Landforms of British Columbia 1976 - Department of Geography

tinctive mesa-like or step-like form. For example, erosion of an extensive flat-lying
bed of cliff-forming sandstone or conglomerate may result in an essentially ‘flat
upland surface surrounded by escarpments related in height to the thickness of the
resistant bed (see Plate XXXVIII) . Alternatively, areas underlain by flat-lying
shales, which are more readily eroded, commonly display a uniformly low relief.
Plateaus, plains, and areas of low relief are commonly associated with areas of flat-
lying sedimentary rocks.
Figure 3. Diagram showing the main areas of flat-lying or gently dipping sedimentary rocks.
Areas of flat-lying or only very gently dipping sedimentary rocks are shown
on Figure 3. By far the largest area is in the northeastern corner of the Province,
where such rocks underlie the country east of the foothills of the Rocky Mountains.
Other areas are the Spatsizi Plateau at the head of the Stikine River, the north-
east tip of Graham Island, and the delta of the Fraser River downstream from
Chilliwack.
(3) Molten lava erupted from a single volcanic vent or from a number of
fissures may spread over considerable areas. When erupted, the lava is fluid and
mobile, and extensive individual flows may be essentially flat or have a slope that
represents the original flow gradient. Lava flowing out over the land surface fills
in the lower areas and eventually may accumulate to considerable thicknesses. In
this way a flat surface may be built up on an older surface of some relief, and a
plateau of constructional origin is formed.
Consolidated lava is resistant to erosion, and flat-lying flows resemble flat-
lying resistant sedimentary beds inasmuch as they produce a plateau topography.
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