Landforms of British Columbia 1976 - Department of Geography

ROCKY MOUNTAIN FOOTHILLS
The Rocky Mountain Foothills (see Plates XXXVIIB and XXXIXA) lie along
the eastern margin of the Rocky Mountains in a continuous belt from the 49th
parallel to the Liard River. In the south the foothills are in Alberta, but the belt
enters British Columbia just north of latitude 54 degrees, at the headwaters of the
Narraway and Wapiti Rivers, and continues northwestward for about 400 miles to
their northern termination at the Liard River. The belt of foothills ranges from a
width of 1 S miles where crossed by the Muskwa River to 45 miles at the head of the
Pine and Sekunka Rivers.
The western boundary of the foothills for the most part is along a structural
line which follows the trace of the easternmost major fault that thrusts Palzeozoic over
Mesozoic formations. This fault normally brings thick cliff-forming Palreozoic limestones
into position to form typical grey limestone Front Ranges, which stand out
prominently on topographic maps and are easily distinguished on the ground. The
eastern boundary of the foothills, especiaJly between the Narraway and Peace Rivers,
is a series of southwesterly dipping in echelon thrust faults which separate strongly
folded and faulted sedimentary formations of the foothills from flat-lying or gently
dipping formations of the plains.
The foothills are entirely underlain by sedimentary rocks that in northeastern
British Columbia are largely of Mesozoic age, but throughout their length range. from
Precambrian to Tertiary. The rocks are folded about northerly to northwesterly
trending axes and are cut by southwesterly dipping thrust faults. Bedrock has a
strongly developed structural grain which is closely reflected by the character of the
topography.
There is a lack of uniformity in the foothill belt because of the occurrence of
Triassic limestones and calcareous siltstones and sandstones in one section and of
Cretaceous sandstones and shales in another. In addition, the intensity of structural
deformation, by folding and faulting, becomes less pronounced eastward from the
mountains. These geologic differences are expressed topographically, and as a consequence
it is possible to divide the foothills into a belt of inner foothills, on the west,
that are higher and more rugged than a belt of outer foothills, on the east, that have
a more moderate relief and a more subdued topography. In general the inner foothills
are composed of closely folded and faulted Triassic and Lower Cretaceous
resistant formations and the outer foothills of more gently folded and only slightly
faulted Cretaceous rocks.
The highest peaks and longitudinal ridges occur where resistant quart&e of
Lower Cretaceous age or silty limestones of Triassic age are prominent, whereas
the lower peaks and valleys are eroded in the softer interbedded shales.
Elevations of summits and high longitudinal ridges mostly range from 6,000 to
7,000 feet, the highest being Mount Laurier (7,712 feet) near the head of the Halfway
River. Summits increase in height southward from the Liard River to the area
between the heads of the Prophet and Pine Rivers and then diminish southward where
the Mont&h quart&e is thinner.
The structural grain of the bedrock has resulted in prominently developed northwesterly
trending longitudinal ridges and a trellis pattern of drainage (see N.T.S.
Sheet 94o). Valleys are eroded along belts of soft rock and along fault zones and
are generally wide and flaring. They lie at 2,500 to 3,000 feet elevation, producing
a variable and moderate relief.
The foothills were occupied by continental ice during the Pleistocene, and easterly
trending valleys were glaciated by ice moving outward from the mountains. The
northwesterly trenclmg valleys generally were not eroded, but they, like the others,
received a mantle of drift when the ice disappeared. Cirque glaciers developed
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