Teeth of Iguanodon.Fig. 230. Partially worn tooth of a young animal. (Mantell.)Fig. 231. Crown of tooth in adult, worndown. (Mantell.)
Teeth of Iguanodon.
Fig. 230. Partially worn tooth of a young animal. (Mantell.)
Fig. 231. Crown of tooth in adult, worndown. (Mantell.)
Occasionally bands of limestone, called Sussex Marble, occur in the Weald Clay, almost entirely composed of a species ofPaludina, closely resembling the commonP. viviparaof English rivers.
Fig. 232.Cypris spinigera, Fitton.
Fig. 232.
Cypris spinigera, Fitton.
Fig. 233.Cypris Valdensis, Fitton. (C. faba, Min. Con. 485.)
Fig. 233.
Cypris Valdensis, Fitton. (C. faba, Min. Con. 485.)
Fig. 234.Cypris tuberculata, Fitton.
Fig. 234.
Cypris tuberculata, Fitton.
Fig. 235.
Fig. 235.
Shells of theCypris, an animal belonging to the Crustacea, and before mentioned (p. 31.) as abounding in lakes and ponds, are also plentifully scattered through the clays of the Wealden, sometimes producing, like the plates of mica, a thin lamination (seefig. 235.). Similar cypriferous marls are found in the lacustrine tertiary beds of Auvergne (see above,p. 183.).
This middle division of the Wealden consists of sand, calciferous grit, clay, and shale; the argillaceous strata, notwithstanding the name, being nearly in the same proportion as the arenaceous. The calcareous sandstone and grit of Tilgate Forest, near Cuckfield, in which the remains of the Iguanodon and Hyleosaurus were first found, constitute an upper member of this formation. The white "sand-rock" of the Hastings cliffs, about 100 feet thick, is one of the lower members of the same. The reptiles, which are very abundant in it, consist partly of saurians, already referred by Owen and Mantell to eight genera, among which, besides those already enumerated, we find the Megalosaurus and Plesiosaurus. The Pterodactyl, also a flying reptile, is met with in the same strata, and many remains of Testudinata of the generaTrionyxandEmys, now confined to tropical regions.
Fig. 236.Lepidotus Mantelli, Agass. Wealden.a.palate and teeth.b.side view of teeth.c.scale.
Fig. 236.
Lepidotus Mantelli, Agass. Wealden.
The fishes of the Wealden belong partly to the generaPycnodusandHybodus(see figure of genus inChap. XXI.), forms common to the Wealden and Oolite; but the teeth and scales of a species ofLepidotusare most widely diffused (seefig. 236.). The general form of these fish was that of the carp tribe, although perfectly distinct in anatomical character, and more allied to the pike. The whole body was covered with large rhomboidal scales, very thick, and having the exposed part covered with enamel. Most of the species of this genus are supposed to have been either river fish, or inhabitants of the coasts, having not sufficient powers of swimming to advance into the deep sea.
Fig. 237.Corbula alata,Fitton. Magnified.
Fig. 237.
Corbula alata,Fitton. Magnified.
The shells of the Hastings beds belong to the generaMelanopsis,Melania,Paludina,Cyrena,Cyclas,Unio, and others, which inhabit rivers or lakes; but one band has been found in Dorsetshire indicating a brackish state of the water, and, in some places, even a saltness, like that of the sea, where the generaCorbula(seefig. 237.),Mytilus, andOstreaoccur. At different heights in the Hastings Sand, in the middle of the Wealden, we find again and again slabs of sandstone with a strongripple-mark, and between these slabs beds of clay many yards thick. In some places, as at Stammerham, near Horsham, there are indications of this clay having been exposed so as to dry and crack before the next layer was thrown down upon it. The open cracks in the clay have served as moulds, of which casts have been taken in relief, and which are, therefore, seen on the lower surface of the sandstone (seefig. 238.).
Fig. 238.Underside of slab of sandstone about one yard in diameter. Stammerham, Sussex.
Fig. 238.
Underside of slab of sandstone about one yard in diameter. Stammerham, Sussex.
