Shearing.—In the lower portion of a glacier where normally the thrusts are greatest, the granules fewest, and their interlocking most intimate, shearing takes place within the ice itself. This is illustrated by the accompanying Figs., 292–295. The shearing results in the foliation of the ice and in the forcing of débris between the sheared layers. Thus the ice becomes loaded in a special englacial or baso-englacial fashion, as previously mentioned and illustrated in Fig. 268.
Within the zone of shearing, it is probable that the gliding planes of the crystals come into effective function. It is thought that the combined effect of the vertical pressure, the forward thrust, and the basal drag of the ice, may be to increase the number of granules whose gliding planes are parallel to the glacier’s bottom. At any rate, Drygalski reports[136] that there is a tendency to such an arrangement in the basal portion of the Greenland glaciers at their borders. It is conceived that where strong thrusts are brought to bear upon such a mass of granules, those whose gliding planes are parallel to the direction of thrust are strained with sufficient intensity to cause the plates to slide over each other, while those which are not parallel to the direction of thrust are either rotated into parallelism—when they also yield—or are pressed aside out of the plane of shear. As previously noted, shearing is observed to occur chiefly where the ice below the plane of shearing is protected more or less from the force of the thrust. It perhaps also occurs where the basal ice becomes so overloaded with débris that it is incapable of ready movement.
It is also probable that sharp differential strain and shearing are developed at the level where the surface-water of the warm season, descending into the ice, reaches the zone of freezing. The expanding of the freezing water at the upper limit of the cold zone may cause the layer expanded by it to shear over that below. As the level of freezing is lowered with the advance of the warm season, the zone of shearing also sinks. This may be regarded as an auxiliary agency of shearing, of application to a special horizon.
High temperature and water.—In the zone of waste, a higher temperature and more water lend their aid to the fundamental agencies of movement, and there is need for these aids to promote a proportionate movement, for here the granules are more intimately interlocked and the ice more compact and inherently more solid and rigid. The average temperature is, however, near the melting-point (p. 276), and during the warm season the ice is bathed in water so that the necessary changes in the crystals are facilitated, and movement apparently takes place even more readily than in the more open granular portion of lower temperature and dryer state. The extraordinary movements of certain tongues of ice in some of the great fiords of Greenland are probably due to the convergence of very thick slow-moving ice from the interior into basins leading down to the fiords. Into the same basins a large amount of surface-water is concentrated at the same time, with the result that the thick ice, bathed with water and having a high gradient, develops unusual velocity during the warm season.
Applications.—By a studious consideration of the coöperation of the auxiliary agencies with the fundamental ones, the peculiarities of glacial movement may apparently be explained. In regions of intense cold, where a dry state and low temperature prevail, as in the heart of Greenland, the snow-ice mass may accumulate to extraordinary thicknesses, for the burden of movement seems to be thrown almost wholly upon compression, with the slight aid of molecular changes due to internal evaporation and allied inefficient processes. Since the temperature in the upper part of the ice is very adverse (see p. 277), the compression must be great before it becomes effective in melting the ice, and hence the great thickness of the mass antecedent to much motion. Similar conditions more or less affect the heads of alpine glaciers, though here the high gradients favor motion with lesser thicknesses of ice; but in the lower reaches of alpine glaciers, where the temperatures are near the melting-point, and the ice is bathed in water, movement may take place in ice which is thin and compact.
If the views here presented are correct, there is also, near the end or edge of a glacier, the coöperation of rigid thrust from behind with the tendency of the mass to move on its own account. The latter is controlled by gravity, and conforms in its results to laws of liquid flow. The former is a derived factor, and is a mechanical thrust. This thrust is different from the pressure of the upper part of a liquid stream on the lower part, because it is transmitted through a body whose rigidity is effective, while the latter is transmitted on the hydrostatic principle of equal pressure in all directions.
The conception of the glacier and its movement here presented explains some of the anomalies that otherwise seem paradoxical. While a glacier in a sense flows over a surface, it often cuts long, deep furrows in firm rock. It is difficult to explain this if the ice be so yielding as to flow under its own weight on a surface which is almost flat. If the mass is really viscous, its hold on its imbedded débris should also be viscous, and a bowlder in the bottom should be rotated in the yielding mass when its lower point catches on the rock beneath, instead of being firmly held while a deep groove is cut. This is more to the point since viscous fluids flow by a partially rotary movement. If, on the other hand, the ice is always a rigid body which yields only as its interlocking granules change their form by loss and gain, a rigid hold on the imbedded rock at some times, and a yielding hold at others, is intelligible, for on this view the nature of its hold is dependent on the temperature and dryness of the ice. Stones in the base of a glacier may be held with very great rigidity when the ice is dry, scoring the bottom with much force, while they may be rotated with relative ease when the ice is wet. In short, the relation of the ice to the bowlders in its bottom varies radically according to its dryness and temperature. A dry glacier is a rigid glacier. A dry glacier is necessarily cold, and a cold glacier is necessarily dry.
