Geology Reference
In-Depth Information
Figure
2.4.
Oblique air view of the Beaufort Plain, northwest Banks Island, showing fl uvial dis-
section of the plain, winter snow remnants, mass-wasting on lower slopes, and large-scale polygonal
patterns on the upland surfaces. Photo taken in early July, 1969.
The annotated aerial photograph (Figure 2.3) suggests this never-glaciated periglacial
terrain is not all that different from terrain existing in many non-periglacial regions of the
world. However, it is unusual. First, extensive areas of the plain are covered by large poly-
gons, the borders of which form shallow troughs. These are similar to the “tessellations”
fi rst described by Griffi th Taylor (1916, 1922) from Antarctica (see p. 10).
The valleys that dissect the Beaufort Plain are the most conspicuous landscape feature
(Figure 2.4). Many are asymmetrical with steeper slopes facing west or southwest (French,
1970, 1971b). During summer, the gradual melt of snow banks on east-facing slopes pro-
vides moisture for mass wasting (solifl uction) to occur. Thus, the stream in the valley
bottom moves laterally towards the slope producing the least colluvium and an asymmetry
is produced in which the two valley-side slopes are constantly adjusting to each other and
to the basal stream channel. Slope asymmetry is interpreted, therefore, as a quasi-
equilibrium form. Despite its fl uvial dissection, the upland surface of the Beaufort Plain
is relatively featureless. Evidence of ground ice is conspicuously lacking; pingos and frost
mounds are absent, and thaw lakes and standing water bodies are non-existent. Patterned
ground is restricted to the occasional non-sorted circle. A wind-veneer of small pebbles
covers the surface and vegetation is sparse.
In summary, the Beaufort Plain suggests that a periglacial landscape is one of low relief
and fl uvial dissection. Wind, snow, and running water are the crucial geomorphic proc-
esses. Permafrost is indicated by the occurrence of thermal-contraction crack polygons
but evidence of high ice content in near-surface sediments is lacking. Mass wasting is
confi ned to snowbank localities. The braided nature of the stream channels refl ects the
unconsolidated nature of the bedrock, the availability of abundant bedload, and the highly
variable and seasonal (nival) discharge regime.
2.3.2. Barn Mountains, Northern Interior Yukon Territory, Canada
The Barn Mountains (latitude 68°30
W) are part of the British
Mountains and rise to elevations in excess of 1200 m a.s.l. In the foothills, sequences of
gently folded Mesozoic-age shale and sandstone produce anticlinal escarpments and syn-
′
N: longitude 138°30
′
