Fig. 84.Junction of unconformable strata near Mons, in Belgium.
Fig. 84.
Junction of unconformable strata near Mons, in Belgium.
It often happens that in the interval between the deposition of two sets of unconformable strata, the inferior rock has not only been denuded, but drilled by perforating shells. Thus, for example, at Autreppe and Gusigny, near Mons, beds of an ancient (paleozoic) limestone, highly inclined, and often bent, are covered with horizontal strata of greenish and whitish marls of the Cretaceous formation. The lowest and therefore the oldest bed of the horizontal series is usually the sand and conglomerate,a, in which are rounded fragments of stone, from an inch to two feet in diameter. These fragments have often adhering shells attached to them, and have been bored by perforating mollusca. The solid surface of the inferior limestone has also been bored, so as to exhibit cylindrical and pear-shaped cavities, as atc, the work of saxicavous mollusca; and many rents, as atb, which descend several feet or yards into the limestone, have been filled with sand and shells, similar to those in the stratuma.
Fractures of the strata and faults.—Numerous rents may often be seen in rocks which appear to have been simply broken, the separated parts remaining in the same places; but we often find a fissure, several inches or yards wide, intervening between the disunited portions. These fissures are usually filled with fine earth and sand, or with angular fragments of stone, evidently derived from the fracture of the contiguous rocks.
The face of each wall of the fissure is often beautifully polished, as if glazed, and not unfrequently striated or scored with parallel furrows and ridges, such as would be produced by the continued rubbing together of surfaces of unequal hardness. These polished surfaces are called by miners "slickensides." It is supposed that the lines of the striæ indicate the direction in which the rocks were moved. During one of the minor earthquakes in Chili, which happened about the year 1840, and was described to me by an eye-witness, the brick walls of a building were rent vertically in several places, and made to vibrate for several minutes during each shock, after which they remained uninjured, and without any opening, although the line of each crack was still visible. When all movement had ceased, therewere seen on the floor of the house, at the bottom of each rent, small heaps of fine brickdust, evidently produced by trituration.
Fig. 85.Faults. AB perpendicular, C D oblique to the horizon.
Fig. 85.
Faults. AB perpendicular, C D oblique to the horizon.
It is not uncommon to find the mass of rock, on one side of a fissure, thrown up above or down below the mass with which it was once in contact on the other side. This mode of displacement is called a shift, slip, or fault. "The miner," says Playfair, describing a fault, "is often perplexed, in his subterraneous journey, by a derangement in the strata, which changes at once all those lines and bearings which had hitherto directed his course. When his mine reaches a certain plane, which is sometimes perpendicular, as in A B,fig. 85., sometimes oblique to the horizon (as in C D, ibid.), he finds the beds of rock broken asunder, those on the one side of the plane having changed their place, by sliding in a particular direction along the face of the others. In this motion they have sometimes preserved their parallelism, as infig. 85., so that the strata on each side of the faults A B, C D, continue parallel to one another; in other cases, the strata on each side are inclined, as ina,b,c,d(fig. 86.), though their identity is still to be recognized by their possessing the same thickness, and the same internal characters."[62-A]
Fig. 86.E F, fault or fissure filled with rubbish, on each side of which the shifted strata are not parallel.
Fig. 86.
E F, fault or fissure filled with rubbish, on each side of which the shifted strata are not parallel.
In Coalbrook Dale, says Mr. Prestwich[62-B], deposits of sandstone, shale, and coal, several thousand feet thick, and occupying an area of many miles, have been shivered into fragments, and the broken remnants have been placed in very discordant positions, often at levels differing several hundred feet from each other. The sides of the faults, when perpendicular, are commonly separated several yards, but are sometimes as much as 50 yards asunder, the interval being filled with brokendébrisof the strata. In following the course ofthe same fault it is sometimes found to produce in different places very unequal changes of level, the amount of shift being in one place 300, and in another 700 feet, which arises, in some cases, from the union of two or more faults. In other words, the disjointed strata have in certain districts been subjected to renewed movements, which they have not suffered elsewhere.
We may occasionally see exact counterparts of these slips, on a small scale, in pits of fine loose sand and gravel, many of which have doubtless been caused by the drying and shrinking of argillaceous and other beds, slight subsidences having taken place from failure of support. Sometimes, however, even these small slips may have been produced during earthquakes; for land has been moved, and its level, relatively to the sea, considerably altered, within the period when much of the alluvial sand and gravel now covering the surface of continents was deposited.
