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LESSONS IN GEOLOG Y.-N0. XXIX.
ON THE DENUDING AGENCY OF THE OCEAN.
DENUDATION is a word in frequent use among geologists. It rather than excavating, their beds. In most longitudinal val. means the act of laying bare some rocks that had been formerly leys, which form the bed of the principal river, there are covered up, the removal of the overlying masses being affected frequently transverse valleys which run across them in such a by water. It is used for the agency of rivers in scooping out way, that the water must have originally passed through them their own channels in the bed or beds of a rock. Its principal instead of excavating their present channels. application is to the agency of the ocean and sea currents Depressions of land, called valleys, are not always easily in wearing down, and removing, rocks that were beneath the accounted for. The diversity of their form would suggest & waves.
diversity of origin. Mountain valleys resemble large cracks
The Denudutiunt of Rucas in Stan Sicitzerland. in a former lesson, I intimated that the formation of valleys | produced in the strata of the earth's crust, either when was a citicult problem in geology. It is evident that rivers, tracting, or when suddenly elevated from the bed of the ocean. in general, have not excavated their own beds, but flow in They are longitudinal, following the direction of the mountain valleys which have been formed, for the most part, by other chain; or they are transverse, running across that direction agents. In the majority of instances, rivers are filling up, ! Their sides are generally rugged, mostly steep, and their edge VOL. III.
Its appearance now presents a broken surface of a country, with a series of hills, cliffs, valleys, and mountains in the landscape-one of the mountains rising to a height of 1,400 feet above the level of the sea.
Imagine, again, that instead of these beds being horizontal,
they have been contorted by some powerful disturbing force Horizontal Bede, first cracked by Upheaval, and then laid bare from below, as is represented in fig. 66. by Denudation.
Fig. 65. Fig. 63 represents the upheaving power acting against the beds of the crust AB. Ate it has so swelled up the lower rocks as to produce at d a crack, which was probably very small at first. On this the currents and waves began to act immediately, until they denuded all the beds down to the bed b. When the same subterranean force acted on the crust at f, it produced a fissure or a fault throughout the whole of the beds, and tilted a part of them on their edges. The rupture thus produced at a would be widened and deepened by the denuding agency of the ocean.
When this district would be raised to the elevation of several hundred or thousand feet, and appear as high plateaus, they
Contorted Beds remored by Denudation. would present the appearance of a wild and desolate landscape of broken and shattered hills, separated by deep and gloomy ravines, that speak distinctly of a period of convulsion, when
It is seen that, under the church at 3, the beds are synclinal. upheaving fires from the abyss, and ocean currents above, had we are sure that the beds of a «y are continuous under , for contended in sublime antagonism—the subterranean power show that they are now continuous downward, the question
they all crop ont at D and E. As their dip and their outcrop slowly elevating the entire district, and the ocean grinding arises whether they were once continuous and unbroken in the down the rocks and sweeping them away. As the
region of direction of the
dotted lines between A and e? The reguler upper surface consisted exclusively, from one extremity to position of these beds at A and
at e, together with the amount the other, of a continuous tract, say of old red sandstone of their curvature, prove that they were once continuous and though, ere the land fully emerged, the ocean currents of ages
unbroken upwards as well as downwards. had swept it away, all except in the lower and last-raised beds, What, again, has become of all the materials of the rocks and in the detached localities where it still remains in pyra. | between A ando? The whole masses marked by the dotted midal hills, to show the amazing depth to which it had once overlaid the inferior rocks.
Synclinical means having the same inolinatlon, or rather inollning To understand this, imagine a large district of land several together
lines have been removed by denudation. Sections in South It is not to be supposed that what are called valleys of Wales and Gloucestershire show that enormous beds of old red denudation have been produced by the streams which flow sandstone and mountain limestone, have been thus removed through them now. In many parts of the world there are from the surface of the underlying rocks by the denuding valleys without any water at all running in them. It is obvious action of sea currente.
that these valleys are not the result of river action, England In mountainous countries, these denudations are sometimes abounds with examples of these dry valleys, especially in the marked by rocks many thousand feet in height, which are combes of chalk districts, and in the numerous depressions separated from each other by intervals or valleys many miles, found in the slate districts of Devonshire. Even in Jamaica, and even leagues in breadth. Of this there is a grand speci- where heavy tropical rains are
frequent, there are valleys in men on the north-west coast of Ross-shire, in Scotland. Those which the waters are immediately swallowed up by subterrathree stupendous mountains, Suil Veinn, Coul Beg, and neous cavities or sink-holes. On the west coast of Peru, where Coul More, which consist of nearly horizontal strata of red rain never falls, there are remarkable instances of dry valleys, sandstone repose on gneiss, the fundamental rock of the which much resemble the lowland valleys of Europe. country. These red sandstone mountains consist of an immense suc- moving masses of water passing over them, and carrying of
These dry valleys appear as if they had been scooped out by cession of thin layers, forming mere flags, with their surfaces the materials which offered the least resistance. This denu. distinctly ripple marked. They rise up at once, like pyramids, dation might have been effected by great disturbances beneath from the gneiss to the height of about 2000 feet, and to an
an ocean, such as would be caused either by the elevation of a average elevation of about 3000 feet above the level of the sea. long range of mountains from the sea, or by a disruption of It is impossible to look at these three high mountains, now the strata of which the crust of the earth in that region was rising
in scattered and detached portions without inferring composed. They were produced either by a marine earththat, at one time, the whole country was covered
with a great quake, or they may have been formed beneath agitated waters, body of sandstone, and that enormous masses, from 1000 to in which
there were strong currents moving with great velocity more than 3000 feet in thickness, have been washed away by The rocks which were thus worn and denuded beneath such the denuding action of water.
