tertiary erosion surface was incised and dissected in varying<br />
degrees. The lava ulains were slizhtlv eroded before the streams were reiuvcnated.<br />
&l to the west, al&q the mom& &ont, incision <strong>of</strong> the plawxu basalt has dated<br />
the uplift as post early Pliocene. In the course <strong>of</strong> the uplift the land surface now<br />
represented by the uplands <strong>of</strong> the Interior Plateau was elevated to 5,000 feet above<br />
sea-level in the Nechako and Fraser Plateaus, to 6,000 feet at the edge <strong>of</strong> the Chil-<br />
cotin Ranq and in the Quesnel Highland, and to over 7,000 feet in thz Shuswap<br />
Highland. The main valleys were deeply incised as a result. The amount <strong>of</strong> incision<br />
was progressively less as the valley heads were reached, and in some instances, as on<br />
the west side <strong>of</strong> the Fraser Plateau, the amount <strong>of</strong> pwPleistocene incision <strong>of</strong> the<br />
rivers was slight or had scarcely begun.<br />
Dissection <strong>of</strong> the upland surface took place contemporaneously with incision<br />
<strong>of</strong> the streams. Dissection varied widely throughout the Interior Plateau, being<br />
dependent largely upon the depth to which stream incision had progressed and the<br />
elevation to which the late Tertiary surface had been raised. The degree <strong>of</strong> dissec-<br />
tion was greatest in the highlands along the eastern plateau margin, was less in the<br />
Thompson Plateau, and least in the western side <strong>of</strong> the Nechako and Fraser Plateaus,<br />
in which large areas <strong>of</strong> upland to this day remain undissected and scarcely incised.<br />
The amount <strong>of</strong> dissection <strong>of</strong> the land is related to the depth <strong>of</strong> incision <strong>of</strong> the<br />
s@eams, and this in turn dewmines the dominant landform <strong>of</strong> a region, Thus pla-<br />
teaus result where the degree <strong>of</strong> dissection is low to moderate and where consider-<br />
able areas <strong>of</strong> flat or gently sloping upland remain (see Plates XVIII, XXIVB, and<br />
XXVIA), Highlands represent a %msition between plateaus and mountains and<br />
result where the degree <strong>of</strong> dissection is fairly high and where the flat and gently<br />
sloo%g upland surfaces are small and at higher elevations (see Plates XVB and<br />
XXVIIIA). Mountains result where the dissection <strong>of</strong> the old surface is complete or<br />
where only a few small remnants <strong>of</strong> it are preserved (see PI&es XIXB and XXVIIB).<br />
The plateaus and highlands <strong>of</strong> the Interior Plateau were formed by the dissec-<br />
tion in the Pliocene <strong>of</strong> the late Tertiary erosion surface. Most <strong>of</strong> the Interior Plateau<br />
consists <strong>of</strong> plateaus, but on the east, where there is a gradual transition to the<br />
<strong>Columbia</strong> Mountains, a belt <strong>of</strong> highlands has resulted. On the west, however, where<br />
the transition between plateau and Coast Mountains is abrupt (see Plate XIB), there<br />
is no belt <strong>of</strong> highlands.<br />
The Tertiary era ended with the land standing comparatively high, and with<br />
a climatic change that brought on a period <strong>of</strong> refrigeration, the Pleismcene or<br />
Glacial epoch.<br />
In the Pleistocene the Interior Plateau was occupied by an ice-sheet whose<br />
upper surface was in excess <strong>of</strong> 8,000 feet. There was some erosion <strong>of</strong> valleys in the<br />
* MatIm”% w. “., “rli”~m~~Y 0, B.C., D@. Of OeoloEY, llqmrt NO. *, ES3.<br />
75