SUCCESSIVE GLACIER TYPES OF A WANING GLACIATION
=Transition from the ice cap to the mountain glacier.=—A study of
existing glaciers leads inevitably to the conclusion that although
subject to short period advances and retreats, yet, broadly speaking,
glaciers are now gradually wasting away, surrounded by wide areas upon
which are the evidences of their recent occupation. We are thus living
in a receding hemicycle of glaciation.
[Illustration: FIG. 408.—Schematic diagram to show the relationships
of glacier types formed in succession during a receding hemicycle of
glaciation.]
Many mountain districts which now support small glaciers only, or
none at all, were once nearly or quite submerged beneath snow and
ice. If once covered by an ice carapace or cap, our present interest
in them begins at that stage of the receding hemicycle _when the rock
surface has made its reappearance above the surface of the snow-ice
mass_. At this stage intensive frostwork, the characteristic high
level weathering, begins, and cirques develop above the scars of those
earlier amphitheaters formed in the advancing hemicycle.
=The piedmont glacier.=—In this early stage of transition from the
ice cap to the mountain glacier, the ice flows outward to the mountain
front in ill-defined streams divided by the projecting ridges, and
upon reaching the mountain front these streams deploy upon it so as
to coalesce in a great stagnant ice apron whose upper surface slopes
gently forward at an angle of a few degrees at the most (Fig. 408,
stage I). This is the _piedmont glacier_, a type found to-day in the
high latitudes of Alaska and in the southern Andes (Fig. 409 and pl. 18
B).
[Illustration: FIG. 409.—Map of the Malaspina glacier of Alaska, the
best known of existing piedmont glaciers (after Russell).]
During this stage the cirques may be but poorly defined, and ice flows
in both directions from rock divides so that the streams transect the
range, and later, after the glaciers have disappeared, may expose a
pass smoothed and polished upon its floor and with striæ directed in
opposite directions from the highest point. The pass of the Grimsel
in Switzerland furnishes an excellent illustration of such earlier
transection of the range.
[Illustration:
FIG. 410.—Map of the Baltoro glacier of the Himalayas, a typical
glacier of the dendritic type.]
=The expanded-foot glacier.=—As air temperatures continue to become
milder, the glacier streams within the mountains are less deep and
hence more clearly defined, and instead of coalescing upon the mountain
foreland, they now issue from the mountains to form individual aprons
and are described as _expanded-foot glaciers_ (Fig. 408, stage II, and
Fig. 292, p. 264).
[Illustration:
FIG. 411.—The Triest glacier, a hanging glacieret separated from the
Great Aletsch glacier to which it was lately a tributary.]
=The dendritic glacier.=—Still later in the hemicycle nourishment of
the glaciers is diminished as depletion from melting increases, so that
the glacier streams no longer reach to the mountain front. Branches
continue to enter the main valley from the several side valleys like
the short branches of a tall tree, and because of this arrangement such
a glacier may be described as a _dendritic glacier_ (Fig. 408, stage
III, and Fig. 410).
Inasmuch as the depletion from melting increases at a rapid rate in
descending to lower levels, the tributary glacier valleys “hanging”
above the main valley in the lower stretches become separated, and may
continue to exist as series of hanging glacierets upon either side
of the main valley below the glacier front (Fig. 408, stage III, and
Fig. 411). It must be clear from this that any attempt to name each
separated ice stream without regard to its relationship must lead to
endless confusion, for glacier size is in such sensitive adjustment
to air temperature that a fall or rise of a few degrees only in the
average annual temperature of the district may prove sufficient to fuse
many glaciers into one or separate one ice mass into many smaller ones.
When in high latitudes a dendritic glacier descends in fjords to
below the level of the sea, it is attacked by the water in the same
manner as are the outlets of Greenland glaciers, and is then known as
a “tidewater glacier”, which may thus be a subtype or variety of the
dendritic glacier (Fig. 412).
[Illustration:
FIG. 412.—The Harriman fjord glacier of Alaska, a tidewater variety of
dendritic glacier (after a map by Gannett).]
=The radiating (Alpine) glacier.=—In the progressive wastings of
dendritic glaciers, there comes a time when their dendritic outlines
give place to radiating ones. Attention has already been called to the
division of the cirque into subordinate basins separated by small rock
arêtes and yielding a markedly scalloped border (Fig. 394, p. 371).