Near the same place a reddish sandstone occurs in which are innumerable traces of a fossil vegetable, apparentlySphenopteris, the stems and branches of which are disposed as if the plants were standing erect on the spot where they originally grew, the sand having been gently deposited upon and around them; and similar appearances have been remarked in other places in this formation.[230-A]In the same division also of the Wealden, at Cuckfield, is a bed of gravel or conglomerate, consisting of water-worn pebbles of quartz and jasper, with rolled bones of reptiles. These must have been drifted by a current, probably in water of no great depth.
Fig. 239.Sphenopteris gracilis(Fitton), from near Tunbridge Wells.a.portion of the same magnified.
Fig. 239.
Sphenopteris gracilis(Fitton), from near Tunbridge Wells.
a.portion of the same magnified.
From such facts we may infer that, notwithstanding the great thickness of this division of the Wealden (and the same observation applies to the Weald Clay and Purbeck Beds), the whole of it was a deposit in water of a moderate depth, and often extremely shallow. This idea may seem startling at first, yet such would be the natural consequence of a gradual and continuous sinking of the ground in an estuary or bay, into which a great river discharged its turbid waters. By each foot of subsidence, the fundamental rock, such as the Portland Oolite, would be depressed one foot farther from thesurface; but the bay would not be deepened, if newly deposited mud and sand should raise the bottom one foot. On the contrary, such new strata of sand and mud might be frequently laid dry at low water, or overgrown for a season by a vegetation proper to marshes.
Immediately below the Hastings Sands we find a series of calcareous slates, marls, and limestones, called the Purbeck Beds, because well exposed to view in the sea-cliffs of the Peninsula of Purbeck, especially in Durlestone Bay, near Swanage. They may also be advantageously studied at Lulworth Cove and the neighbouring bays between Weymouth and Dorchester. At Meup's Bay in particular, Prof. E. Forbes has recently examined minutely the organic remains of the three members of the Purbeck group, displayed there in a vertical section 155 feet thick. To the information previously supplied in the works of Messrs. Webster, Fitton, De la Beche, Buckland, and Mantell, he has made most ample and important additions, so that it will be desirable to give them at some length, it appearing that the Upper, Middle, and Lower Purbecks are each marked by peculiar species of organic remains, these again being different, so far as a comparison has yet been instituted, from the fossils of the overlying Hastings Sands and Weald Clay. This result cannot fail to excite much wonder, and it leads us to suspect that the Wealden period, which many geologists have scarcely deigned to notice in their classification, may comprehend the history of a lapse of time as great as that of the Oolitic or Cretaceous eras respectively.[231-A]
Upper Purbeck.—The highest of the three divisions is purely freshwater, the strata, about 50 feet in thickness, containing shells of the generaPaludina,Physa,Lymnea,Planorbis,Valvata,Cyclas, andUnio, with cyprides, and fish.
Middle Purbeck.—To these succeed the Middle Purbeck, about 30 feet thick, the uppermost part of which consists of freshwater limestone, with cyprides, turtles, and fish of different species from those in the preceding strata. Below the limestone are brackish-water beds full ofCyrena, and traversed by bands abounding inCorvulæandMelaniæ. These are based on a purely marine deposit, withPecten,Modiola,Avicula, andThracia, all undescribed shells. Below this, again, come limestones and shales, partly of brackish and partly of freshwater origin, in which many fish, especially species ofLepidotusandMicrodon radiatus, are found, and a reptile namedMacrorhyncus. Among the mollusks, a remarkable ribbedMelania, of the sectionChilira, occurs.
Immediately below is the great and conspicuous stratum, 12 feet thick, long familiar to geologists under the local name of "Cinder-bed," formed of a vast accumulation of shells ofOstrea distorta(fig. 240.). In the uppermost part of this bed Mr. Forbes discovered the first echinoderm as yet known in the Purbeck series, a species ofHemicidaris, a genus characteristic of the Oolitic period. It was accompanied by a species ofPerna. Below the Cinder-bed freshwater strata are again seen, filled in many places with species ofCypris,Valvata,Paludina,Planorbis,Lymnea,Physa, andCyclas, all different from any we had previously seen above. Thick siliceous beds of chert, filled with these fossils, occur in a beautiful state of preservation, often converted into chalcedony. Among these Mr. Forbes met with gyrogonites (the spore vesicles ofCharæ), plants never before discovered in rocks older than the Eocene. Again, beneath these freshwater strata, a very thin band of greenish shales, with marine shells and impressions of leaves, like those of a largeZostera, succeeds, forming the base of the Middle Purbeck.
