Folding (geology)

“ If, for example, a section shows comparatively horizontal strata lying across the worn down edges of a series of vertical sedimentary rocks, we may fairly assume that some such changes as the following have taken place in that particular area.
The underlying beds were originally laid down as more or less horizontal deposits; these were gradually hardened and compacted, then elevated above sea-level by a folding of the earth’s crust ”

“ The deposition of the Cretaceous. 2. The folding and erosion of the Cretaceous. 3. The deposition of the Tertiary. 4. The desiccation of the Tertiary lake basin. 5. The erosion which is still in progress. It is evident that the laccolites were not formed until the Cretaceous strata had been deposited; for their uplifts have bent and tilted all Cretaceous rocks up to and including the Masuk sandstone. ”

“ So soon as a fold begins to be lifted, it is, by reason of its position, subject to more rapid erosion than its surroundings. For the same reason the crest of the fold is likely to be degraded more rapidly than its lower slopes, and must suffer more degradation before it is brought to base-level. Folds are usually composed of beds of unequal resistance, and as the degradation of a fold proceeds, successive layers are worn from the top, and the alternating hard and soft layers composing it are exposed. ”

“ An anticlinal fold is attacked by erosion as soon as it begins to rise above the original level, and the higher it is uplifted, and the stronger are its slopes, the faster is it worn away. Even while rising, a young upfold is often thus unroofed, and instead of appearing as a long, Smooth, boat-shaped ridge, it commonly has had opened along the rocks of the axis, when these are weak, a valley which is overlooked by the infacing escarpments of the hard layers of the sides of the fold (Fig. 177) . ”

“ Such I conceive to have been the origin of the sand, gravel, and imbedded plants of our boulder. It may be well, however, in going into the details of the subject, to take a wider view of this interesting branch of geology, and look for a little at the forms and modes of the decomposition of rocks, and the varied manner in which new sedimentary accumulations are formed.
All over the world, wherever a land surface spreads out beneath the sky, there goes on a process of degradation and decay. ”

“ They are like the shingle of a sea-beach, or pebbles formed in the beds of torrents and rivers, which are carried down into the ocean wherever these descend from high grounds bordering a coast. There the gravel is spread out by the waves and currents over a considerable space; but during seasons of drought the torrents and rivers are nearly dry, and have only power to convey fine sand or mud into the sea. Hence, alternate layers of gravel and fine sediment accumulate under water, and such alternations are found by geologists in the interior of every continent. ”

“ It often happens that a formation is not where we expect to find it, this being due to several possible factors. The sediments may not have been deposited there, or they may have been removed by erosion. Where the structure has been disturbed by folding and faulting, a multitude of complications is introduced. The expected sequence is sometimes inverted and repeated through a series of folds. Formations also may be moved miles out of place by faulting. Both thickness and character of sediments may vary considerably within a formation. ”

“ North of the folded belt, and including the greater part of Siberia, Mongolia and northern China, lies another area which is, in general, free from any important folding of Mesozoic or Tertiary age. There are, it is true, mountain ranges which are formed of folded beds; but in many cases the direction of the chains is different from that of the folds, so that the ranges must owe their elevation to other causes ”

“ But the mechanical erosion effected by running water is what impresses us most with the importance of rivers as geological agencies. This erosive action is due to the gravel, sand, and mud carried along by the water. These ingredients act as files in the hand of a workman, and grind, polish, and reduce the rocks against which they are borne. The beds of some streams that flow over solid rock are often pitted with circular holes, at the bottom of which one invariably [Pg 52] finds a few rounded stones. ”

“ No sooner are residual weathered mantles formed or sedimentary rocks deposited, whether under air or water, than processes of consolidation begin. Settling, infiltration of cementing materials, and new growths, or recrystallization, of the original minerals of the rock all play a part in the process. The mud or clay becomes a shale, the sand becomes sandstone or quartzite, the marl becomes limestone or marble. ”

“ Thus, as the current sweeps along in its course, the sediment which it bears away from the land will be in a manner sorted, and three distinct layers of different materials will be deposited in the bed of the ocean;—first, nearest to the shore, a layer of pebbles and coarse gravel, then a layer of sand, and last of all a layer of fine mud or clay. This is the first step in the construction of stratified rock. To complete the work nothing more is necessary than the consolidation of these loose and incoherent materials. ”

“ For the same reason a thin-bedded formation is more easily eroded than a thick-bedded one.
Fig. 103 and 104.—Diagrams to illustrate the fact that a stream crosses many more cleavage-planes when the beds of rock are inclined than when they are horizontal.
The beds of stratified rock may be horizontal, vertical, or inclined, and inclined strata may stand at any angle between horizontality and verticality. In indurated formations the rate of erosion is influenced both by the position of the strata and by the relation of the direction of the flowing water to their dip and strike. ”

“ Subsequent erosion has obliterated in great part the inequality of the surface, and the higher-lying blocks do not stand as mountains, but are outlined by zones of tilted strata which mark the flexures by which the blocks are separated. Along the zones of flexure it frequently happens that a hard stratum outcropping between two that are softer will be preserved from erosion and form a long, continuous ridge. Such ridges, and other forms produced by the erosion of the flexures, are conspicuous features of the topography, and the tracing out of the limits of the blocks is a simple matter. ”

“ An unconformity between two marine formations records the fact that between the periods when they were deposited in the sea the area emerged as land and suffered erosion (p. 227) . The attitude and structure of the strata tell also of the foldings and fractures, the deformation and the metamorphism, which they have suffered; and the igneous rocks associated with them as lava flows and igneous intrusions add other details to the story. Each formation is thus a separate local chapter in the geological history of the [292] earth, and its strata are its leaves. ”

