The Story of a Boulder; or, Gleanings from the Note-book of a Field Geologist

by Archibald Geikie

and in conjuring up visions of earthquakes, and frightful abysses from which there ever rose a lurid glare as hill after hill of molten rock came belching up from the fires below. But while far from denying that such appearances may have been sometimes seen during the long lapse of the geological ages, and that they give no little vividness and sublimity to a geological picture, we claim for the doctrine of the tranquil and uniform operation during past time of existing laws and forces, an element not less poetic. In the former case the pervading idea is that of unlimited expenditure of power, in the latter that of unlimited lapse of time. In the one case the action is Titanic but transient, in the other it is tranquil but immensely protracted. The two doctrines in this way counterbalance each other; yet I cannot but think that however impressive it may be to stand in some lone glen, and while gazing at its dark jagged precipitous cliffs, to dream about the paroxysmal convulsions of some hour far back in the distant past, the scene becomes yet more impressive when we look on its nakedness and sublimity not as the sudden and capricious creation of a day, but as the gradual result of a thousand centuries. These cliffs may once have been low-browed rocks rising but a little way out of a broad grassy plain, and serving as a noon-tide haunt for animals of long extinct races. Thousands of years pass away and we see these same rocks higher and steeper in their outline, brown with algæ and ever wet with surf, while around them stretches a shoreless sea. Ages again roll on, and we mark still the same rocks shooting up as bleak crags covered with ice and snow. Another interval of untold extent elapses, and rock, snow, and ice have all disappeared beneath a broad ocean cumbered with ice-floes and wandering bergs. Again the curtain drops upon the scene, and when once more it rises, the cliffs stand out in much the same abrupt precipitous aspect with that which they now present, save that their bald foreheads look less seamed and scarred than now, and their dark sides show no trace of bush or tree. The white cascades that to-day pour down from their summits and sides—seeming in the distance like the white hairs of age—are insensibly deepening the scars and furrows on these ancient hills, and thus slowly but yet surely carrying on the process of degradation and decay. Musing on all this long series of stages in the formation of one single cliff-line, is there not something more sublime, something yet more impressive than if we pictured but the chance random result of the gigantic paroxysm of an hour?

Let us not be deterred then from seeking an explanation of the origin of the ravine among some of the quieter and more unobtrusive forces of Nature. Give them but an unlimited period to work in and they will abundantly satisfy all our demands.

We return again to the rocky ledge in mid-channel, and proceed to ascend the course of the stream, marking as we go the changes in the character and features of the stone that forms the cliff on either hand. We come to a bare part of the ravine where brushwood and herbage find but a scanty footing and where accordingly the rocks can be attentively studied. The face of the escarpment shows a number of beds of pale grey sandstone alternating with courses of a dark crumbly shale. The sandstones being harder and firmer in texture stand out in prominent relief while the shales between have been wasted away, covering the bottom of the slope with loose debris. We can mark too that, as this decay goes on, the harder beds continually lose their support, cracking across chiefly along the lines of joint, and rolling down in huge angular blocks into the stream. In truth we cannot doubt that every year adds to this decay and thus slowly widens the dell, for the broken fragments do not form in heaps over the solid rock below so as to protect it from the weather, but are evidently carried away by the stream and hurried down the ravine onwards to the sea. From what has been said above relative to the disintegration of rocks by percolating water, frosts, and other causes, the reader will easily see how this rotting away of the sides of the ravine must be carried on; and he will not fail to mark that here we have at work an agency not yet considered, that of running water. The effects of the weather are seen in the crumbling, ruinous cliffs overhead; the effects of the streamlet are observable in the continual removal of the rubbish whereby a fresh surface is ever exposed to the decomposing forces, while at some points we can mark the water actually undermining an overhanging part of the cliff from which there are ever and anon vast masses precipitated into the channel where eventually they get worn down and carried away out to sea. "Still," you may remark, "these forces are at work only in widening a channel already made. How was the ravine formed at first?"

We continue our ascent. A scrambling walk through briars and hazel-bushes, sometimes on rocky ledges high among the cliffs, sometimes among the prostrate blocks that dam up the stream, brings us at last full in front of a sparkling waterfall that dashes over a precipitous face of rock some twenty feet high. The appearances observable here deserve a careful attention. Our eyes have not been long employed noting the more picturesque features of the scene ere they discover that the dark-brown band of rock forming the summit of the ledge over which the water tumbles is continuous all round the sides of the dell. There is consequently no break or dislocation here. Approaching the cascade we note the rock behind it so hollowed out that its upper bars project several feet beyond the under ones. In this way the body of water is shot clear over the top of the cliff without touching rock till it comes splashing down among the blocks in the channel. And yet this hollowed surface is never dry; the spray of the fall constantly striking on it keeps it always dank and dripping. In some parts the rock stands out bare and worn, while on the less exposed portions there gathers a thick green scum which is replaced on the drier ledges by the soft cellular leaves of the liver-wort. Now our examination of the influence of percolating water upon even the hardest rocks teaches us that this moist soaked surface is just the very best condition for favouring the decay of the rock. Nay more, the green vegetation that mantles over the stone serves to prevent the water from running off too rapidly, and keeps the rock in a still more moist state than would otherwise happen. So that the portion of sandstone behind the cascade comes to be in a still more favourable situation for speedy decay than the ledge over which the water is rapidly driven. We can see, therefore, how in the lapse of years the corrosion may go on until the upper projecting part of the cliff loses its support and falls with a crash into the rocky pool below, while the form of the waterfall becomes thus greatly altered, and new surfaces are exposed to the wear and tear of the stream.

