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.

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).

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.

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).

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).

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).

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.

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.

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.