Fig. 240.Ostrea distorta. Cinder-bed.
Fig. 240.
Ostrea distorta. Cinder-bed.
Lower Purbeck.—Beneath the thin marine band last mentioned, purely freshwater marls occur, containing species ofCypris,Valvata, andLymnea, different from those of the Middle Purbeck. This is the beginning of the Inferior division, which is about 80 feet thick. Below the marls are seen more than 30 feet of brackish-water beds, at Meup's Bay, abounding in a species ofSerpula, allied to, if not identical with,Serpula coacervites, found in the Wealden of Hanover. There are also shells of the genusRissoa(of the subgenusHydrobia), and a littleCardiumof the subgenusProtocardium, in the same beds, together withCypris. Some of the cypris-bearing shales are strangely contorted and broken up, at the west end of the Isle of Purbeck. The great dirt-bed or vegetable soil containing the roots and stools ofCycadeæ, which I shall presently describe, underlies these marls, resting upon the lowest freshwater limestone, a rock about 8 feet thick, containingCyclades,Valvata, andLymnea, of the same species as those of the uppermost part of the Lower Purbeck. This rock rests upon the top beds of the Portland stone, which is purely marine, and between which and the Purbecks there is no passage.
The most remarkable of all the varied successions of beds enumerated in the above list, is that called by the quarrymen "the dirt," or "black dirt," which was evidently an ancient vegetable soil. It is from 12 to 18 inches thick, is of a dark brown or black colour, and contains a large proportion of earthy lignite. Through it are dispersed rounded fragments of stone, from 3 to 9 inches in diameter, in such numbers that it almost deserves the name of gravel. Many silicified trunks of coniferous trees, and the remains of plants allied toZamiaandCycas, are buried in this dirt-bed (see figure of livingZamia,fig. 241.).
These plants must have become fossil on the spots where they grew. The stumps of the trees stand erect for a height of from 1 to 3 feet, and even in one instance to 6 feet, with their roots attached to the soil at about the same distances from one another as the trees in amodern forest.[233-A]The carbonaceous matter is most abundant immediately around the stumps, and round the remains of fossilCycadeæ.[233-B]
Fig. 241.Zamia spiralis; Southern Australia.[233-C]
Fig. 241.
Zamia spiralis; Southern Australia.[233-C]
Besides the upright stumps above mentioned, the dirt-bed contains the stems of silicified trees laid prostrate. These are partly sunk into the black earth, and partly enveloped by a calcareous slate which covers the dirt-bed. The fragments of the prostrate trees are rarely more than 3 or 4 feet in length; but by joining many of them together, trunks have been restored, having a length from the root to the branches of from 20 to 23 feet, the stems being undivided for 17 or 20 feet, and then forked. The diameter of these near the roots is about 1 foot.[233-D]Root-shaped cavities were observed by Professor Henslow to descend from the bottom of the dirt-bed into the subjacent freshwater stone, which, though now solid, must have been in a soft and penetrable state when the trees grew.[233-E]
Fig. 242.Section in Isle of Portland,Dorset. (Bucklandand De la Beche.)
Fig. 242.
Section in Isle of Portland,Dorset. (Bucklandand De la Beche.)
The thin layers of calcareous slate (fig. 242.) were evidently deposited tranquilly, and would have been horizontal but for the protrusion of the stumps of the trees, around the top of each of which they form hemispherical concretions.
The dirt-bed is by no means confined to the island of Portland, where it has been most carefully studied, but is seen in the same relative position in the cliffs east of Lulworth Cove, in Dorsetshire, where, as the strata have been disturbed, and are now inclined at an angle of 45°, the stumps of the trees are also inclined at the same angle in an opposite direction—a beautiful illustration of a change in the position of beds originally horizontal (seefig. 243.). Traces of the dirt-bed have also been observed by Dr. Buckland, about two miles north of Thame, in Oxfordshire; and by Dr. Fitton, in the cliffs of the Boulonnois, on the French coast; but, as might be expected, this freshwater deposit is of limited extent when compared to most marine formations.
