Rock (geography)

“ PRIMARY ROCKS.
Although the earth has not been actually penetrated to a greater depth than three thousand feet, the nature of its substance can, in many instances, be inferred for the depth of many miles by other means of observation. We see a mountain composed of a particular substance, with strata, or beds of other rock, lying against its sloped sides; we, of course, infer that the substance of the mountain dips away under the strata which we see lying against it. ”

“ To obtain a general idea of the nature and method of our science before beginning its study in detail, we may visit some valley, such as that illustrated in the frontispiece, on whose sides are rocky ledges. Here the rocks lie in horizontal layers. Although only their edges are exposed, we may infer that these layers run into the upland on either side and underlie the entire district; they are part of the foundation of solid rock which everywhere is found beneath the loose materials of the surface. ”

“ Others again are nothing but rock and granite from base to summit, from earth almost to heaven, and down their sides there are marked deep slides, where the rock and limestone has crumbled into an avalanche of stone and dust. The changes on their unchanging surfaces are the most beautiful. Like human nature, hard on the surface, they have hidden soft and susceptible moods. The pine-clad mountains are sunnier and more pleasing, but it is those of adamantine rock that fascinate you. ”

“ Mountain ranges observe a singular uniformity of direction, and the cleavage planes of rock are evidently determined by some all-pervading power. Mineral bodies are not distributed in all rocks indiscriminately. ”

“ The nature of the rock, whether sandstone, shale, limestone, or greenstone, must be set down at the same place, and, to save room, a system of marks for the different rocks may be conveniently used. When a sufficient area of ground has been thus traversed, the student may find, say a row of arrows on his map all pointing due west, and indicating a set of quarries about a quarter of a mile distant from one another, the rock in each of them dipping to the west. ”

“ And hence if the geologist would form an accurate conception of the origin and structure of many of the stratified rocks, he must study them not in hand specimens neatly trimmed and arranged along the shelves and drawers of a cabinet, nor even in the ponderous blocks daily exhumed by the quarryman, but along some surf-beaten cliff-line or down some precipitous ravine where the rock for centuries has been exposed to the wear and tear of the elements. ”

“ They lie in all positions—horizontal, tilted, vertical—with heaps of scoria revealing their igneous origin. Rock is piled on rock, hill upon hill, ridge upon ridge, mountain upon mountain—serried, castellated, turreted, . ”

“ No geologist can doubt that such relationship is the result of denudation. The mountains are — 19 — monuments of erosion—they are the wreck of an old table-land—the upper surface and original inclination of which are approximately indicated by the summits of the various mountain-masses and the direction of the principal water-flows. If we in imagination fill up the valleys with the rock-material which formerly occupied their place, we shall in some measure restore the general aspect of the Highland area before its mountains began to be shaped out by Nature’s saws and chisels. ”

“ In this little valley we see slow changes now in progress. We find also in the composition, the structure, and the attitude of the rocks, and the land forms to which they have been sculptured, the record of a long succession of past changes involving the origin of sand grains and their gathering and deposit upon the bottom of some ancient sea, the cementation of their layers into solid rock, the uplift of the rocks to form a land surface, and, last of all, the carving of a valley in the upland. ”

“ Also the coasts of continents and islands suffer continual wear and tear at the hands of sea waves; and thus the supply of sediment is increased.
When the geologist comes to study the great rock-masses—hundreds, and even thousands, of feet in thickness—of which mountain-ranges are composed, he finds all those kinds of rock which we have just been considering,—sandstones, shales (or hardened clays) , pebble-beds, and limestones,—and endeavours to picture to himself their gradual growth in the ways we have described. ”

“ My favorite one is a huge projecting rock, protected by large upright ones nearly as immense, and shooting out over tier upon tier of rocks, down a hundred feet below, to where the river runs like life—now in sunshine, now in shadow. ”

“ A tremendous ascent among rocks and pines to a height of 9,000 feet brought us to a passage seven feet wide through a wall of rock, with an abrupt descent of 2,000 feet, and a yet higher ascent beyond. I never saw anything so strange as looking back. It was a single gigantic ridge which we had passed through, standing up knife-like, built up entirely of great brick-shaped masses of bright-red rock, piled one on another by Titans. ”

“ If we dig down through the soil and subsoil to a sufficient depth, we always come sooner or later to the solid rock. In many places the rock actually reaches the surface, or rises in cliffs, hills, or ridges, far above it. ”

“ I also had a clear view of the coast mountains for a hundred miles or thereabouts, all of which are tellingly glaciated in harmony with the above generalization. Most of the rock is granite with cleavage planes that cause it to weather rapidly into flat blocks. One conical black hill, fifteen hundred feet high, is volcanic rock, close-grained and dense like some kinds of iron ore. ”

“ In mountainous regions it frequently occurs that the foundations of a rock, undermined by filtering waters, give way, and that huge masses of stone and earth, now no longer reposing on a solid basis, are precipitated into the valley below. ”

“ In their older and wider valleys they cut narrow, steep-walled inner gorges, in which they flow swiftly over rocky floors, broken here and there by falls and rapids where a harder layer of rock has been discovered. ”

“ Therefore, whether we suppose the mountains formed of a rock in mass, or in that of regular beds, this must have an influence in the form of this decaying surface of the earth, and may be distinguished in the shape of mountains. It is but rarely that we find mountains formed altogether of rock, although we often find them of the other sort, where little or nothing of rock is to be seen. ”