I have already stated that a geologist must be on his guard, in a region of disturbed strata, against inferring repeated alternations of rocks, when, in fact, the same strata, once continuous, have been bent round so as to recur in the same section, and with the same dip. A similar mistake has often been occasioned by a series of faults.
Fig. 87.Apparent alternations of strata caused by vertical faults.
Fig. 87.
Apparent alternations of strata caused by vertical faults.
If, for example, the dark line A H (fig. 87.) represent the surface of a country on which the strataa b cfrequently crop out, an observer, who is proceeding from H to A, might at first imagine that at every step he was approaching new strata, whereas the repetition of the same beds has been caused by vertical faults, or downthrows. Thus, suppose the original mass, A, B, C, D, to have been a set of uniformly inclined strata, and that the different masses under E F, F G, and G D, sank down successively, so as to leave vacant the spaces marked in the diagram by dotted lines, and to occupy those marked by the continuous lines, then let denudation take place along the line A H, so that the protruding masses indicated by the fainter lines are swept away,—a miner, who has not discovered the faults, finding the massa, which we will suppose to be a bed of coal four times repeated, might hope to find four beds, workable to an indefinite depth, but first on arriving at the fault G he is stopped suddenly in his workings,upon reaching the strata of sandstonec, or on arriving at the line of fault F he comes partly upon the shaleb, and partly on the sandstonec, and on reaching E he is again stopped by a wall composed of the rockd.
Fig. 88.
Fig. 88.
The very different levels at which the separated parts of the same strata are found on the different sides of the fissure, in some faults, is truly astonishing. One of the most celebrated in England is that called the "ninety-fathom dike," in the coal-field of Newcastle. This name has been given to it, because the same beds are ninety fathoms lower on the northern than they are on the southern side. The fissure has been filled by a body of sand, which is now in the state of sandstone, and is called the dike, which is sometimes very narrow, but in other places more than twenty yards wide.[64-A]The walls of the fissure are scored by grooves, such as would have been produced if the broken ends of the rock had been rubbed along the plane of the fault.[64-B]In the Tynedale and Craven faults, in the north of England, the vertical displacement is still greater, and has extended in a horizontal direction for a distance of thirty miles or more. Some geologists consider it necessary to imagine that the upward or downward movement in these cases was accomplished at a single stroke, and not by a series of sudden but interrupted movements. This idea appears to have been derived from a notion that the grooved walls have merely been rubbed in one direction. But this is so far from being a constant phenomenon in faults, that it has often been objected to the received theory respecting those polished surfaces called "slickensides" (see above,p. 61.), that the striæ are not always parallel, but often curved and irregular. It has, moreover, been remarked, that not only the walls of the fissure or fault, but its earthy contents, sometimes present the same polished and striated faces. Now these facts seem to indicate partial changes in the direction of the movement, and some slidings subsequent to the first filling up of the fissure. Suppose the mass of rock A, B, C, to overlie an extensive chasmd e, formed at the depth of several miles, whether by the gradual contraction in bulk of a melted mass passing into a solid or crystalline state, or the shrinking of argillaceous strata, baked by a moderate heat, or by the subtraction of matter by volcanic action, or any other cause. Now, if this region be convulsed by earthquakes, the fissuresf g, and others at right angles to them, may sever the mass B from A and from C, so that it may move freely, and begin to sink into the chasm. A fracture may be conceived so clean andperfect as to allow it to subside at once to the bottom of the subterranean cavity; but it is far more probable that the sinking will be effected at successive periods during different earthquakes, the mass always continuing to slide in the same direction along the planes of the fissuresf g, and the edges of the falling mass being continually more broken and triturated at each convulsion. If, as is not improbable, the circumstances which have caused the failure of support continue in operation, it may happen that when the mass B has filled the cavity first formed, its foundations will again give way under it, so that it will fall again in the same direction. But, if the direction should change, the fact could not be discovered by observing the slickensides, because the last scoring would efface the lines of previous friction. In the present state of our ignorance of the causes of subsidence, an hypothesis which can explain the great amount of displacement in some faults, on sound mechanical principles, by a succession of movements, is far preferable to any theory which assumes each fault to have been accomplished by a single upcast or downthrow of several thousand feet. For we know that there are operations now in progress, at great depths in the interior of the earth, by which both large and small tracts of ground are made to rise above and sink below their former level, some slowly and insensibly, others suddenly and by starts, a few feet or yards at a time; whereas there are no grounds for believing that, during the last 3000 years at least, any regions have been either upheaved or depressed, at a single stroke, to the amount of several hundred, much less several thousand feet. When some of the ancient marine formations are described in the sequel, it will appear that their structure and organic contents point to the conclusion, that the floor of the ocean was slowly sinking at the time of their origin. The downward movement was very gradual, and in Wales and the contiguous parts of England a maximum thickness of 32,000 feet (more than six miles) of Carboniferous, Devonian, and Silurian rock was formed, whilst the bed of the sea was all the time continuously and tranquilly subsiding.[65-A]Whatever may have been the changes which the solid foundation underwent, whether accompanied by the melting, consolidation, crystallization, or desiccation of subjacent mineral matter, it is clear from the fact of the sea having remained shallow all the while that the bottom never sank down suddenly to the depth of many hundred feet at once.
