The levelling and abrading action of water on rock has an entirely different character. Tides or currents driven powerfully and constantly against a rocky shore, and bringing with them hard materials, may produce blunt, smooth surfaces, such as the repeated blows of a hammer on stone would cause; but they never bring it to a high polish, because, the grinding materials not being held steadily down, in firm, permanent contact with the rocky surfaces against which they move, as is the case with the glacier, but, on the contrary, dashed to and fro, they strike and rebound, making a succession of blows, but never a continuous, uninterrupted pressure and friction. The same is true of all the marks made on rocky shores against which loose materials are driven by water-currents. They are separate, disconnected, fragmentary; whereas the lines drawn by the hard materials set in the glacier, whether light and fine or strong and deep, are continuous, often unbroken for long distances, and rectilinear. Indeed, we have seen[A] that we have beneath every glacier a complete apparatus adapted to all the results described above. In the softer fragments ground to the finest powder under the incumbent mass we have a polishing paste; in the hard materials set in that paste, whether pebbles, or angular rocky fragments of different sizes, or grains of sand, we have the various graving instruments by which the finer or coarser lines are drawn. Not only are these lines frequently uninterrupted for a distance of many yards, but they are also parallel, except when some change takes place in the thickness of the ice, which may slightly modify the trend of the mass, or where lines in a variety of directions are produced by the intermittent action of separate glaciers running successively at different angles over the same surfaces. The deeper grooves sometimes present a succession of short staccato touches, just as when one presses the finger vertically along some surface where the resistance is sufficient to interrupt the action without actually stopping it,—a kind of grating motion, showing how firmly the instrument which produced it must have been held in the moving mass. No currents or sudden freshets carrying hard materials with them, even moving along straight paths down hill-sides or mountain-slopes, have ever been known to draw any such lines. They could be made only by some instrument held fast as in a vice by the moving power. Something of the kind is occasionally produced by the drag of a wheel grating over rocks covered with loose materials.
It has been said that grounded ice or icebergs floating along a rocky shore might produce similar marks; but they will chiefly be at the level of high-water mark, and, if grounded, they will trend in various directions, owing to the rocking or rotating movement of the iceberg. It has also been urged, that, without admitting any general glacier-period, icebergs and floating ice from more northern latitudes might account for the extensive transportation of the loose materials scattered in a continuous sheet over a large portion of the globe. There can be no doubt that an immense amount of débris of all sorts is carried to great distances by floating ice; where their presence is due to this cause, however, they are everywhere stranded along the shore or dropped to the sea-bottom. Large boulders are frequently left by the ice along the New-England coast, and we shall trace them hereafter among the sand-dunes of Cape Cod. But before it can be admitted that the drift-phenomena, and the polished and engraved surfaces with which they are everywhere intimately associated, are owing to floating ice or icebergs, it must be shown that all these appearances have been produced by some agency moving from the sea-board towards the land, and extending up to the very summits of the mountains, or else that all the countries exhibiting glacial phenomena have been sunk below the ocean to the greatest height at which glacier-marks are found, and have since gradually emerged to their present level. Now, though geologists are lavish of immersions when something is to be accounted for which they cannot otherwise explain, and a fresh baptism of old Mother Earth is made to wash away many obstacles to scientific theories, yet the common sense of the world will hardly admit the latter assumption without positive proof, and all the evidence of the kind we have, at the period under consideration, indicates only a comparatively slight change of relative level between sea and land within a narrow belt along the shores; and even this is shown to be posterior, not anterior, to the glacial phenomena. As to the supposition that the motion proceeded from the sea towards the land, all the facts are against it, since the whole trend of these phenomena is from inland centres toward the shore, instead of being from the coast upward.
Certainly, no one familiar with the facts could suppose that floating ice or icebergs had abraded, polished, and furrowed the bottom of narrow valleys as we find them worn, polished, and grooved by glaciers. And it must be remembered that this is a theory founded not upon hypothesis, but upon the closest comparison. I have not become acquainted with these marks in regions where glaciers no longer exist, and made a theory to explain their presence. I have, on the contrary, studied them where they are in process of formation. I have seen the glacier engrave its lines, plough its grooves and furrows in the solid rock, and polish the surfaces over which it moved, and was familiar with all this when I found afterwards appearances corresponding exactly to those which I had investigated in the home of the present glaciers. I could therefore say, and I think with some reason, that "this also is the work of the glacier acting in ancient times as it now acts in Switzerland."