On the view here presented, a glacier should be more rigid in winter than in summer, and the whole thickness of the glacier should experience this rigidity chiefly at the ends and edges, where the relative thinness of the ice permits the low temperature to reach its bottom. The motion in these parts during the winter is, therefore, very small.
In this view may also be found an explanation of the movement of glaciers for considerable distances on upward slopes, even when the surface as well as the base is inclined backwards. So far does this go that superglacial streams sometimes run for some distance backwards, i.e. toward the heads of the glaciers, while in other places surface-waters are collected into ponds and lakelets. Such a slope of the surface of ice is not difficult to understand if the movement be due to thrust from behind, or if it be occasioned by internal crystalline changes acting upon a rigid body; but it must be regarded as very remarkable if the movement be that of a fluid body, no matter how viscous, for the length of the acclivity is sometimes several times the thickness of the ice. Crevassing and other evidences of brittleness and rigidity find a ready elucidation under the view that the ice is a really solid body at all times, and that its apparent fluency is due to the momentary fluidity of small portions of the mass assumed in succession as compression demands.
In addition to the considerations already adduced, it may be urged that a glacier does not flow as a stiff liquid because its granules are not habitually drawn out into elongated forms, as are cavities in lavas and plastic lumps in viscous bodies. Flowage lines comparable to those in lavas are unknown in glaciers.
All this is strictly consistent with our primary thesis, that a glacier is a crystalline rock of the purest and simplest type, and that it never has other than the crystalline state. This strictly crystalline character is incompatible with viscous liquidity.
While these views of glacial motion seem to us to best accord with the known facts, they are not to be regarded as established in scientific opinion, or as the views most commonly held. The mode of glacial motion has long been a mooted question, and is still so regarded. The main alternative interpretations that have been entertained are the following:
(1) In the early days of glacial studies De Saussure thought that glaciers slid bodily on their beds;
(2) Charpentier and Agassiz referred the movement to the expansion of descending water freezing within the glacier;
(3) Rendu and Forbes, followed by many, perhaps most, modern writers, believed ice to be viscous, and that in sufficiently large masses it flows under the influence of its own weight, like pitch or asphalt;
(4) Others, realizing the fundamental difference between crystalline ice and a true viscous body, have fallen back on a vague notion of plasticity which scarcely amounts to a definite hypothesis at all;
(5) Tyndall urged that the movement was accomplished by minute repeated fracturing and regelation, appealing to the fact that broken pieces of ice slightly pressed together at melting temperatures freeze together, but neglecting the fact that this would destroy the integrity of the crystals;
(6) Moseley assigned the movement to a bodily expansion and contraction of the glacier, analogous to the creeping of a mass of lead on a roof;
(7) James Thompson demonstrated that pressure lowers the melting-point, and while this effect is so small as probably to be ineffectual, it is correlated with the very important fact that compression may cause melting, which is not the case in most other rocks. He recognized that under pressure partial liquefaction took place, that the water so liberated might be refrozen as it escaped from pressure, and appears to have regarded this as a vital factor;
(8) Croll held that the movement was due to a consecutive series of molecular changes somewhat like the chain of chemical combinations in electrolysis;
(9) Hugi, Eli de Beaumont, Bertin, Forel, and others thought that the growth of the granules was the leading factor in the ice movement;
(10) McConnel and Mügge have made the gliding planes of the ice crystals serve an important function in glacial movement.
It will be seen that the principle of partial liquefaction for which Thompson laid the basis, the crystallization of descending water, urged by Charpentier and Agassiz, and the granular growth on which Hugi, Beaumont, Forel, and others founded their hypotheses, are incorporated in the view already presented. Probably the agencies on which some of the other views are based may also be participants in producing glacial motion, sometimes as incidental factors, and sometimes perhaps as important ones, for under different conditions, different agencies may play rôles of varying importance. For example, in going over the brinks of precipices of sufficient height, glaciers break into fragments which are re-cemented below, and the “reconstructed” glacier moves on as before. Here fracture and regelation are evident. The movement of the gliding planes of the ice crystals over each other, which has been looked upon as a special kind of viscoid movement, probably plays a large part in the shearing movements in certain cases. But neither of these is probably a large factor in ordinary glacial movement, and it seems highly improbable that any of them are essential factors in the primary movements in the snow-fields where glacial action begins.