Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

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]).

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]).

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]).