There is another character of glacial action distinguishing it from any abrasions caused by water, even if freighted with a large amount of loose materials. On any surface over which water flows we shall find that the softer materials have yielded first and most completely. Hard dikes will be left standing out, while softer rocks around them are worn away,—furrows will be eaten into more deeply,—fissures will be widened,—clay-slates will be wasted,—while hard sandstone or limestone and granite will show greater resistance. Not so with surfaces over which the levelling plough of the glacier has passed. Wherever softer and harder rocks alternate, they are brought to one outline; where dikes intersect softer rock, they are cut to one level with it; where rents or fissures traverse the rock, they do not seem to have been widened or scooped out more deeply, but their edges are simply abraded on one line with the adjoining surfaces. Whatever be the inequality in the hardness of the materials of which the rock consists, even in the case of pudding-stone, the surface is abraded so evenly as to leave the impression that a rigid rasp has moved over all the undulations of the land, advancing in one and the same direction and levelling all before it.
Among the inequalities of the glacier-worn surfaces which deserve especial notice, are the so-called "roches moutonnées." They are knolls of a peculiar appearance, frequent in the Alps, and first noticed by the illustrious De Saussure, who designated them by that name, because, where they are numerous and seen from a distance, they resemble the rounded backs of a flock of sheep resting on the ground. These knolls are the result of the prolonged abrasion of masses of rocks separated by deep indentations wide enough to be filled up by large glaciers, overtopping the summits of the intervening prominences, and passing over them like a river, or like tide-currents flowing over a submerged ledge of rock. It is evident that water rushing over such sunken hills or ledges, adapting itself readily to all the inequalities over which it flows, and forming eddies against the obstacles in its course, will scoop out tortuous furrows upon the bottom, and hollow out rounded cavities against the walls, acting especially along preëxisting fissures and upon the softer parts of the rock,—while the glacier, moving as a solid mass, and carrying on its under side its gigantic file set in a fine paste, will in course of time abrade uniformly the angles against which it strikes, equalize the depressions between the prominent masses, and round them off until they present those smooth bulging knolls known as the "roches moutonnées" in the Alps, and so characteristic everywhere of glacier-action. A comparison of any tide-worn hummock with such a glacier-worn mound will convince the observer that its smooth and evenly rounded surface was never produced by water.
Besides their peculiar form, the roches moutonnées present all the characteristic features of glacier-action in their polished surfaces accompanied with the straight lines, grooves, and furrows above described. But there are two circumstances connected with these knolls deserving special notice. They frequently present the glacial marks only on one side, while the opposite side has all the irregularities and roughness of a hill-slope not acted upon by ice. It is evident that the polished side was the one turned towards the advancing glacier, the side against which the ice pressed in its onward movement,—while it passed over the other side, the lee side as we may call it, without coming in immediate contact with it, bridging the depression, and touching bottom again a little farther on. As an additional evidence of this fact, we frequently find on the lee side of such knolls accumulations of the loose materials which the glacier carries with it. It is only, however, when the knolls are quite high, and abrupt enough to allow any rigid substance to bridge over the space in its descent from the summit to the surface below, that we find these conditions; when the knolls are low and slope gently downward in every direction, they present the characteristic glacier-surfaces equally on all sides. This circumstance should be borne in mind by all who investigate the traces of glacier-action; for this inequality in the surfaces presented by the opposite sides of any obstacle in the path of the ice is often an important means of determining the direction of its motion.
The other characteristic peculiarity of these roches moutonnées consists in the direction of the glacier-scratches, which ascend the slope to its summit in a direct line on one side, while they deviate to the right and left on the other sides of the knoll, more or less obliquely according to its steepness. Occasionally, large boulders may be found perched on the very summit of such prominences. Their position is inexplicable by the supposition of currents as the cause of their transportation. Any current strong enough to carry a boulder to such a height would of course sweep it on with it. This phenomenon finds, however, an easy explanation in the glacial theory. The thickness of such a sheet of ice is of course less above such a hill or mound than over the lower levels adjoining it. Not only will the ice melt, therefore, more readily at this spot, but, as ice is transparent to heat, the summit of the prominence will become warmed by the rays of the sun, and will itself facilitate the melting of the ice above it. On the breaking up of the ice, therefore, such a spot will be the first to yield, and allow the boulders carried on the back of the glacier to fall into the hollow thus formed, where they will rest upon the projecting rock left uncovered. This is no theoretical explanation; there are such cases in Switzerland, where holes in the ice are formed immediately above the summit of hills or prominences over which the glacier passes, and into which it drops its burdens. Of course, where the ice is constantly renewed over such a spot by the onward progress of the glacier, these materials may be carried off again; but if we suppose such a case to occur at the breaking up of the glacier-period, when the ice was disappearing forever from such a spot, it is easy to account for the poising of these large boulders on prominent peaks or ledges.
