Physiography and Glaciation

Tipper, 1949). In Nechako River area, silt and clay are the most common de-
posits, occurring as thin varves one quarter inch to 8 inches with an average
thickness of about 2 inches. They are grey to buff, compact, relatively imper-
meable, and turn into a greasy, gumbo-like substance when wet. Considerable
contortion occurs in the beds, particularly near the top of most sections. Pebbles
common near the top of the deposits were previously considered to have been
ice-rafted, but are now believed to have been deposited by ice. The surface of
the deposits is undulating and frequently grooved parallel with the known direc-
tion of ice movement.

Because of the distortion of the upper beds and their contained pebbles,
and the grooving of the surface, it is thought that these deposits were overridden
by ice. These contorted upper beds presumably represent one poorly developed
till sheet with an older till below the lake beds. The period of recession during
which the lake deposits were laid down may have been very brief. The lacustrine
deposits around Lily Lake do not appear to have been overridden by ice, sug-
gesting either that these are later deposits or that the ice did not re-advance in
this sector. The latter explanation is preferred.

Ablation Moraine and Pitted Outwash

Several outlined areas are characterized by deposits believed to be ablation
moraine and pitted outwash, and include numerous eskers, kettles, and abandoned
channels and gullies. These areas have poorly organized drainage systems and
swampy conditions are common. No relation to bedrock is apparent to explain
the chaotic arrangement of these deposits and the writer believes these areas to
be sites of decay of large masses of stagnant ice.

The ablation moraine has an irregular, hummocky topography with low
relief and no obvious arrangement. In some areas the surface is gently undulat-
ing. The material suggests a till from which the fine sand, silt, and clay have been
washed. The resulting coarse gravel may be stratified but slump features are
common. Little or no clay-rich till is present although it should be exposed below
the ablation moraine if any valleys are cut sufficiently deep.

Pitted outwash occurs as small isolated patches, not as extensive areas.
These patches apparently were a result of local conditions and no general ex-
planation pertains to all areas. The kettles are 20 to 30 feet deep, rarely deeper,
with swamps or meadows in the bottoms.

Eskers are numerous within these areas and leading away from their edges.
They are thought to have been deposited in crevasses in the ice, or in tunnels
under the ice while the ice was melting.

Drumlins, where present, are poorly formed and indistinct, rock drumlins
are not present, and till grooves are irregular and indistinct. Presumably these
features have been masked by the ablation moraine and outwash.

These areas where ice is believed to have stagnated are almost entirely re-
stricted to the southeast half of the area and within this region most of the areas

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