But we have not yet exhausted all that the rocks at the cascade can teach us. By dint of some exertion we climb the cliff and gain the upper edge of the fall. The rocks that form the bed of the stream are now seen to be deeply grooved and worn, every exposed surface having a smoothed blunted aspect. We can mark how the stone has split up along the natural lines of joint, whereby great facility is given to the removing power of the current, and how large irregular angulated blocks become detached and are swept down the stream. In not a few parts, too, we may notice circular holes of greater or less depth, in the bottom of each of which lie perhaps a pebble or two, that with a constant gyratory movement, caused by the eddying water, have eaten their way downwards into the solid rock. When the stream is in flood and comes roaring down the rocky gorge bearing along with it a vast amount of mud, gravel, and stones, one can easily see how the friction of the transported material must wear down the hard bed and sides of the channel, and how this process repeated month after month and year after year, must aid the decomposing forces in scooping out a deep ravine. From the cascade the ascent of the stream becomes steeper and the run of water is consequently more rapid. Soon however we emerge from the woody copse, and find ourselves on a flat alluvial cultivated plain through which the rivulet winds in a tortuous meandering course, bending back upon itself into loops that almost meet and well-nigh form broad flat islets. Strolling along this winding route we can mark the effects of the stream in eating away the soft clay and sand at one part of the bend and piling them up at another. Such loose material can present but little resistance to a stream swollen with rains, and consequently a large quantity of the mud and gravel along with the interspersed boulders must be swept away down into the dell at every season of flood. The matter thus removed will of course be still further comminuted in its passage, and at the same time will help to grind down the hard rock surfaces over which it is driven.

Here then may be found the whole history of the ravine. Originally the streamlet wound its devious course through a flat alluvial country with a channel sunk but a foot or two below the level of the plain. Such continued its character till it reached a low bluff, down which the water flowed more rapidly to gain a second level undulating region. The part of this bluff crossed by the stream was ere long bared of its covering of soil and clay, and the rock below came to be washed by a group of little cascades. Once exposed to the decomposing and disintegrating forces, the stone soon began to decay and the cascades ere long merged into one. By slow degrees the rock gave way and the waterfall retreated from the bluff. For perchance thousands of years the same process has been going on, now with greater, now with less rapidity, according to the nature of the rocks encountered and other modifying causes, until the fall has eaten its way back for well-nigh three miles and scooped out a wild rocky gorge some fifty or sixty feet deep. This is but a solitary and insignificant instance of what may be seen all over the world, for the process remains the same whether we stand beside a tiny rivulet in some lone Highland glen or listen to the roar of the falls of Niagara.

There is but one other principal agency at work in the demolition of rock-masses, the waves and currents of the ocean. But we have already noted the effects thus produced, and need not now retrace our steps further than to recall the vast amount of devastation which can be shown to have been effected in our own country by marine causes, both in breaching the existing shores and in scooping out valleys and grinding down hills at former periods when the land was either rising above or sinking below the level of the sea.

Having now satisfied ourselves that there goes on all over the world an incessant waste of the solid lands, that the disintegrated debris is washed down by rains and transported seawards by rivers, and that the waves are ever eating their way into the iron-bound coast-line as well as into the low alluvial shore, we naturally come to ask the result and end of all this decay. What becomes of that vast amount of mineral matter annually removed from the land? To be able to answer this question clearly and distinctly, let us look for a little at what takes place in lakes, at river-mouths, and in open sea.

The river Rhone rises among the Bernese Alps, and after a course of about 100 miles through the Canton of Valais, it enters the upper end of the Lake of Geneva. Its waters, where they mingle with those of the lake, are muddy and discoloured, but where they pass out at the town of Geneva are limpid and clear. The mud, therefore, which they bring into the lake must be deposited there, and as the stream may have continued to flow for thousands of years, we may reasonably expect to find some trace of the large amount of sediment necessarily deposited during the whole or part of that long period. Accordingly, careful examination of the Lake of Geneva has shown that such accumulations have really been formed, and that their progress and amount during part of the historic period can be approximately calculated. Where the turbid current of the Rhone enters the still water of the lake, the mud slowly sinks to the bottom. In the lapse of centuries layer after layer has been thrown down, rendering the lake at this part sensibly shallower, until a large area or delta has been filled up and converted into a flat alluvial plain. Thus, a town which in the time of the Romans formed a harbour on the water's edge, now stands more than a mile and a half inland. This new-formed land is entirely the work of the stream, and if we could obtain a complete section of it from the surface to the bottom, "we should see a great series of strata, probably from 600 to 900 feet thick (the supposed original depth of the head of the lake), and nearly two miles in length, inclined at a very slight angle." These strata, which are said to have taken about eight centuries to form, "probably consist of alternations of finer and coarser particles; for, during the hotter months, from April to August, when the snows melt, the volume and velocity of the river are greatest, and large quantities of sand, mud, vegetable matter, and drift-wood, are introduced; but, during the rest of the year, the influx is comparatively feeble, so much so that the whole lake, according to Saussure, stands six feet lower."[55] If the present conditions continue for a sufficient length of time, the lake may be eventually filled up with mud, sand, and gravel, deposits that would eventually harden by pressure into shale and sandstone. So that the day may yet arrive when the blue waters of the Leman lake shall have passed away, when the Rhone perchance may have ceased to flow or found its way by some other channel, when the peasant may guide the plough where now the boatman plies the oar, and when the geologist shall trace out in quarries and excavations the successive deposits of hardened sediment with their lacustrine shells and drift-wood, and, musing on the changes of which they are the silent yet impressive witnesses, may sit down to pen a record of the gradual extinction of the Leman lake on that classic ground where an immortal historian described the decline and fall of the empire of Rome.

[55] Lyell's Principles of Geology. Ninth edition, p. 252.