Popular Science Monthly/Volume 14/April 1879/The Origin of Upland Lakes
|THE ORIGIN OF UPLAND LAKES.|
WHO has not felt a sudden and intense pleasure when, rounding the end of some mighty mountain or towering crag, the still waters of an upland lake or tarn have first met the eye? Perhaps, on approach, wild birds have started from the smooth surface and left it a little sea of shimmering gold, as the sun's light has been reflected from each tiny wavelet. The raven's croak among the overhanging cliffs, the patch of snow lying unmelted deep in a rocky fissure, the scattered sheep browsing carelessly on the few grassy slopes, while all around are masses of tumbled rock, and the light veil of cloud that ever and anon sweeps the cliff-tops and adds an air of mystery and wonder to the whole—all combine to make a scene which can not but send a thrill of pleasure and perhaps of happy awe to every heart. Instinctively one feels, if the power of expression be not present, what Nature's true poet hath so truly sung:
. . . . How divine
The liberty, for frail, for mortal man,
To roam at large among unpeopled glens
And mountainous retirements, only trod
By devious footsteps; regions consecrate
To oldest time! and, reckless of the storm
That keeps the raven quiet in her nest,
Be as a presence or a motion—one
Among the many there.
No one can wander over rugged and beautiful mountains without being led to love and admire these calm sheets of water, which lie nestled in hollows, and are ofttimes blackened by the shadow of encircling cliffs. Love for such solitary spots soon excites our curiosity as to the origin of these miniature upland lakes. In the Cumbrian lake district they are scattered broadcast over the country in far greater numbers than most people imagine, and at a period not vastly remote their number must have been more than double what it is now. But the yearly waste of mountain-side and the matter brought down by every stream have filled up many a mountain pool, and frequent peat-mosses mark the spot where once the waters danced in the mountain breeze. Whence these hollows? What is their origin? Do we see in them the relics of volcanic effort? Are the combs (cwm), coves, or corries in which they lie the vestiges of volcanic craters, as the form of many at first, perhaps, suggests? Or have we here hollows produced directly by surface action? Again, are these hollows of great depth, or are they shallow? What is their general form? Now, there is little doubt that most people, if asked to draw the form of the hollow in which the waters of a tarn now lie so placidly, would grossly exaggerate its true depth, or perhaps liken it to the basin formed by placing the two hands together, side by side, curved, with the palms uppermost. Some years since I took a number of soundings among the Cumbrian lakes and tarns, and communicated the results of my examination to the Geological Society ("Quarterly Journal of the Geological Society," vol. xxx., p. 96, and vol, xxxi., p. 152). Hold out one hand, palm uppermost, and straighten it as much as possible—the hollow in the palm is yet far too deep to represent with truth the natural rock basin. Soundings taken in lakes throughout the district all show the same thing—the basins are very shallow compared with their size and the height of the surrounding hills.
Next, let us search out the origin of these shallow basins. At the outset we distinguish two classes of action, one of which must have been at work. Either the matter formerly filling the hollow has been dug out and carried away by some agent working at the surface; or force from below has here sought a vent, and dispersed the matter far and wide; or, from failing support, the ground has sunk at this spot into a hollow.
First we will consider the upward or downward theory. If these numerous mountain hollows, with included tarns, be of volcanic origin, then it is clear we shall find the signs of a crateral hollow such as we see them in many parts of the world at the present day. There are no such signs. It is true that in many cases the surrounding rocks are of volcanic origin; but the volcanic beds, in their he and position, show no manner of relation to the tarn-hollows; and a little study of the rocks of the district and the form of the ground clearly shows that the volcanoes which gave rise to the ashes and lavas forming many of Cumbria's highest mountains, were active, not as but yesterday, but in untold ages past. Then, as to the downward or special depression theory, when we can conceive such minute subsidences taking place at a great number of almost microscopic spots without affecting the rocks around, or leaving an}' evidence of a sinking away, we may admit it as possible.
If not produced by expulsion of matter outward or sinking of matter inward, these hollows must be the effect of some surface-working agent. The sea planes away along the coast-line, and the material goes to fill up ocean-hollows; therefore the sea can not be the agent. and any force in an ocean current is clearly out of the question over these scattered spots. Streams and rivers work along lines, form ravines and gorges, but never a more or less circular basin of great size in comparison with the stream, or river; hence they can not be the agents. The atmospheric powers—rain, snow, wind, and chemical action—weather the rocks indeed, form tiny basins on almost every stone; but this is but nature's fretwork, the delicate carving around the sculptured craggy tower or spire and smooth-scooped rocky front. Yet there is one surface agent remaining, the moving glacier. Most people are familiar with the proofs of former glacial action in Cumberland and Wales—proofs as clear as are those of the former greater extension of the Swiss glaciers. Now by far the greater number of our tarns lie in true rock basins—hollows completely inclosed by rocky sides, which are, moreover, smoothed and grooved in a manner peculiar to ice-action. At the sides of many a tarn and lake you may see the ice grooves and scratches passing beneath the water, so as to leave no doubt whatever that ice has once occupied the rocky hollow. The question is. Did the ice-movement form the hollow? I believe that in most cases it did, and for these reasons: 1. The tarn lies almost invariably in the path of old ice streams or glaciers, as is proved by the direction of the scratches in the surrounding rocks. 2. They frequently occur at the foot of slopes more or less steep, or where the ice-pressure can be shown to have been great. 3. The position of the deepest points in the larger tarns and lakes occurs almost invariably where, from the confluence of two or more glaciers or the narrowing of the valley, the ice-pressure must have been somewhat increased. 4. The depth of these tarns is very slight as compared with the thickness of the ice which can be proved to have passed over them. 5. There is every gradation from a tiny, rock-bound pool, glaciated on all sides, and which all will admit must have been scooped out by the ice, to the tarn or lake showing precisely similar phenomena on a larger scale.
Since the ice-plow passed over our land the atmospheric powers have been at work for a long period; and while many rock basins are now completely filled up by stream-borne matter, all are being so filled, and each age must witness a decrease in the number and size of those sheets of water which form so marked a character of our Cumbrian scenery.
Before quitting this subject, however, I must remark that there are a few tarns which seem to me to owe the whole or a part of their depth to a moraine dam. That is to say, the rock basin is imperfect on one side, and there an old glacial moraine may have helped to dam the waters back ever since the retreat of the glacier which threw off the moraine. It frequently happens that a little moraine material has been left upon ice-rounded rocks at the foot of a tarn, and in such cases a hasty observation might lead one to believe that the whole mound was a moraine. Let us remember, then, that a tarn may lie in a complete rock basin, ice-formed; in a glaciated hollow dammed on the lower side by a moraine or other accumulation of rocky débris; or it may owe part of its depth to a rock-inclosed hollow, and part to a morainic dam. Therefore, on a summer's day, as we lie dreamily gazing upon the rippling waters of these mountain tarns, we may sometimes think of an age which is past, when the ice-sheet moved majestically over the now heather-clad fells, and all the country lay "clad in white samite, mystic, wonderful."—Popular Science Review.