i

19

Evidence presented by Armstrong and Tipper (1948) has indicated a
great movement of glacial ice eastward across north-central British Col-
umbia in Pleistocene time from gathering grounds in the Coast Mountains.
On reaching the Rocky Mountain Trench some of this ice escaped easterly
to the plains by way of Peace River and Liard River Valleys, and perhaps
also by way of the low mountain passes, such as Pine Pass. Just where
the continental and Cordilleran ice-sheets met and mingled in the Plains
region has not been determined. Williams (1944) believes that the con-
tinental ice-sheet extended into the Foothills valleys for a time in the
aCe west of Fort Nelson. He writes as follows regarding glaciation
there:

“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 Lake-
Trout River systems. The upper, laked valley. of Toad River is contin-
uous with the valley now followed northward 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 and of mature development. Trom
the bend of the Toad, 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
system. 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 was probably 600 to
700 feet lower than at present, and the valleys were clogged with morainal
and glaciofluviatile material to that depth. With the passing of the ice,
the land gradually rose and re-excavation of valleys started. That pro-
cess 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 valley. 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.”

Bostock (1948) makes the following observation on areas of the Rocky
Mountains that may have suffered little glaciation:

“During Pleistocene time the Rocky Mountains appear to have been
an area of relatively light precipitation as they are today. In the south,
valley glaciation was extensive and the level of the ice was high. North-
ward the effects of glaciation seem to have been less pronounced and in some
areas north of Peace River no features attributable to glaciation can be