18

A large lateral moraine of the trunk glacier in Finlay River Valley
completely blocks the through valley of Zygadine Creek and Pelly Lake
and, in a single, steep-sided, very narrow ridge 150 feet high, forms the
‘barrier’ of Barrier Pass.

The medial moraine formed at the junction of the Lay Creek Valley
and Mesilinka River Valley ice streams can be traced down Mesilinka
Valley for about 5 miles. This moraine, backed by the rock bastion of the
hanging floor of Lay Creek Valley projecting into Mesilinka Valley,
together with morainal deposits in Mesilinka Valley itself, has dammed
Mesilinka River to form Aiken Lake (Plate I).

STREAM ACTION

GENERAL STATEMENT

Although the form and detail of most of the landscape features in
Aiken Lake map-area are characteristically the result of glacial processes,
the basic pattern of valleys and mountain masses, which was available to
modification by glaciation, was developed by stream action. The major
valley systems in the area, namely: the Ingenika, with tributaries such
as Wrede Creek and Cutbank Creek; the Mesilinka, with Lay, Kliyul,
and Abraham Creeks and Tutizika River; and the upper Osilinka-Chude-
latsa Lake Valley, each shows a normal, balanced, dendritic pattern that
cuts indiscriminately across major lithologie and structural contacts. It
is evident that these master valley systems are relatively old features, and
their pattern has possibly been superimposed from valley systems developed
in a rock cover, now removed by erosion, of different geological characteris-
tics from the present cover. Except for the Rocky Mountain Trench, the
main valleys do not lie along the transverse or longitudinal faults that
traverse the area, nor are they noticeably offset where crossed by these
faults. These relations suggest two possibilities, both of which may apply
in certain places: (1) the faults along which there has been considerable
movement may be mainly older than the cover on which the present
drainage pattern originally developed, and the position of the valleys had
become established by the time the streams cut down to rocks that had
been affected by the faulting; (2) some faults of relatively small displace-
ment may post-date the drainage pattern; if so, they did not appreciably
affect the main stream valleys, but may have helped localize tributary
streams, and in particular may have been partly instrumental in bringing
about late pre-Glacial (and inter-Glacial ?) stream piracy.

Whatever the relation to faulting or to bedrock formations that have
since been completely removed by erosion, it appears that by late Tertiary
time a normal, dendritic drainage pattern had been established on the
present, relatively complex bedrock. In very late Tertiary, immediate
pre-Glacial time this pattern seems to have been disrupted to some extent
by rejuvenation of some streams, causing significant drainage changes, with
the result that the main rivers are not now always to be found in the large
valleys. Examples of these changes will be described.

Since the onset of glaciation, stream action has played only a minor
part in shaping the land surface, and has been mainly confined to the