When the ice front retires from the main valley into one of these
mature cirques, the now wasted ice stream is broken up into subordinate
glacierets, each of which occupies one of the basins within the larger
cirque, and these ice streams flow together to produce a glacier whose
component elements radiate like the sticks within a lady’s fan (Fig.
408, stage IV, and Fig. 413).
[Illustration:
FIG. 413.—Map of the Rotmoos glacier, a radiating glacier of
Switzerland (after Sonklar).]
=The horseshoe glacier.=—As the glacier draws near to its final
extinction, it is crowded hard against the wall of the amphitheater in
which it has so long been nourished. Up to this stage it has offered
a swelling front outwardly convex as a direct consequence of the laws
controlling its flow. No longer amply nourished, for the first time
its front is hollowed, and it awaits its final dissolution curled up
against the cirque wall (Fig. 408, stage V, and Fig. 414). Practically
all the glaciers of the United States and southern Canada are of this
type.
The above classification is one depending directly upon glacier
nourishment, and hence also upon size, and upon the stage of the
glacial hemicycle. In order to determine the type of any glacier it is
necessary to know the outlines of the mountain valley—its divide—and
those of the glacier or glaciers within it. It is likely that the
types of the advancing hemicycle of glaciation would be much the same,
save only for the _new-born_ or _nivation glacier_, which would be as
different as possible from the horseshoe type, to which in size it
corresponds. Upon the continent of Antarctica, where the absence of any
general melting of the ice, even in the summer season and near the sea
level, introduces special conditions, some additional glacier types are
found, which, however, it is not necessary that we consider here.
[Illustration: FIG. 414.—Outline map of the Asulkan glacier in the
Selkirks, a typical horseshoe glacier.]
=The inherited-basin glacier.=—It may be, however, that glaciers have
developed, not upon mountains shaped in a cycle of river erosion, nor
yet in succession to an ice cap, as in the normal cases which we have
considered. On the contrary, glaciers may develop where basins of
one sort or another have been inherited from the preceding period.
In such cases inherited depressions may become more important than
the auto-sculpture of the glacier. Glaciers which develop under such
conditions may be described as _inherited-basin glaciers_.
[Illustration: FIG. 415.—Outline map of the Illecillewaet glacier, an
inherited-basin glacier in the Selkirks.]
A partly closed basin between ridges may supply a collecting ground for
snows carried from neighboring slopes by the wind, and so may yield a
broad névé, approaching in size a small ice cap, yet without developing
definite ice streams except upon its border. Such a glacier is the
Illecillewaet glacier of the Selkirks (Fig. 415).
Again in low latitudes the high and pointed volcanic peaks may push
up beyond the snow line into the upper atmosphere, and so become
snow-capped. Definite cirques do not develop well under these
circumstances, and the loose materials of which such peaks are always
composed are attacked in somewhat irregular fashion from the different
sides. This is the case of Mount Rainier and similar peaks of the
Cascade range of North America.
=Summary of types of mountain glacier.=—In tabular form the various
types of mountain glacier may be arranged as follows:—
MOUNTAIN GLACIERS
_Piedmont glacier._ Mountain valleys entirely occupied and largely
submerged, with overflow upon the foreland to form a common ice apron
through coalescence of neighboring streams.
_Expanded-foot glacier._ Valley entirely occupied and an overflow upon
the foreland sufficient to produce individual ice apron.
_Dendritic glacier._ Valley not completely occupied but with tributary
ice streams ranged along the sides of the main stream, and with
hanging glacierets separated near the glacier foot.
_Radiating glacier._ Glacier largely included in a cirque with
subordinate glacierets converging below like the sticks in a lady’s
fan.
_Horseshoe glacier._ Small glacier remnants hugging the cirque wall
and having an incurving front.
_Inherited-basin glacier._ Of form dependent on a basin inherited and
not shaped by the glacier itself.
READING REFERENCE FOR CHAPTER XXVII
WILLIAM H. HOBBS. The Cycle of Mountain Glaciation, Geogr. Jour., vol.
37, 1910, pp. 268-284.