It is by assuming such reiterated variations of level, each separately of small vertical amount, but multiplied by time till they acquire importance in the aggregate, that we are able to explain the phenomena of denudation, which will be treated of in the next chapter. By such movements every portion of the surface of the land becomes in its turn a line of coast, and is exposed to the action of the waves and tides. A country which is undergoing such movement is neverallowed to settle into a state of equilibrium, therefore the force of rivers and torrents to remove or excavate soil and rocky masses is sustained in undiminished energy.
Denudation defined — Its amount equal to the entire mass of stratified deposits in the earth's crust — Horizontal sandstone denuded in Ross-shire — Levelled surface of countries in which great faults occur — Coalbrook Dale — Denuding power of the ocean during the emergence of land — Origin of Valleys — Obliteration of sea-cliffs — Inland sea-cliffs and terraces in the Morea and Sicily — Limestone pillars at St. Mihiel, in France — in Canada — in the Bermudas.
Denudation defined — Its amount equal to the entire mass of stratified deposits in the earth's crust — Horizontal sandstone denuded in Ross-shire — Levelled surface of countries in which great faults occur — Coalbrook Dale — Denuding power of the ocean during the emergence of land — Origin of Valleys — Obliteration of sea-cliffs — Inland sea-cliffs and terraces in the Morea and Sicily — Limestone pillars at St. Mihiel, in France — in Canada — in the Bermudas.
Denudation, which has been occasionally spoken of in the preceding chapters, is the removal of solid matter by water in motion, whether of rivers or of the waves and currents of the sea, and the consequent laying bare of some inferior rock. Geologists have perhaps been seldom in the habit of reflecting that this operation has exerted an influence on the structure of the earth's crust as universal and important as sedimentary deposition itself; for denudation is the inseparable accompaniment of the production of all new strata of mechanical origin. The formation of every new deposit by the transport of sediment and pebbles necessarily implies that there has been, somewhere else, a grinding down of rock into rounded fragments, sand, or mud, equal in quantity to the new strata. All deposition, therefore, except in the case of a shower of volcanic ashes, is the sign of superficial waste going on contemporaneously, and to an equal amount elsewhere. The gain at one point is no more than sufficient to balance the loss at some other. Here a lake has grown shallower, there a ravine has been deepened. The bed of the sea has in one region been raised by the accumulation of new matter, in another its depth has been augmented by the abstraction of an equal quantity.
When we see a stone building, we know that somewhere, far or near, a quarry has been opened. The courses of stone in the building may be compared to successive strata, the quarry to a ravine or valley which has suffered denudation. As the strata, like the courses of hewn stone, have been laid one upon another gradually, so the excavation both of the valley and quarry have been gradual. To pursue the comparison still farther, the superficial heaps of mud, sand, and gravel, usually called alluvium, may be likened to the rubbish of a quarry which has been rejected as useless by the workmen, or has fallen upon the road between the quarry and the building, so as to lie scattered at random over the ground.
