Landforms of British Columbia 1976 - Department of Geography

In the short interval, about 10,000 years, since the disappearance of the bulk
of the Pleistocene ice, stream erosion has again become an active agent of landscape
development. The time interval is too short to result in extensive modification of
the landscape, but some very striking small-scale features have nevertheless devel-
oped, such as post-glacial canyons in solid rock and rapid downcutting of gravel
deposits and of valley fill.
Landforms of volcanic origin are comparatively minor features, but volcanic
activity has been important through much of the geological history of British Co-
lumbia, the last eruption being only about 300 years old, As volcanic features have
a singular interest, they are the subject of special treatment on pages 117 to 120.
INFLUENCE OF BEDROCK
Much of the late geological history of the Province is recorded by landforms,
many of which are sculptured on bedrock by the forces of erosion. The correct
interpretation of their origin requires some knowledge of the character and the
distribution of bedrock, both of which influence the course of erosion and to a
degree govern the nature and distribution of the resulting landforms.
The different kinds of rock, whether igneous, sedimentary, or metamorphic,
are affected by erosion in different degree. The properties of rocks, such as hardness,
solubility, and homogeneity, determine their response to erosion. Structures,
including such features as the intensity and orientation of folding, or the presence
of faults, shear zones, joint systems, regionally developed cleavage, and zones of
alteration, all influence the course of erosion and combine to determine the character
of the erosional form, the landform. One has only to contrast the appearance of the
uniform dome-like mountains carved from great masses of almost monolithic granite
in the Coast Mountains (see Plate XA) with the strongly castellated and layered
mountains composed of folded and faulted sedimentary rocks in the Park (Main)
Ranges of the Rocky Mountains (see Plate XXXVA) to appreciate the very great
influence exercised by the nature and characteristics of the bedrock.
In describing bedrock geology it is customary to group rocks according to their
age, but this does not necessarily provide a satisfactory basis for the comparison of
topographic features. Alternatively, in order that direct comparison of one area
with another may be made, it is here proposed to group the rocks of the Province on
the basis of the physical characteristics that influence the over-all form of the topography.
Thus Precambrian and Cretaceous folded sedimentary rocks are grouped in
order to emphasize their physical characteristics rather than their age. As a result,
it is possible to present a series of six diagrams which outline areas having, as a
common denominator, similar bedrock characteristics, The age of any rock is disregarded
in dividing the Province into areas which are dominantly underlain by:-
(I ) Intrusive igneous rocks (Fig. 2).
(2) Flat-lying or gently dipping sedimentary rocks (Fig. 3).
(3) Flat-lying or gently dipping lava flows (Fig. 4).
(4) Folded and faulted sedimentary rocks (Fig. 5).
(5) Folded and faulted volcanic and sedimentary rocks and some igneous
intrusions (Fig. 6).
(6) Foliated metamorphic rocks (Fig. 7).
(1) Intrusive rocks are petrographically variable, but physically they are sufficiently
uniform that, under normal circumstances, they are fundamentally homogeneous
. As a consequence, landforms developed on this sort of rock by similar
forces look remarkably alike though they may be geographically far apart. The
homogeneity of large intrusions may be partly destroyed by the presence of roof
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