Landforms of British Columbia 1976 - Department of Geography

The tuyas consist of nearly horizontal beds of basaltic lava capping outward-
dipping beds of fragmental volcanic rocks; they have a most interesting origin.
It is thought that they were formed by volcanic eruptions in lakes which had been
thawed through the Pleistocene ice-sheet by underlying volcanic heat. The lavas
capping the mountains were extruded after the volcanoes were built above lake-
level, and the outward-dipping beds were formed by the chilling of the lava when
it reached the water’s edge.
In the Pleistocene the area was covered by several thousand feet of ice, which
moved westward across the plateau to the head of the Nahlin River and then swung
northwest and north to escape down the Teslin Valley. In its wake it left a wide-
spread mantle of drift, which is modelled into numerous drumlin-like hills and into
depressions occupied by the numerous small lakes between Tachilta Lakes and
Teslin Lake.
[References: Watson, K. Dep., and Mathews, W, H., “ Tuya-Teslin Area,
Northern British Columbia,” B.C. Dept. of Mines, Bull. No. 19, 1944; Mathews,
W. H., “ Tuyas, Flat Topped Volcanoes in Northern British Columbia,” Am. Jour.
Sci., Vol. 245, 1947, pp. 560~570.1
[Photographs: B.C. 692:l10; B.C. 695:22, 44, 56; B.C. 699:92.]
The A~sL& Runge (,see Plate XVIIA) is 40 miles long by 15 miles wide
between the Jennings River and the Teslin River. It lies within the Kawdy Plateau
and rises above the general plateau level of 5,900 feet to a maximum elevation of
7,056 feet.
The range has an unroofed core of granitic rocks intrusive into older, closely
folded Paheozoic and Mesozoic Formations. The sharp peaks are glacially sculp-
tured, but many gently sloping upland surfaces are remnants of the uplifted and
dissected Tertiary erosion surface of the Kawdy Plateau.
“ Extensive modification of the land forms of the Tuya-Teslin Area by the
ice-sheet is not evident. The mountain valleys of the area appear to have been
widened and deepened, but it is believed that much of this erosion was accomplished
by Inca1 glaciers which existed before and after the ice-sheet. The numerous
cirques and small hanging valleys on the northern and eastern slopes of many of
the mountains are likewise attributable to local glaciers. The ridges which appar-
ently were not buried by these glaciers show little or no rounding, although the
presence of erratics testifies that they were covered by the ice-sheet.“*
[Photographs: B.C. 692:96; B.C. 695:2.]
Nahlin Plateau
The Nahlin Plateau (see Plate XVIIB) lies between the Sheslay River on the
west and Tuya River on the east. It adjoins the Tahltan Highland on the southwest,
and on the northeast is arbitrarily separated from the Kawdy Plateau along a line
running from Nahlin Crossing up the Nahlin River and across to the Tuya River
just south of the Tachilta Lakes.
The area is drained by the Nahlin and Sheslay Rivers flowing into the Taku
River, and by the Tuya and Tahltan Rivers flowing into the Stikine River,
The area is dominated by the great shield volcano of Level Mountain, whose
unwooded, low dome-shaped mass, almost circular and 20 miles in diameter,
culminates in Meszah Peak, elevation 7,101 feet (see Plate XVIIB) . The gentle
slopes of the mountains parallel the gently outward-dipping thin basaltic lava flows.
There has been some dissection of the dome by stream erosion, and late-stage cirque
glaciers have scalloped some of the valley headwalls.
* Watson, K. Dep., and Mathews, W. H., B.C. Dept. of Mines, Bull. No. 19, 1944, p. 37.
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