If, then, the entire mass of stratified deposits in the earth's crust is at once the monument and measure of the denudation which hastaken place, on how stupendous a scale ought we to find the signs of this removal of transported materials in past ages! Accordingly, there are different classes of phenomena, which attest in a most striking manner the vast spaces left vacant by the erosive power of water. I may allude, first, to those valleys on both sides of which the same strata are seen following each other in the same order, and having the same mineral composition and fossil contents. We may observe, for example, several formations, as Nos. 1, 2, 3, 4, in the accompanying diagram (fig. 89.); No. 1. conglomerate, No. 2. clay, No. 3. grit, and No. 4. limestone, each repeated in a series of hills separated by valleys varying in depth. When we examine the subordinate parts of these four formations, we find, in like manner, distinct beds in each, corresponding, on the opposite sides of the valleys, both in composition and order of position. No one can doubt that the strata were originally continuous, and that some cause has swept away the portions which once connected the whole series. A torrent on the side of a mountain produces similar interruptions; and when we make artificial cuts in lowering roads, we expose, in like manner, corresponding beds on either side. But in nature, these appearances occur in mountains several thousand feet high, and separated by intervals of many miles or leagues in extent, of which a grand exemplification is described by Dr. MacCulloch, on the north-western coast of Ross-shire, in Scotland.[67-A]The fundamental rock of that country is gneiss, in disturbed strata, on which beds of nearly horizontal red sandstone rest unconformably. The latter are often very thin, forming mere flags, with their surfaces, distinctly ripple-marked. They end abruptly on the declivities of many insulated mountains, which rise up at once to the height of about 2000 feet above the gneiss of the surrounding plain or table land, and to an average elevation of about 3000 feet above the sea, which all their summits generally attain. The base of gneiss varies in height, so that the lower portions of the sandstone occupy different levels, and the thickness of the mass is various, sometimes exceeding 3000 feet. It is impossible to compare these scattered and detached portions without imagining that the whole country has once been covered with a great body of sandstone, and that masses from 1000 to more than 3000 feet in thickness have been removed.
Fig. 89.Valleys of denudation.a.alluvium.
Fig. 89.
Valleys of denudation.a.alluvium.
Fig. 90.Denudation of red sandstone on north-west coast ofRoss-shire. (MacCulloch.)
Fig. 90.
Denudation of red sandstone on north-west coast ofRoss-shire. (MacCulloch.)
In the "Survey of Great Britain" (vol. i.), Professor Ramsayhas shown that the missing beds, removed from the summit of the Mendips, must have been nearly a mile in thickness; and he has pointed out considerable areas in South Wales and some of the adjacent counties of England, where a series of palæozoic strata, not less than 11,000 feet in thickness, have been stripped off. All these materials have of course been transported to new regions, and have entered into the composition of more modern formations. On the other hand, it is shown by observations in the same "Survey," that the palæozoic strata are from 20,000 to 30,000 feet thick. It is clear that such rocks, formed of mud and sand, now for the most part consolidated, are the monuments of denuding operations, which took place on a grand scale at a very remote period in the earth's history. For, whatever has been given to one area must always have been borrowed from another; a truth which, obvious as it may seem when thus stated, must be repeatedly impressed on the student's mind, because in many geological speculations it is taken for granted that the external crust of the earth has been always growing thicker, in consequence of the accumulation, period after period, of sedimentary matter, as if the new strata were not always produced at the expense of pre-existing rocks, stratified or unstratified. By duly reflecting on the fact, that all deposits of mechanical origin imply the transportation from some other region, whether contiguous or remote, of an equal amount of solid matter, we perceive that the stony exterior of the planet must always have grown thinner in one place whenever, by accessions of new strata, it was acquiring density in another. No doubt the vacant space left by the missing rocks, after extensive denudation, is less imposing to the imagination than a vast thickness of conglomerate or sandstone, or the bodily presence as it were of a mountain-chain, with all its inclined and curved strata. But the denuded tracts speak a clear and emphatic language to our reason, and, like repeated layers of fossil nummulites, corals or shells, or like numerous seams of coal, each based on its under clay full of the roots of trees, still remaining in their natural position, demand an indefinite lapse of time for their elaboration.
No one will maintain that the fossils entombed in these rocks did not belong to many successive generations of plants and animals. In like manner, each sedimentary deposit attests a slow and gradual action, and the strata not only serve as a measure of the amount of denudation simultaneously effected elsewhere, but are also a correct indication of the rate at which the denuding operation was carried on.
