Boulder (geology)

“ But no explanation was offered why such agency should have been developed more energetically in modern times than at former periods of the earth's history, or why it should be displayed in its fullest intensity in northern latitudes; for it is important to insist on the fact, that the boulder formation is a northern phenomenon. Even the southern extension of the drift, or the large erratics found in the Alps and the surrounding lands, especially their occurrence round the highest parts of the chain, offers such an exception to the general rule as confirms the glacial hypothesis ”

“ We return again to the boulder, and shall now seek to learn the lessons which the sand and pebbles have to teach us. Their subject belongs to what is called physical geology, and will bring before us some of the more mechanical operations of nature, such as the slow but constant action of air, rain, and rivers, upon hard rock, the grinding action of the waves, and the consequent accumulation of new masses of sedimentary rock.
In all the departments of nature that come under the cognizance of man, there is seen to be an endless succession of mutations. ”

“ It is not all stones. Clay and earth mingle with it, often enclosing the stones; and the debris left by extinct glaciers of ages ago is sometimes called the boulder-clay. This is the deposit, earthy and stony, that the glacier leaves on the floor of the valley as it shrinks—unless the river which usually springs from the end of glaciers sweeps it away. Most of the stones thus left are smoothed or polished and covered with scratches or ruts, such as would be made by rubbing against other hard pointed fragments of stone. ”

“ At a distance of 110 miles from the coast, the plain has risen to the height of 1,416 feet above the sea; and the gravel, with the associated great boulder formation, has attained a thickness of 212 feet. The plain, apparently with its usual gravel covering, slopes up to the foot of the Cordillera to the height of between 3,200 and 3,300 feet. In ascending the valley, the gravel gradually becomes entirely altered in character: high up, we have pebbles of crystalline feldspathic rocks, compact clay-slate, quartzose schists, and pale-coloured porphyries ”

“ As the burrows excavated directly beneath the stone after a time collapse, the stone sinks a little. Hence it is, that boulders which at some ancient period have rolled down from a rocky mountain or cliff on to a meadow at its base, are always somewhat imbedded in the soil; and, when removed, leave an exact impression of their lower surfaces in the under-lying fine mould. If, however, a boulder is of such huge dimensions, that the earth beneath is kept dry, such earth will not be inhabited by worms, and the boulder will not sink into the ground. ”

“ But it is the formation of the many hills of giant granite boulders that make the scenes so astonishing, so rugged, and so unique—you might be on the roughest sea coast in the world, and not find scenes to surpass those here in desolation and wildness. They are hills that appear to the eye as if a mighty energy underneath had at some time heaved and shouldered boulder upon boulder of colossal proportion into position, until huge, wide-based, solid masses were raised upon the plateau. ”

“ We are on the very brink of a deep narrow gorge, the upper part of which is so thickly clad with pines as to resemble the crest of some gigantic helmet, but beneath the naked granite stands out in all its grim barrenness, lashed by the spray of the mighty torrent that roars between its projecting rocks. Just below us, the river, forced back by a huge boulder in the centre of its course, literally piles itself up into a kind of liquid mound, foaming, flashing, and trembling incessantly, the ceaseless motion and tremendous din of the rapids having an indescribably bewildering effect.... ”

“ The scratches on the rock floor are parallel grooves, all of which trend southward. They are unmistakable tracks left by glaciers, and the boulders are like the stones perched on glacial ice for a ride to the terminal moraine.
The land above the old lake shore is bare scratched rock or rocky soil called boulder till. Every hill farm has been cleared of more stones than trees, and it is only with the vogue of the rock garden that these erratics have found any merit in man’s estimation. ”

“ It must contain, in heterogeneous admixture, the great boulders, the lesser rock fragments, the gravel chips, the sand, and the slimy mud; these settling down quietly in the cold, gloomy waters, overshadowed by the great ice-sheet, must form just such an agglomeration as we find in the boulder clay and tills, and lie just in those places where these deposits abound, provided the relative level of land and sea during the glacial epoch were suitable. ”

“ Every now and then a boulder, that appeared firmly seated until burdened with the weight of one of them, would give way and go crashing and thundering down with great leaps behind them until lost in the forest below. ”

“ A deep cleft on a mountain-crest, a bowl-shaped hollow scooped out of a hillside, a profound ravine, a conical mound or a group of such mounds, rising conspicuously above a bare moorland, a solitary boulder of gigantic size, or a line of large boulders—these and many other prominent elements in the scenery, alike of the Lowlands and the Highlands, have arrested attention from the earliest times. ”

“ The broad moonlight fell full on a huge boulder, whose mighty top, a thousand feet above the Cove, was fringed with a tall forest growth that looked in the distance like stunted berry-bushes, and whose rounded granite side was streaked with black storm-stains where the rains of centuries had coursed down. ”

“ In other localities, currents brought in a continual supply of sand, or washed off the boulder mud and sand, and then re-deposited them in irregular beds; hence resulted those local deposits of stratified sand and gravel so frequently to be seen resting over the boulder clay. At length, by degrees, the land emerged from the sea, yet glaciers still capped its hills and choked its valleys; but eventually a warmer and more genial climate arose, plants and animals, such as those at present amongst us, and some, such as the wolf, no longer extant, were ere long introduced ”

“ Some of the limestone boulders lying on the surface of the plateau are perforated in the most extraordinary way. The driving sand apparently eats its way into the softer portions of the stone, boring holes into its surface. Small pebbles are often to be seen which have been blown into these holes. These fly round and round in the excavation under the influence of a strong wind, and presumably continue the erosion of the sand blast, in the same way as a stone wears a pot-hole in a stream. In course of time the whole boulder becomes so riddled with holes as to resemble a gigantic sponge. ”