currents were afterwards protruded above the level of the sea, One of the most splendid generalisations in geological science is presented in the “Survey of Great Britain," by these currents are constant, some are periodical, and some
The ocean abounds in currents strong and deep. Some of have been washed away from the summits of the Mendip Hills, bodies of water, moving with different velocities, and in Professor Ramsey. He shows that the missing, beds which
are only occasional. These currents consist of immense between Wells and Bristol, must have been originally about a mile in thickness. In considerable districts of Monmouthshire strears of different breadths and depths. Of the character of and Breconshire on the west, and Gloucestershire and Here these currents we have a well-known specimen in the Gulf fordshire on the east, he shows that a series of ancient sedi: Stream of the Atlantic Ocean. High winds and heavy gales mentary rocks, no less than eleven thousand feet
in thickness greatly affect the velocity and the strength of these currentshave been stripped off by denudation.
sometimes diminishing their breadth and augmenting their The stupendousness of this generalisation is in the two facts currents are supposed to be certain prevalent winds—such as
velocity, and vice versa. The principal causes of all ocean --that all these materials have been removed and transported the trade winds, the monsoons, &c. to some other regions to compose rocks of a new formation ; and that these paleozoic rocks are from twenty to thirty
Among the periodical currents of the ocean we must place thousand feet thick. It is evident that whatever has been the tides. These have great power in scooping shallow banks, contributed to one area on the face of the globe, must have and
in abrading our coasts. The general velocity of the tide been derived and taken from another.
is about a mile and a-half in an hour. When any obstacles One of the most magnificent, and, at the same time, most are presented to its currents, its abrading and transporting clear and palpable specimens of denudation, is furnished by power becomes much augmented, and the process of denuda. Saxon Switzerland, å district of Germany about ten miles tion takes place very rapidly and extensively. beyond Dresden. The rock of the district is what the German It is often found that at greater or less distances from the geologists call quadersandstein, corresponding to the green sand shore, a great discoloration of the sea frequently takes place. formation of England. The rocks on each side of the river This discoloration is produced by heavy gales and powerful Elbe are cut in all directions into chasms, gorges, and passages, hurricanes, and is due to the action of the sea on the rocks as if mechanical tools had been used to hew them into particular beneath, and not to the sands and mud which the ebbing ride sbapes. Some of these passages among the rocks look like brings with it. Darrow lanes,--80 narrow are the openings and so smoothly Since strong winds are generally the causes of currents in perpendicular do the gigantic walls of rock rise on both sides. the ocean, it is obvious that the ocean streams thus produced The walls of these rocks are cut vertically into separate masses will not extend deeper than the depth to which the propelling by narrow passages, reaching from the summit to the very power of the wind or gale extends. All hydrographers have bottom, as if a cement that once united them had been washed demonstrated that the waters of the ocean vary in density away.
according to their depth. A wind, then, sufficient to agitate The perpendicular masses of these separate rocks or cliffs and propel the surface water to a certain depth beneath, will are divided horizontally into distinct layers, like blocks regu. reach a point below at which it can produce no change or larly laid upon each other in a massive work of artificial movement, as all water beneath that depth would, as far as masonry. The terminations or perpendicular extremities of surface causes are concerned, be immoveable, and would con. these magses or columns are very rarely sharp or angular, but sequently exert no denuding agency, Hence, the denuding are almost invariably well rounded, which is a clear proof of power of ocean currents depends on the depth of the sea. The subaqueous action. Some of them appear as if two sugar- smaller the depth, the greater is their denuding power, and loaves were put together, the small end of one resting on the consequently the greatest amount of denudation has, in every small end of the other,
geological era, taken place on shoals and near coasts, From what is called the Bastei (on the right of the engraving, Sea currents have always their greatest velocity and force in 6g. 66), and 600 feet above the Èlbe, the country looks as an shallow water and in contracted channels. It is, therefore, in amphitheatre, studded with lofty and rounded ranges of moun these situations that, among ancient rocks, we always discover tains. From the bosom of this amphitheatre, huge columnar the greatest effects of their denuding power. Their geological hills start up at once from the ground, at a considerable dis- importance depends on two things : on the relative depth of tance from each other. All these are monuments of the the sea which they traverse, and on their proximity to land, denuding agency of water-not that of the present Elbe, but By their shallowness, their velocity is increased ; and by their of the sea-at a time when this part of Germany was slowly néarness to coasts, they wear away and remove the rocks that rising from the ocean. The identity of structure and of com- resist them. At very great depths in the bed of the ocean we position in all these columnar eminences prove that they once have no reason to suppose that this denuding power exists, formed one body, and that all the softer parts of their beds If it does exist, its cause must differ from all surfa bare been removed bsdondtion,