Tales

but with the general northward decline of the upland surface and the
widespread effects of glaciation the lowland benches for the most eee
lose their identity. b,

The valley system of the region is deeply entrenched below the
old land surfaces. The pre-glacial valleys were deeper and probably
better graded than those of the present system, although the immediate
pre-glacial uplift resulted in rapid down-cutting and stream adjustment.
An excellent example may be seen in the Toad River-Muncho [ake-Trout
River systems. The upper, laked valley of Toad River is continuous with
the valley now followed northward from mile 159 to Liard River. Except
for a short tributary of the Toad, the valley is occupied by the Muncho
Lake-Trout River drainage, and is wide ond of mature developments From
the bend of the Toad at mile 159, the river descends through a canyon of
youthful character for 10 miles to the east before entering the wide,.
mature valley of Racing River. The canyon was filled with till and ill--
sorted gravel, and is clearly pre-glacial, but it was evidently developed
during the period of maximum uplift and consequent drainage adjustment
immediately preceding Pleistocene time.

The Ice Age itself modified, rather than developed, the drainage
systeme Widened valleys, scarped spurs, hanging valleys, cirque formation,
arrétes, and other typical glacial forms resulted.

The continental ice-sheet extended into the Foothills valleys
almost to the mountain front, damming back the mountain glaciers with their
loads of boulders, gravel, sand, and rock flour. The continental sheet
retreated first, for remnants of mountain glaciers still remain south of
the pass. At the close of the Ice Age, the whole land ws probably 600 to
700 feet lower than at present and the valleys were clogged with morainal
and glaciofluviatile material to that depth. With tho passing of the ice,
the land ‘pradudily rose*and re-excavation~ of valleys” started. ~ That process
is still continuing rapidly, but at few places is the solid rock being
attacked by stream erosion, as glacial debris covers most of the valley
floors. Mountain streams are cutting gorges, and falls occur where they
enter the main valleys. Gorges and rapids occur in rivers where new
channels have crossed rock spurs in old valleys, but in general post-
glacial erosion is closely controlled by the established pattern.

The unstable (youthful) condition of the present stream
system was amply demonstrated during the floods of 1943, to which
reference has been made. Besides mud slides, erosion of river bars and
river banks and widespread destruction of road grades and culverts, rock
slides occurred such as are described belowe

Near mile 94, the pioneer road location crossed a small stream
valley some distance up the hillside, The road grade descends in both
directions from the culverte For 100 yards or more the stream bed is in
angular slide rock from the hill to the north, but this is covered with
small timber. During the flood of July 10 the stream overflowed its banks,
loosened a large quantity of coarse rocks and piled it up 6 feet high
across the roadway, covering some 50 yards of road. Blocks 2 feet across
were thus moved. A remarkable feature of the slide is that the base of
the rock pile is above the level of the stream and higher than the source
of much of the material,

Mountain Building and Crustal Movements
The structure of the Northerm Rocky Mountains is similar to

that of the system farther south. Compression from the west resulted in
overturned folds and thrust faulting toward the east along the mountain