“ From the bottom of the ravine came a constant roar of falling water, though we could spy it only now and then leaping down from one chasm to another; and more than once our guides would cry to us to stop (and that where our mules had to keep shifting their feet to get a hold) while some huge boulder, loosened by the night's rain, flew down across our path in terrific bounds from the heights above, making the very mountain tremble with the shock. ”

“ In this beauty work, every boulder is prepared and measured and put in its place more thoughtfully than are the stones of temples. If for a moment you are inclined to regard these taluses as mere draggled, chaotic dumps, climb to the top of one of them, tie your mountain shoes firmly over the instep, and with braced nerves run down without any haggling, puttering hesitation, boldly jumping from boulder to boulder with even speed. You will then find your feet playing a tune, and quickly discover the music and poetry of rock piles,—a fine lesson; and all nature’s wildness tells the same story. ”

“ A great boulder, losing its balance, rolled from the crest above, and, loosening a great quantity of stones, mud, and sand, came rolling down near where we stood, making a fearful uproar. At the same moment the tall cliff above us let loose some immense fragments, which, bursting in pieces like bomb-shells, bounded down the steep slope at its base; and the two avalanches, meeting in the gorge, changed their direction, and went crashing down to the valley at a fearful rate, directly over the track which we had pursued in coming up. ”

“ We see from this letter that the thought of the mystery of the great boulder haunted Darwin for years, but the young geologists of the Survey were confronted not with one boulder, but, day after day, week after week, with an accumulation of only half-explained mysteries. When they started work the view that a large part of the British Islands had been covered by land ice, and that the boulder-clay was the record of its passage, had, after a period of neglect, again come into prominence; but it was [Pg 28] very far from being universally accepted (see the historical discussion in Part II.) . ”

“ And how could water carry boulders right across a deep mountain valley and pile them high up on the mountains on the other side? How could water perch one boulder on another or on a flat ledge of rock or on the summits of the cliffs? Boulders so perched are very common, and often they are so nicely balanced that a man can set them rocking; and sometimes a small boy can do it. Every young man who goes to Dartmouth College knows about the rocking stone some half mile east of the college. ”

“ On the beach, a little above high-water mark, just under Appin House, and within an easy stone’s cast of the public road, there is an immense spherical boulder of granite, to which there is attached a curious old story, which invests with additional interest an object deserving enough of attention for its own sake—for the sake, that is, of its huge size and almost perfect spherical form, this latter peculiarity, in the huge solid mass, making it the most remarkable thing of the kind on the mainland, at least of the West Highlands. ”

“ But every period of life has its own playthings; and I was now chiefly engaged with the deep chasm and the huge boulder. Chasm and boulder had come to have greatly more of interest to me than the delicate berries, or than even that sovereign dispeller of ennui and low spirits, an adventurous scramble among the cliffs.
In what state did the chasm exist when the huge boulder,—detached, mayhap, at the close of a severe frost, from some island of the archipelago that is now the northern Highlands of Scotland,—was suffered to drop beside it, from some vast ice-floe drifting eastwards on the tide? ”

“ Long before he showed up I had collected a number of large rocks and cemented them into one large boulder. The boulder itself was poised on the edge of the summit. Around its middle I had deposited a ring of iron, grooved to fit a rail of the same mineral. This rail led from the boulder to a point halfway down the slope. Thus, when the mobile had reached that ridge of iron and was following it up the slope, I removed with my tentacles the little rocks that kept the boulder from toppling over the edge of the summit. ”

“ A stratified form of boulder clay, which not infrequently rests upon, and is therefore younger than, the more typical variety, is usually regarded as a deposit formed by water from the material (englacial, innenmorän) held in suspension within the ice, and set free during the process of melting. Besides the innumerable boulders, large and small, embedded in the boulder clay, isolated masses of rock, often of enormous size, have been borne by ice-sheets far from their original home and stranded when the ice melted. ”

“ Have you sometimes seen great boulder stones, as big as a small house, that stand alone by themselves in some field, or on some seashore, where no other rocks are near? Well, the Frost Giants carried these boulders about, and dropped them down miles away from their homes, as you might take a pocketful of pebbles, and drop them along the road as you walk. Sometimes they roll great rocks down the mountain-sides, playing a desperate game of ball with each other. Sometimes they are sent to make a bridge over Niagara Falls, or to build a dam across a mountain torrent in an hour's time. ”

“ Before proceeding to open the volume itself, it will be well that we clearly mark the wide interval in time between the ages represented by the surface-striation and those indicated by the interior of the boulder. When we proceed from the groovings on the outside to the plants within, we pass, to be sure, over scarcely an inch of space, but we make a leap over untold millenniums in point of time. ”

“ Yes, it was exactly as King Tommy of Yoolaman had described it—a great black bluff boulder, fashioned by Nature into the rude semblance of a human head, the back part of which, being somewhat corrugated and affording a deposit ground for drift and windblown particles, had become overgrown with grey hanging lichen that in the distance gave the appearance of an old man's hair. ”

“ The beauty and richness of these valleys increase as we leave Virginia, and everywhere the green spots are becoming the homes of thrifty farmers. On the divide near Boulder Creek are wonderful proofs of the gradual levelling of the mountains, in the huge blocks of rock piled up in the most grotesque shapes. Many of these are colossal pillars, surmounted by boulders weighing many tons. The softer rock and gravel have washed down the ravines, leaving these as monuments of the primal ages. The ravines penetrate the mountain on every side, and little by little wear the monster away. ”