The appearances about the roches moutonnées, especially the straight scratches and grooves on the side up which the ice ascended, have led to a mistaken view of the mode in which large boulders are transported by ice. It has been supposed, by those who, while they accepted the glacial theory, were not wholly conversant with the mode of action of glaciers, that, in passing through the bottom of a valley, for instance, the glacier would take up large boulders, and, carrying them along with it, would push them up such a slope and deposit them on its summit. It is true that large boulders may sometimes be found in front of glaciers among the materials of their terminal moraines, and may, upon any advance of the glacier, be pushed forward by it. But I know of no example of erratic boulders being carried to considerable distances and raised from lower to higher levels by this means. All the angular boulders perched upon prominent rocks must have fallen upon the surface of the glacier in the upper part of its course, where rocky ledges rise above its surface and send down their broken fragments. The surface of any boulder carried under the ice, or pushed along for any distance at its terminus, would show the friction and pressure to which it had been subjected. In this connection it should be remembered that in the case of large glaciers low hills form no obstacle to their onward progress, especially when the glacier is thick enough to cover them completely, and even to rise far above them. The roches moutonnées about the Grimsel show that hills many hundred feet high have been passed over by the great glacier of the Aar, when it descended as far as Meyringen, without having seemingly influenced its onward progress.
But in enumerating the evidences of glacier-action, we have to remember not only the effects produced upon the surface of the ground by the ice itself, but also the deposits it has left behind it. The loose materials scattered over the face of the earth may point as distinctly to the source of their distribution as does the character of the rocky surfaces on which they rest indicate the different causes of abrasion. In characteristic localities the loose materials deposited by glaciers may readily be recognized at first sight, and distinguished from water-worn pebbles; nor is it difficult to distinguish both from loose materials resulting from the decomposition of rocks on the spot,—the latter always agreeing with the rocks on which they rest, while the decomposition to which they owe their separation from the solid rock is often still going on. Such débris are found everywhere about disintegrating rocks, and they constantly mingle with the loose fragments brought from a distance by various agencies. They are found upon and among the glacier-worn pebbles, especially where the latter have themselves been disturbed since their accumulation. They are also found among water-worn pebbles, wherever the rocky beds of our rivers or the rocky bluffs of our sea-shores crumble down. In investigating the character of loose materials transported from greater or less distances, either by the agency of glaciers or by water-currents, it is important at the very outset to discriminate between these deposits of older date and the local accessions mingling with them.
Occasionally we may have also to distinguish between all these deposits and the débris brought down by land-slides, or by sudden freshets transporting to a distance a vast amount of loose materials which are neither ice-worn nor water-worn. At Rossberg, for instance, in the Canton of Schwitz, the land-slide which buried the village of Goldau under a terrific avalanche, and filled a part of the Lake of Lauertz, spread an immense number of huge boulders across the valley, some of which even rolled up the opposite side to a considerable height. Many of these boulders might easily be mistaken for erratic boulders, were not the aggregate of these loose materials traceable to the hills from which they descended. In this case water had no part in loosening or bringing down this mass of fragments. They simply rolled from the declivity, and stopped when they had exhausted the momentum imparted to them by their weight. In the case of the débâcle of Bagnes, above Martigny, in a valley leading to the St. Bernard, the circumstances were very different. A glacier, advancing beyond its usual limits and rising against the opposite mountain-slope, dammed up the waters of the torrent and caused a lake to be formed. The obstruction gave way in the course of time, and the waters of the lake rushed out, carrying along with them huge boulders and a mass of loose materials of all sorts, and scattering them over the plain below. Such an accumulation of débris differs from the pebbles and loose fragments found in river-beds. The comparatively short distance over which they are carried, and the suddenness of the transportation, allow no time for the abrasion which produces the smooth surfaces of water-worn pebbles or the polished and scratched surfaces of glacier-worn ones. In the latter case, we have seen that the pebbles, being so set in the ice as to expose only one side, may be only partially polished, while others, more loosely held and turning in their sockets, may receive the same high polish on every side. In such a case the lines will intersect one another, in consequence of the different position in which the stone has been held at different times. No such appearances exist in the water-worn pebbles: their blunt surfaces, smoothed and rounded uniformly by the action of the water in which they have been rolled or tossed about, present everywhere the same aspect.