“ After the land had been subjected for many long ages to the wearing action of the denuding agents, it is evident that the softer rocks—those which were least capable of withstanding weathering and erosion—would be more worn away than the less easily decomposed masses. The latter would, therefore, tend to form elevations, and the former hollows. This is precisely what we find in nature. The great majority of isolated hills and hilly tracts owe their existence as such merely to the fact that they are formed of more durable materials than the rock-masses by which they are surrounded. ”

“ It is also observed that strata, in proportion as they are of newer date, bear the nearest resemblance in mineral character to those which are now in the progress of formation in seas or lakes, the newest of all consisting principally of soft mud or loose sand, in some places full of shells, corals, or other organic bodies, animal or vegetable, in others wholly devoid of such remains. The farther we recede from the present time, and the higher the antiquity of the formations which we examine, the greater are the changes which the sedimentary deposits have undergone. ”

“ Experiment has reproduced all forms of folds by subjecting to lateral thrust layers of plastic material such as wax.
Vast as the force must have been which could fold the solid rocks of the crust as one may crumple the leaves of a magazine in the fingers, it is only under certain conditions that it could have produced the results which we see. Rocks are brittle, and it is only when under a heavy load and by great pressure slowly applied, that they can thus be folded [206] and bent instead of being crushed to pieces. ”

“ The Pinney Hollow formation, which is an estimated 2,300 or more feet thick, is primarily composed of pale-green schist [5] with abundant white quartz layers and lenses sandwiched between the dominantly greenish colored rock. Originally these metamorphic rocks were sand, mud and sandy mud deposited on a relatively shallow sea floor. After burial these sediments hardened into thin layers and lenses of sandstone, shale, and sandy shale. ”

“ In folds where the compression has been great the layers are often found thickened at the crest and thinned along the limbs (174) . Where strong rocks such as heavy limestones are folded together with [205] weak rocks such as shales, the strong rocks are often bent into great simple folds, while the weak rocks are minutely crumpled.
Fig. 173. Fan Folds, the Alps
Systems of folds. ”

“ In all these cases, it is only necessary that there should be some deposition of sedimentary matter, however minute, such as may be supplied by rivers draining a continent, or currents preying on a line of cliffs, in order that stratified formations, hundreds of feet in thickness, and replete with organic remains, should result in the course of ages. ”

“ In the Western Hemisphere, both in Canada and as far south as the 40th and even 38th parallel of latitude in the United States, we meet with a repetition of all the peculiarities which distinguish the European boulder formation. Fragments of rock have travelled for great distances from north to south; the surface of the subjacent rock is smoothed, striated, and fluted; unstratified mud or till containing boulders is associated with strata of loam, sand, and clay, usually devoid of fossils. ”

“ Sometimes bedding seems to have been determined by strong currents which temporarily not only prevented deposition over a given area, but even cut away the loose surface of deposits already made, giving a firm surface from which succeeding deposits are distinct. This sequence of events is sometimes shown by the truncation of laminæ, and by other signs of erosion. The commoner sorts of bedded rock are limestones, shales, sandstones, and conglomerates. ”

“ The oldest known sedimentary strata, wherever they are exposed by uplift and erosion, are found to be involved with a mass of crystalline rocks which possesses the same characteristics in all parts of the world. It consists of foliated rocks, gneisses, and schists of various kinds, which have been cut with dikes and other intrusions of molten rock, and have been [306] broken, crumpled, and crushed, and left in interlocking masses so confused that their true arrangement can usually be made out only with the greatest difficulty if at all. ”

“ When the sand is loose and incoherent, as in the case here represented, the deviation from parallelism of the slanting laminæ cannot possibly be accounted for by any rearrangement of the particles acquired during the consolidation of the rock. In what manner, then, can such irregularities be due to original deposition? We must suppose that at the bottom of the sea, as well as in the beds of rivers, the motions of waves, currents, and eddies often cause mud, sand, and gravel to be thrown down in heaps on particular spots, instead of being spread out uniformly over a wide area. ”

“ For the most part, the deposits of mud, sand, and gravel are made under the sea or in lakes and estuaries, but they are also formed on the land in lodgment basins, in low-gradient valleys, and on base-plains. The deposits of sediment on land have received less recognition than they deserve. When formed under the sea or in other life-sustaining waters, shells and other organic material are liable to be entrapped and to form a part of the rock. These organic remains, or fossils, greatly aid in interpreting the deposits in which they occur. ”

“ We have but to note the effects of a vigorous shower on a steep slope, or of a swift stream on its channel, or of waves on the cliffs which face lakes and seas, to understand at least one of the reasons why loose materials are frequently absent from steep slopes. The very general exposure of solid rock where conditions favor surface erosion suggests that rock is everywhere present beneath the soil or subsoil. Fortunately there is an easy way of testing the universality of the crust beneath the mantle. ”

“ The steeper the channel, other conditions being equal, the more rapid will be the erosion. Geological structure also affects the character and rate of the excavation. Where the rocks are so arranged as to favour the formation and persistence of a waterfall, a long chasm may be hollowed out like that of the Niagara below the falls, where a hard thick bed of nearly flat limestone lies on softer and more easily eroded shales. ”

“ In many of these orthogneisses the foliation is primitive, being an original character of the rock which was produced either by fluxion movements in a highly viscous, semi-solid mass injected at great pressure into the surrounding strata, or by folding stresses acting immediately after consolidation. ”