Perhaps the most convincing evidence of denudation on a magnificent scale is derived from the levelled surfaces of districts where large faults occur. I have shown, infig. 87.p. 63., and infig. 91., how angular and protruding masses of rock might naturally have been looked for on the surface immediately above great faults, although in fact they rarely exist. This phenomenon may be well studied in those districts where coal has been extensively worked, for there the former relation of the beds which have shifted their positionmay be determined with great accuracy. Thus in the coal field of Ashby de la Zouch, in Leicestershire (seefig. 91.), a fault occurs, on one side of which the coal bedsa b c drise to the height of 500 feet above the corresponding beds on the other side. But the uplifted strata do not stand up 500 feet above the general surface; on the contrary, the outline of the country, as expressed by the linez z, is uniform and unbroken, and the mass indicated by the dotted outline must have been washed away.[69-A]There are proofs of this kind in some level countries, where dense masses of strata have been cleared away from areas several hundred square miles in extent.
Fig. 91.Faults and denuded coal strata, Ashby de laZouch. (Mammat.)
Fig. 91.
Faults and denuded coal strata, Ashby de laZouch. (Mammat.)
In the Newcastle coal district it is ascertained that faults occur in which the upward or downward movement could not have been less than 140 fathoms, which, had they affected equally the configuration of the surface to that amount, would produce mountains with precipitous escarpments nearly 1000 feet high, or chasms of the like depth; yet is the actual level of the country absolutely uniform—affording no trace whatever of subterranean movements.[69-B]
The ground from which these materials have been removed is usually overspread with heaps of sand and gravel, formed out of the ruins of the very rocks which have disappeared. Thus, in the districts above referred to, they consist of rounded and angular fragments of hard sandstone, limestone, and ironstone, with a small quantity of the more destructible shale, and even rounded pieces of coal.
Allusion has been already made to the shattered state and discordant position of the carboniferous strata in Coalbrook Dale (p. 62.). The collier cannot proceed three or four yards without meeting with small slips, and from time to time he encounters faults of considerable magnitude, which have thrown the rocks up or down several hundred feet. Yet the superficial inequalities to which these dislocated masses originally gave rise are no longer discernible, and the comparative flatness of the existing surface can only be explained, as Mr. Prestwich has observed, by supposing the fractured portions to have been removed by water. It is also clear that strata of red sandstone, more than 1000 feet thick, which once covered the coal, in the same region, have been carried away fromlarge areas. That water has, in this case, been the denuding agent, we may infer from the fact that the rocks have yielded according to their different degrees of hardness; the hard trap of the Wrekin, for example, and other hills, having resisted more than the softer shale and sandstone, so as now to stand out in bold relief.[70-A]
Origin of valleys.—Many of the earlier geologists, and Dr. Hutton among them, taught that "rivers have in general hollowed out their valleys." This is true only of rivulets and torrents which are the feeders of the larger streams, and which, descending over rapid slopes, are most subject to temporary increase and diminution in the volume of their waters. The quantity of mud, sand, and pebbles constituting many a modern delta proves indisputably that no small part of the inequalities now existing on the earth's surface are due to fluviatile action; but the principal valleys in almost every great hydrographical basin in the world, are of a shape and magnitude which imply that they have been due to other causes besides the mere excavating power of rivers.
Some geologists have imagined that a deluge, or succession of deluges, may have been the chief denuding agency, and they have speculated on a series of enormous waves raised by the instantaneous upthrow of continents or mountain chains out of the sea. But even were we disposed to grant such sudden upheavals of the floor of the ocean, and to assume that great waves would be the consequence of each convulsion, it is not easy to explain the observed phenomena by the aid of so gratuitous an hypothesis.
On the other hand, a machinery of a totally different kind seems capable of giving rise to effects of the required magnitude. It has now been ascertained that the rising and sinking of extensive portions of the earth's crust, whether insensibly or by a repetition of sudden shocks, is part of the actual course of nature, and we may easily comprehend how the land may have been exposed during these movements to abrasion by the waves of the sea. In the same manner as a mountain mass may, in the course of ages, be formed by sedimentary deposition, layer after layer, so masses equally voluminous may in time waste away by inches; as, for example, if beds of incoherent materials are raised slowly in an open sea where a strong current prevails. It is well known that some of these oceanic currents have a breadth of 200 miles, and that they sometimes run for a thousand miles or more in one direction, retaining a considerable velocity even at the depth of several hundred feet. Under these circumstances, the flowing waters may have power to clear away each stratum of incoherent materials as it rises and approaches the surface, where the waves exert the greatest force; and in this manner a voluminous deposit may be entirely swept away, so that, in the absence of faults, no evidence may remain of the denuding operation. It may indeed be affirmed that the signs of waste will usually be least obvious where the destruction has beenmost complete; for the annihilation may have proceeded so far, that no ruins are left of the dilapidated rocks.