“ To all these Professor Forbes assigns the characteristic regions of depth, nature of favourite ground, and geographical distribution. The most positive and determining influence is that of the ground or sea-bottom. "According as rock, sand, mud, weedy or gravelly ground prevails, so will the numbers of the several genera and species vary. The presence of the sponges of commerce often depends on the rising up of peaks of rock in the deep water near the coast. ”

“ The old rock stands for the mountains of the Old World, like the Welsh mountains, or the Mendip Hills. The slates stand for the new rocks, which have been piled up against these, one over the other. But, you see, each slate is lower than the one before it, and slopes more; till the last slate which you are putting on is the lowest of all, though it overlies all. ”

“ Yet, to try out the theory, geologists in the universities make layers of different material, corresponding to the strata of different kinds of stone, and then subject this composition to pressure at both ends, as the earth crust is supposed to be pressed in the crumpling process. The result is that these artificial strata take similar forms to those we see in mountain rock. And that's the answer!
Notice the similarity of the rock wrinkles in the baby mountain in Virginia and these imitation mountains of the laboratory.
WHY MOUNTAINS RUN NORTH AND SOUTH
Look at your relief map once more. ”

“ The underlying rock, especially if massive, hard, and of a fine grain, has often been ground down to a smooth surface and rubbed to a polish as perfect as that seen on the rock beside an Alpine glacier where the ice has recently melted back. ”

“ Rock-taluses, and all the varied alluvium carried seaward as the outwash of continental land, well deserve a chapter to themselves. But there is really no end to the subject, which embraces all the accumulative processes of the earth. A few vacation-journeys, judiciously planned out, teach us that text-books are merely signposts to set us on what is believed to be the way. When the path enters the great forest, or rises above green lakelets to the crags, we find there those who went before us, pointing to unconquered lands. ”

“ Near the mountains it is filled with coarse rock particles, and under the action of the elements these become disproportionately prominent on the surface. Receding from the mountains, it becomes gradually finer, until gravel and bits of broken stone are no longer seen. ”

“ It has long been a well recognized fact in the science of geology, that the most important mountain ranges lift up and sustain upon their sides the beds of rock which form the inferior groups of hills around them in the manner roughly shown in the section Fig. ”

“ Beneath this coverlet of soil, extending as far as man has penetrated into the earth, is rock. Excepting in regions overflowed by lava poured out from beneath, or along the backbones of continents where the surface rocks have been upheaved into folds and carried away by denudation, the rocks immediately beneath the soil, to a thickness often of thousands of feet, are in the form of layers, or sheets, arranged one above another. ”

“ The Hills and Valleys.—Sir Charles Warren was the first to point out the necessity of ascertaining the depth of the rock below the present surface, in as many places as possible, and of referring all the measurements to one fixed datum, the level of the sea. In the study of the ancient topography the original appearance of the ground is the first consideration, for although a certain amount of soil may always have existed, still the ancient surface must have conformed far more closely to that of the rock than does the present. ”

“ Thus it is that where geologists find considerable fields of rock, where limestones are intercalated with sandstones and deposits of clay, they are justified in assuming that the strata were laid down near some ancient shore. ”

“ On the land, the mantle of loose material is sometimes absent, and in such places the surface of solid rock of the crust appears. Bare surfaces of rock are most commonly seen where the topography is rough, especially on the slopes of steep-sided valleys and mountains, and on the slopes of cliffs which face seas or lakes. Solid rock, without covering of soil or loose material of any sort, is also frequently seen in the channels of streams, especially where there are falls or rapids. ”

“ Look at the first piece of bluff rock you come near, and observe the clear pencil-like markings of layer above layer—not often indeed lying flat, one over another, and this must be explained later, but however irregularly slanting, still plainly visible. You can examine these lines of stratification on the nearest cliff, the nearest quarry, the nearest bare headland, in your neighborhood.
But how can this be? If all these stratified rocks are built on the floor of the ocean out of material taken from the land, how can we by any possibility find such rocks upon the land? ”

“ The artists who sketched the hills must indeed have had good eyes for form. So carefully has their work been done, that it is often not difficult to distinguish upon their maps hills formed of such rocks as sandstone from those that are composed of more durable kinds. The individual characteristics of mountains of schist, of granite, of quartz-rock, of slate, are often well depicted: nay, even the varieties of igneous rock which enter into the formation of the numerous hills and knolls of the Lowlands can frequently be detected by the features which the artists have so intelligently caught. ”

“ Led by this observation, the geologist, after comparing the rocks of different countries with one another, compared next the soils of various districts with the rocks on which they immediately rest. The [Pg 81] general result of this comparison has been, that in almost every country the soils have as close a resemblance to the rocks beneath them—as the loose earth derived from the crumbling of a rock before our eyes, bears to the rock of which it lately formed a part. ”

“ The beginner, however, must constantly bear in mind that the term rock by no means implies that a mineral mass is in an indurated or stony condition.
The most natural and convenient mode of classifying the various rocks which compose the earth’s crust, is to refer, in the first place, to their origin, and in the second to their relative age. ”

“ Under the drift lie layers of stratified rock, and [Pg 56] under these are the granites and other fire-formed rocks, the beginning of those rock masses which form the solid bulk of the globe. We know little about the core of the earth, but the granites that are exposed in mountain ridges are found to have a great capacity for absorbing water, so it is not unlikely that much surface water soaks into the rock foundations and is never drained away into the sea. ”

“ By wind action alone there appears to be incontestable testimony that from the entire area of the vast arid region there has been lifted and exported in very recent geologic times a prodigious layer of rock not less than 5000 feet in thickness. ”