Landforms of British Columbia 1976 - Department of Geography
Landforms of British Columbia 1976 - Department of Geography
Landforms of British Columbia 1976 - Department of Geography
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ROCKY MOUNTAIN FOOTHILLS<br />
The Rocky Mountain Foothills (see Plates XXXVIIB and XXXIXA) lie along<br />
the eastern margin <strong>of</strong> the Rocky Mountains in a continuous belt from the 49th<br />
parallel to the Liard River. In the south the foothills are in Alberta, but the belt<br />
enters <strong>British</strong> <strong>Columbia</strong> just north <strong>of</strong> latitude 54 degrees, at the headwaters <strong>of</strong> the<br />
Narraway and Wapiti Rivers, and continues northwestward for about 400 miles to<br />
their northern termination at the Liard River. The belt <strong>of</strong> foothills ranges from a<br />
width <strong>of</strong> 1 S miles where crossed by the Muskwa River to 45 miles at the head <strong>of</strong> the<br />
Pine and Sekunka Rivers.<br />
The western boundary <strong>of</strong> the foothills for the most part is along a structural<br />
line which follows the trace <strong>of</strong> the easternmost major fault that thrusts Palzeozoic over<br />
Mesozoic formations. This fault normally brings thick cliff-forming Palreozoic limestones<br />
into position to form typical grey limestone Front Ranges, which stand out<br />
prominently on topographic maps and are easily distinguished on the ground. The<br />
eastern boundary <strong>of</strong> the foothills, especiaJly between the Narraway and Peace Rivers,<br />
is a series <strong>of</strong> southwesterly dipping in echelon thrust faults which separate strongly<br />
folded and faulted sedimentary formations <strong>of</strong> the foothills from flat-lying or gently<br />
dipping formations <strong>of</strong> the plains.<br />
The foothills are entirely underlain by sedimentary rocks that in northeastern<br />
<strong>British</strong> <strong>Columbia</strong> are largely <strong>of</strong> Mesozoic age, but throughout their length range. from<br />
Precambrian to Tertiary. The rocks are folded about northerly to northwesterly<br />
trending axes and are cut by southwesterly dipping thrust faults. Bedrock has a<br />
strongly developed structural grain which is closely reflected by the character <strong>of</strong> the<br />
topography.<br />
There is a lack <strong>of</strong> uniformity in the foothill belt because <strong>of</strong> the occurrence <strong>of</strong><br />
Triassic limestones and calcareous siltstones and sandstones in one section and <strong>of</strong><br />
Cretaceous sandstones and shales in another. In addition, the intensity <strong>of</strong> structural<br />
deformation, by folding and faulting, becomes less pronounced eastward from the<br />
mountains. These geologic differences are expressed topographically, and as a consequence<br />
it is possible to divide the foothills into a belt <strong>of</strong> inner foothills, on the west,<br />
that are higher and more rugged than a belt <strong>of</strong> outer foothills, on the east, that have<br />
a more moderate relief and a more subdued topography. In general the inner foothills<br />
are composed <strong>of</strong> closely folded and faulted Triassic and Lower Cretaceous<br />
resistant formations and the outer foothills <strong>of</strong> more gently folded and only slightly<br />
faulted Cretaceous rocks.<br />
The highest peaks and longitudinal ridges occur where resistant quart&e <strong>of</strong><br />
Lower Cretaceous age or silty limestones <strong>of</strong> Triassic age are prominent, whereas<br />
the lower peaks and valleys are eroded in the s<strong>of</strong>ter interbedded shales.<br />
Elevations <strong>of</strong> summits and high longitudinal ridges mostly range from 6,000 to<br />
7,000 feet, the highest being Mount Laurier (7,712 feet) near the head <strong>of</strong> the Halfway<br />
River. Summits increase in height southward from the Liard River to the area<br />
between the heads <strong>of</strong> the Prophet and Pine Rivers and then diminish southward where<br />
the Mont&h quart&e is thinner.<br />
The structural grain <strong>of</strong> the bedrock has resulted in prominently developed northwesterly<br />
trending longitudinal ridges and a trellis pattern <strong>of</strong> drainage (see N.T.S.<br />
Sheet 94o). Valleys are eroded along belts <strong>of</strong> s<strong>of</strong>t rock and along fault zones and<br />
are generally wide and flaring. They lie at 2,500 to 3,000 feet elevation, producing<br />
a variable and moderate relief.<br />
The foothills were occupied by continental ice during the Pleistocene, and easterly<br />
trending valleys were glaciated by ice moving outward from the mountains. The<br />
northwesterly trenclmg valleys generally were not eroded, but they, like the others,<br />
received a mantle <strong>of</strong> drift when the ice disappeared. Cirque glaciers developed<br />
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