Although denudation has had a levelling influence on some countries of shattered and disturbed strata (seefig. 87.p. 63.andfig. 91.p. 69.), it has more commonly been the cause of superficial inequalities, especially in regions of horizontal stratification. The general outline of these regions is that of flat and level platforms, interrupted by valleys often of considerable depth, and ramifying in various directions. These hollows may once have formed bays and channels between islands, and the steepest slope on the sides of each valley may have been a sea-cliff, which was undermined for ages, as the land emerged gradually from the deep. We may suppose the position and course of each valley to have been originally determined by differences in the hardness of the rocks, and by rents and joints which usually occur even in horizontal strata. In mountain chains, such as the Jura before described (seefig. 71.p. 55.), we perceive at once that the principal valleys have not been due to aqueous excavation, but to those mechanical movements which have bent the rocks into their present form. Yet even in the Jura there are many valleys, such as C (fig. 71.), which have been hollowed out by water; and it may be stated that in every part of the globe the unevenness of the surface of the land has been due to the combined influence of subterranean movements and denudation.
I may now recapitulate a few of the conclusions to which we have arrived: first, all the mechanical strata have been accumulated gradually, and the concomitant denudation has been no less gradual: secondly, the dry land consists in great part of strata formed originally at the bottom of the sea, and has been made to emerge and attain its present height by a force acting from beneath: thirdly, no combination of causes has yet been conceived so capable of producing extensive and gradual denudation, as the action of the waves and currents of the ocean upon land slowly rising out of the deep.
Now, if we adopt these conclusions, we shall naturally be led to look everywhere for marks of the former residence of the sea upon the land, especially near the coasts from which the last retreat of the waters took place, and it will be found that such signs are not wanting.
I shall have occasion to speak of ancient sea-cliffs, now far inland, in the south-east of England, when treating in Chapter XIX. of the denudation of the chalk in Surrey, Kent, and Sussex. Lines of upraised sea-beaches of more modern date are traced, at various levels from 20 to 100 feet and upwards above the present sea-level, for great distances on the east and west coasts of Scotland, as well as in Devonshire, and other counties in England. These ancient beach-lines often form terraces of sand and gravel, including littoral shells, some broken, others entire, and corresponding with species now living on the adjoining coast. But it would be unreasonable to expect to meet everywhere with the signs of ancient shores, since no geologist can have failed to observe how soon all recent marks of thekind above alluded to are obscured or entirely effaced, wherever, in consequence of the altered state of the tides and currents, the sea has receded for a few centuries. We see the cliffs crumble down in a few years if composed of sand or clay, and soon reduced to a gentle slope. If there were shells on the beach they decompose, and their materials are washed away, after which the sand and shingle may resemble any other alluviums scattered over the interior.
Fig. 92.Section of inland cliff at Abesse, near Dax.a.Sand of the Landes.b.Limestone.c.Clay.
Fig. 92.
Section of inland cliff at Abesse, near Dax.
The features of an ancient shore may sometimes be concealed by the growth of trees and shrubs, or by a covering of blown sand, a good example of which occurs a few miles west from Dax, near Bordeaux, in the south of France. About twelve miles inland, a steep bank may be traced running in a direction nearly north-east and south-west, or parallel to the contiguous coast. This sudden fall of about 50 feet conducts us from the higher platform of the Landes to a lower plain which extends to the sea. The outline of the ground suggested to me, as it would do to every geologist, the opinion that the bank in question was once a sea-cliff, when the whole country stood at a lower level. But this is no longer matter of conjecture, for, in making excavations in 1830 for the foundation of a building at Abesse, a quantity of loose sand, which formed the sloped e, was removed; and a perpendicular cliff, about 50 feet in height, which had hitherto been protected from the agency of the elements, was exposed. At the bottom appeared the limestoneb, containing tertiary shells and corals, immediately below it the clayc, and above it the usual tertiary sanda, of the department of the Landes. At the base of the precipice were seen large partially rounded masses of rock, evidently detached from the stratumb. The face of the limestone was hollowed out and weathered into such forms as are seen in the calcareous cliffs of the adjoining coast, especially at Biaritz, near Bayonne. It is evident that, when the country was at a somewhat lower level, the sea advanced along the surface of the argillaceous stratumc, which, from its yielding nature, favoured the waste by allowing the more solid superincumbent stonebto be readily undermined. Afterwards, when the country had been elevated, part of the sand,a, fell down, or was drifted by the winds, so as to form the talus,d e, which masked the inland cliff until it was artificially laid open to view.
