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alive 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 softor rocks around them are worn away, — furrows will be eaten into more deeply,—fissures will be widened,—clayslates 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 glacierworn surfaces which deserve especial notice, are the so-called " roches moutonnees." They are knolls of a peculiar appearance, frequent in the Alps, and first noticed by the illustrious De Saussurc, 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 preexisting 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 moutonnees" in the Alps, and so characteristic everywhere of gla.ieraction. A comparison of any tide-worn hammock 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 tnouionne'es 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 ride, 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-eurfaces equally on all sides. This circumstance should be borne in mind by all who investigate the traces of glacieraction ; 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 mautonneet 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 or.ly 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 raj-s 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 »ver 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 glacierperiod, 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 ledgts. The appearances about the rochet mcnttonne'es, 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. Jn 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 rochet moutonnees about the Grimscl 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. Snch debris 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 glacierworn 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 debris 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 waterworn. 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 debacle 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 courae of time, aad 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 debris 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 ever)* 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.
The correlation between these different loose materials and the position in which they are found helps us also to detect their origin. The loose materials bearing glacier-marks are always found resting upon surfaces which have been worn, abraded, and engraved in the same manner, while the water-worn pebbles are everywhere found resting upon* rocks the abrasion of which may be traced to water. It is true that in some localities, as, for instance, in the gravel-pit of Mount Auburn, near Cambridge, large masses of glacier-worn pebbles alternate with beach - shingle; but it is easy to show that there was here a glacier advancing into the sea, crowding its front moraine and the materials carried under it over and into the shingle washed up by the waves upon the beach. Not infrequently, also, river - pebbles may be found among glacial materials. This is especially the case where, after the disappearance of large glaciers, rivers have occu
pied their beds. Examples of this kind may be seen in all the valleys of the Alps.
But, besides the special character of the individual fragments, the true origin of any accumulation of glacier-rfeJrw, commonly called drift, may be detected by the total absence of stratification, so essential a feature in all water - deposits. This absence of stratification throughout its mass is, after all, the great and important characteristic of the drift; and though I have alluded to it before, I reiterate it here, as that which distinguishes it from all like accumulations Under water. I may be pardoned for dwelling upon this point, because the great controversy among geologists respecting the nature and origin of the sheet of loose materials scattered over a great part of the globe turns upon it. The debris of which the drift consists are thrown together pell-mell, without any arrangement according to size or weight, larger and smaller fragments being mixed so indiscriminately that the heaviest materials may be on the very summit of the mass, and the lightest at the bottom in immediate contact with the underlying rock, or the larger pieces may stand at any level in the mass of finer ones. Impalpable powder, coarse sand, rounded, polished, and scratched fragments of every size are mixed together in a homogeneous paste, in which the larger materials are imbedded, to use a homelv, but expressive comparison, like raisins and currants in a pudding. The adhesive paste holding all these fragments together is, no doubt, the result of the friction to which the whole was subjected under the glacier, and which has worked some of the softer materials into a kind of cement.
The mode of aggregation of water-worn materials is very different. Examine the shingle along our beaches: we find it so distributed as to show that the fading tide-wave has carried the lighter materials farther than the heavier ones, and the successive deposits exhibit an imperfect cross-stratification resulting from changes in the height of the tide and the direction of the wind. Moreover, in any materials collected under water we find the heavier ones at the bottom, the lighter on the top. It is true that large angular boulders may occasionally be found resting upon beach - shingle, but their presence in such a connection is easily explained. They may have been dropped there by floating icebergs, or have fallen from c'rumhling drift-cliffs.
I should add, in speaking of drift-materials, that, while we find the large angular boulders resting atibve them, we occasionally fmd boulders of unusual size mingled with them; but, when this is the case, such massive fragments are more or less rounded, polished, and marked in the same way as the smaller pebbles, or as the surfaces over which the glacier has passed. This is important to remember, because, when we examine the drift in countries where the ice, during the glacier-period, overtopped nearly all the mountains, so that few fragments could fall from them upon its surface,- we find scarcely any angular boulders, while the drift is interspersed with larger fragments of this character, carried under the ice, instead of on its back. Another distinction between water-worn deposits and drift consists in the feet that the former are washed clean, while the latter always retains the mud gathered during its journey and spread throughout its mass.
In summing up the glacial evidences, I must not omit the moraines, though I have described them so fully in a previous article that I need not do more than allude to them here; but any argument for the glacial theory which did not include these characteristic walls erected by glaciers would be most imperfect. We need hardly discuss the theory of currents with reference to the formation of terminal moraines, extending across the valleys from side to side. Any current powerful enough to bring the boulders and d£bris of all sorts of which these walls are composed to the places where they are found would certainly not build them up with such regularity, but would sweep them away or scatter them along thejiot
VOL. XIII. 47
tom of the valley. That this is actually the case is seen in the lower course of the valley of the Rhone, where there are no transverse moraines, while they are frequent and undisturbed in the upper part of the valley. This is no doubt owing, to the fact, that, when the main glacier had already retreated considerably up the valley, the lateral glaciers from the chains of the Combin and the Diablerets still reached the valley of the Rhone at a lower point, and barred the outlet of the waters from the glaciers above. A lake was thus formed, which, when the lower glaciers retreated up the lateral valleys, swept away all the lower transverse moraines, and formed the flat bottom of Martigny. In this case, the moraines were totally obliterated; but there are many other instances in which the materials have been only broken up and scattered over a wider surface by currents. In such remodelled moraines, the glaciermud has, of course, been more or less washed away. We have here a blending of the action of water with that of the glacier; and, indeed, how could it be otherwise, when the colossal glaciers of past ages gradually disappeared or retreated to the mountain - heights? The wasting ice must have occasioned immense freshets, the action of which we shall trace hereafter, when examining the formation of our drift-ponds, of our river-beds and estuaries, as well as the river-terraces standing far above the present waterlevel.
And now, if it be asked how much of this evidence for the former existence of glaciers is to be found in Great Britain, I answer, that there is not a valley in Switzerland where all these traces are found in greater perfection than in the valleys of the Scotch Highlands, or of the mountains of Ireland and Wales, or of the lake-region in England. Not a link is wanting to the chain. Polished surfaces, traversed by striae, grooves, and furrows, with a sheet of drift resting immediately upon them, extend throughout the realm,—the roches moutonnc'es raise their rounded backs from the ground there as