Physiography and Glaciation

where prominences are higher the summit levels rise gradually; particularly in
the northerly part, the hills merge into a fairly uniform horizon. The tentative
conclusion is that this is a very old dissected surface, but no proof is available.

Glaciation

Nechako River map-area was completely covered by glacier ice whose
movement, at its maximum, was seemingly unaffected by the bedrock surface.
The suggested thickness of 6,000 feet (Prest, 1957, p. 460) for the ice-sheet
is not unreasonable, as in order to cover all hills and mountains it must have
been at least 3,500 feet thick but to move with the freedom suggested by the
glacial features it must have been much thicker. South of the area evidence
indicates ice thicknesses in excess of 5,000 feet.

Glacial features are prominent, and most of the glacial information available
from air photos and ground observations has been plotted on Figure 2. Greater
detail, however, could be obtained by further ground studies.

Glacial Striae, Drumlins, Rock Drumlins, and Till Grooves

These features are grouped together because they all originate from the
movement of the ice and because they offer the most precise means of determining
the direction of the last movement. Glacial striae are fairly abundant, the more
clearly defined ones being developed on Mesozoic volcanic rocks and Tertiary
rhyolite.

Numerous drumlins occur throughout the area and possess the usual
inverted teaspoon-shape. They are best developed in thick glacial deposits and
in areas underlain by flat-lying Miocene basalt, where they form large ‘fields of
drumlins’. Elsewhere they occur singly, surrounded by rock drumlins and bed-
rock hills. Commonly 50 to 200 feet high, about a mile long and up to one
quarter of a mile wide, the drumlins rise gently to a crest near the stoss-end
and taper gently to the lee end, merging with depressions that, although distinct,
are less prominent than the hills. These drumlins are composed essentially of
till, although a bedrock core may be present. Included under drumlins are the
more elongate drumlinoid ridges which, although differing in shape, are believed
to be similar in origin (see Pl. IV).

Rock drumlins, commonly described as ‘crag and tail’, are similar in form
and origin to the drumlins but differ in several aspects. Rock drumlins possess
a bedrock ‘crag’, which forms the highest part of the feature, and a ‘tail’ that is
moulded by ice on the lee of the ‘crag’. They are strikingly linear features, with
a much smaller width relative to length than the drumlins. The intervening grooves
are narrow, sharply defined, and are as prominent as the ‘crag and tail’ features.
Rock drumlins are best developed where glacial drift is thin and where the bed-
rock is hard and well jointed with an irregular surface. Areas of Tertiary rhyolites
and Mesozoic volcanic rocks are well suited to their development.