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One of the characteristic depositional features of the trunk valleys and
through valleys is the presence of kame terraces, and terrace-like deposits
of ice-dammed lakes along the valley walls (Plate III B). These terraces
occur at all elevations from near the valley floor to the summits of ridges
more than 6,500 feet above sea-level. Some of the higher terraces are
found on peaks that must have been merely small nunataks at the time
the terraces were deposited. ;

An unusual feature of many of these terraces is their persistence from
the walls of trunk valleys up to the heads of small tributary valleys. It
is evident that at the time the terraces were formed many of the smaller
valleys did not support alpine glaciers tributary to the main glaciers.
Instead, they were apparently filled by a ‘backwater’ of ice from the
trunk valley. The mouths of other tributaries were blocked by the ice in
the main valley, and the tributary valleys contained, at times, pro-glacial
lakes. Upon the disappearance of ice from the main valley, there was no
retreat of ice up these tributary valleys in the form of independent glaciers,
to end as small cirque glaciers at the heads of the valleys. Instead, the
‘backwater’ ice merely stagnated, to leave kame terraces and silt deposits
essentially undisturbed. Most of these tributary valleys containing kame
terraces or silt deposits, however, show signs of an earlier valley glaciation.
The west fork of Orion Creek, for example, is a normal glaciated valley
heading in a cirque. Its floor at its lower end contains many kettle-holes
and chaotic morainal deposits apparently indicative of stagnation of the
last ice fill in that part of the valley. Its upper part contains at least
ten levels of well-developed terraces, some of which are continuous around
the head of the cirque, at elevations slightly above the cirque floor. It is
evident that these terraces were formed after the last glacier in the cirque;
in other words, the last ice occupying the valley retreated down, rather
than up, the valley.

The kame terraces in the tributary valleys would appear to indicate
that some of the tributary valleys had been heavily glaciated and sculptured
to more or less their present form before the trunk valleys were nearly or
completely filled with ice for the last time. The preservation of the terraces
in the vailley heads suggests that the last period of extensive ice occupation
ceased comparatively abruptly, and that the ice-cover wasted rapidly from
large valley glaciers to smail cirque glaciers present in only the higher
mountains.

Erosion by small cirque glaciers has been active in all of the mountain
ranges of the area, although most pronounced in the higher, southwestern
part. The degree of cirque development would appear to indicate that the
extent of glacier cover has been, for a considerable length of time, only
slightly greater than at present. Many of the mountain massifs contain
compound cirques, with two, or in some instances three, distinct levels of
cirque incision (Plate IV A). The floors of the abandoned cirques are at
about 5,600 to 6,000 and 6,300 to 6,800 feet above sea-level respectively,
whereas most of the present glaciers head in cirques whose floor level
Suggests a regional snow-line at about 7,200 feet above sea-level. Many
of the glaciers have well-developed, unbreached terminal moraines a short
distance down the valley from their snouts, indicating a stage of compara-
tive stability at a time in the relatively recent past when the glaciers were

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