When we are considering the various causes which, in the course of ages, may efface the characters of an ancient sea-coast, earthquakes must not be forgotten. During violent shocks, steep and overhanging cliffs are often thrown down and become a heap ofruins. Sometimes unequal movements of upheaval or depression entirely destroy that horizontality of the base-line which constitutes the chief peculiarity of an ancient sea-cliff.
It is, however, in countries where hard limestone rocks abound, that inland cliffs retain faithfully the characters which they acquired when they constituted the boundary of land and sea. Thus, in the Morea, no less than three, or even four, ranges of what were once sea-cliffs are well preserved. These have been described, by MM. Boblaye and Virlet, as rising one above the other at different distances from the actual shore, the summit of the highest and oldest occasionally exceeding 1000 feet in elevation. At the base of each there is usually a terrace, which is in some places a few yards, in others above 300 yards wide, so that we are conducted from the high land of the interior to the sea by a succession of great steps. These inland cliffs are most perfect, and most exactly resemble those now washed by the waves of the Mediterranean, where they are formed of calcareous rock, especially if the rock be a hard crystalline marble. The following are the points of correspondence observed between the ancient coast lines and the borders of the present sea:—1. A range of vertical precipices, with a terrace at their base. 2. A weathered state of the surface of the naked rock, such as the spray of the sea produces. 3. A line of littoral caverns at the foot of the cliffs. 4. A consolidated beach or breccia with occasional marine shells, found at the base of the cliffs, or in the caves. 5. Lithodomous perforations.
In regard to the first of these, it would be superfluous to dwell on the evidence afforded of the undermining power of waves and currents by perpendicular precipices. The littoral caves, also, will be familiar to those who have had opportunities of observing the manner in which the waves of the sea, when they beat against rocks, have power to scoop out caverns. As to the breccia, it is composed of pieces of limestone and rolled fragments of thick solid shell, such asStrombusandSpondylus, all bound together by a crystalline calcareous cement. Similar aggregations are now forming on the modern beaches of Greece, and in caverns on the sea-side; and they are only distinguishable in character from those of more ancient date, by including many pieces of pottery. In regard to thelithodomiabove alluded to, these bivalve mollusks are well known to have the power of excavating holes in the hardest limestones, the size of the cavity keeping pace with the growth of the shell. When living they require to be always covered by salt water, but similar pear-shaped hollows, containing the dead shells of these creatures, are found at different heights on the face of the inland cliffs above mentioned. Thus, for example, they have been observed near Modon and Navarino on cliffs in the interior 125 feet high above the Mediterranean. As to the weathered surface of the calcareous rocks, all limestones are known to suffer chemical decomposition when moistened by the spray of the salt water, and are corroded still more deeply at points lower down where they are just reached by the breakers. By this action the stone acquires a wrinkled and furrowed outline, andvery near the sea it becomes rough and branching, as if covered with corals. Such effects are traced not only on the present shore, but at the base of the ancient cliffs far in the interior. Lastly, it remains only to speak of the terraces, which extend with a gentle slope from the base of almost all the inland cliffs, and are for the most part narrow where the rock is hard, but sometimes half a mile or more in breadth where it is soft. They are the effects of the encroachment of the ancient sea upon the shore at those levels at which the land remained for a long time stationary. The justness of this view is apparent on examining the shape of the modern shore wherever the sea is advancing upon the land, and removing annually small portions of undermined rock. By this agency a submarine platform is produced on which we may walk for some distance from the beach in shallow water, the increase of depth being very gradual, until we reach a point where the bottom plunges down suddenly. This platform is widened with more or less rapidity according to the hardness of the rocks, and when upraised it constitutes an inland terrace.
But the four principal lines of cliff observed in the Morea do not imply, as some have imagined, four great eras of sudden upheaval; they simply indicate the intermittence of the upheaving force. Had the rise of the land been continuous and uninterrupted, there would have been no one prominent line of cliff; for every portion of the surface having been, in its turn, and for an equal period of time, a sea-shore, would have presented a nearly similar aspect. But if pauses occur in the process of upheaval, the waves and currents have time to sap, throw down, and clear away considerable masses of rock, and to shape out at certain levels lofty ranges of cliffs with broad terraces at their base.
There are some levelled spaces, however, both ancient and modern, in the Morea, which are not due to denudation, although resembling in outline the terraces above described. They may be called Terraces of Deposition, since they have resulted from the gain of land upon the sea where rivers and torrents have produced deltas. If the sedimentary matter has filled up a bay or gulf surrounded by steep mountains, a flat plain is formed skirting the inland precipices; and if these deposits are upraised, they form a feature in the landscape very similar to the areas of denudation before described.
In the island of Sicily I have examined many inland cliffs like those of the Morea; as, for example, near Palermo, where a precipice is seen consisting of limestone at the base of which are numerous caves. One of these called San Ciro, about 2 miles distant from Palermo, is about 20 feet high, 10 wide, and 180 above the sea. Within it is found an ancient beach (b,fig. 93.), formed of pebbles of various rocks, many of which must have come from places far remote. Broken pieces of coral and shell, especially of oysters and pectens, are seen intermingled with the pebbles. Immediately above the level of this beach,serpulæare still found adhering to the face of the rock, and the limestone is perforated bylithodomi. Within the grotto, also, at the same level, similar perforations occur; and sonumerous are the holes, that the rock is compared by Hoffmann to a target pierced by musket balls. But in order to expose to view these marks of boring-shells in the interior of the cave, it was necessary first to remove a mass of breccia, which consisted of numerous fragments of rock and an immense quantity of bones of the mammoth, hippopotamus, and other quadrupeds, imbedded in a dark brown calcareous marl. Many of the bones were rolled as if partially subjected to the action of the waves. Below this breccia, which is about 20 feet thick, was found a bed of sand filled with sea-shells of recent species; and underneath the sand, again, is the secondary limestone of Monte Grifone. The state of the surface of the limestone in the cave above the level of the marine sand is very different from that below it.Above, the rock is jagged and uneven, as is usual in the roofs and sides of limestone caverns;below, the surface is smooth and polished, as if by the attrition of the waves.
Fig. 93.a.Monte Grifone.b.Cave of San Ciro.[75-A]c.Plain of Palermo, in which are Newer Pliocene strata of limestone and sand.d.Bay of Palermo.
Fig. 93.
The platform indicated atc,fig. 93., is formed by a tertiary deposit containing marine shells almost all of living species, and it affords an illustration of the terrace of deposition, or the last of the two kinds before mentioned (p. 74.).
There are also numerous instances in Sicily of terraces of denudation. One of these occurs on the east coast to the north of Syracuse, and the same is resumed to the south beyond the town of Noto, where it may be traced forming a continuous and lofty precipice,a b,fig. 94., facing towards the sea, and constituting the abrupt termination of a calcareous formation, which extends in horizontal strata far inland. This precipice varies in height from 500 to 700 feet, and between its base and the sea is an inferior platform,c b, consisting of similar white limestone. All the beds dip towards the sea, but are usually inclined at a very slight angle: they are seen to extend uninterruptedly from the base of the escarpment into the platform, showing distinctly that the lofty cliff was not produced by a fault or vertical shift of the beds, but by the removal of a considerable mass of rock. Hence we may conclude that the sea, which is now undermining the cliffs ofthe Sicilian coast, reached at some former period the base of the precipicea b, at which time the surface of the terracec bmust have been covered by the Mediterranean. There was a pause, therefore, in the upward movement, when the waves of the sea had time to carve out the platformc b; but there may have been many other stationary periods of minor duration. Suppose, for example, that a series of escarpmentse,f,g,h, once existed, and that the sea, during a long interval free from subterranean movements, advances along the linec b, all preceding cliffs must have been swept away one after the other, and reduced to the single precipicea b.