Fig. 243.Section in cliff east of LulworthCove. (Bucklandand De la Beche.)
Fig. 243.
Section in cliff east of LulworthCove. (Bucklandand De la Beche.)
From the facts above described, we may infer, first, that the superior beds of the Oolite, called "the Portland," which are full of marine shells, were overspread with fluviatile mud, which became dry land, and covered by a forest, throughout a portion of the space now occupied by the south of England, the climate being such as to admit the growth of theZamiaandCycas. 2dly. This land at length sank down and was submerged with its forests beneath a body of fresh water, from which sediment was thrown down enveloping fluviatile shells. 3dly. The regular and uniform preservation of this thin bed of black earth over a distance of many miles, shows that the change from dry land to the state of a freshwater lake or estuary, was not accompanied by any violent denudation, or rush of water, since the loose black earth, together with the trees which lay prostrate on its surface, must inevitably have been swept away had any such violent catastrophe then taken place.
The dirt-bed has been described above in its most simple form, but in some sections the appearances are more complicated. The forest of the dirt-bed was not everywhere the first vegetation which grew in this region. Two other beds of carbonaceous clay, one of them containingCycadeæ, in an upright position, have been found below it, and one above it[234-A], which implies other oscillations in the level of the same ground, and its alternate occupation by land and water more than once.
Table showing the changes of medium in which the strata were formed, from the Lower Greensand to the Portland Stone inclusive, in the south-east of England.
The annexed tabular view will enable the reader to take in at a glance the successive changes from sea to river, and from river to sea, or from these again to a state of land, which have occurred in this part of England between the Cretaceous and Oolitic periods. That there have been at least four changes in the species of testacea during the deposition of the Wealden, seems to follow from the observations recently made by Professor E. Forbes, so that, should we hereafter find the signs of many more alternate occupations of the same area by different elements, it is no more than we might expect. Even during a small part of a zoological period, not sufficient to allow time for many species to die out, we find that the same area has been laid dry, and then submerged, and then again laid dry, as in the deltas of the Po and Ganges, the history of which has been brought to light by Artesian borings.[235-A]We also know that similar revolutions have occurred within the present century (1819) in the delta of the Indus in Cutch[235-B], where land has been laid permanently under the waters both of the river and sea, without its soil or shrubs having been swept away. Even, independently of any vertical movements of the ground, we see in the principal deltas, such as that of the Mississippi, that the sea extends its salt waters annually for many months over considerable spaces, which, at other seasons, are occupied by the river during its inundations.
It will be observed that the division of the Purbecks into upper, middle, and lower, has been made by Professor E. Forbes, strictly on the principle of the entire distinctness of the species of organic remains which they include. The lines of demarcation are not lines of disturbance, nor indicated by any striking physical characters or mineral changes. The features which attract the eye in the Purbecks, such as the dirt-beds, the dislocated strata at Lulworth, and the Cinder-bed, do not indicate any breaks in the distribution of organized beings. "The causes which led to a complete change of life three times during the deposition of the freshwater and brackish strata must," says this naturalist, "be sought for, not simply in either arapid or a sudden change of their area into land or sea, but in the great lapse of time which intervened between the epochs of deposition at certain periods during their formation."
Each dirt-bed may, no doubt, be the memorial of many thousand years or centuries, because we find that 2 or 3 feet of vegetable soil is the only monument which many a tropical forest has left of its existence ever since the ground on which it now stands was first covered with its shade. Yet, even if we imagined the fossil soils of the Lower Purbeck to represent as many ages, we need not expect on that account to find them constituting the lines of separation between successive strata characterized by different zoological types. The preservation of a layer of vegetable soil, when in the act of being submerged, must be regarded as a rare exception to a general rule. It is of so perishable a nature, that it must usually be carried away by the denuding waves or currents of the sea or by a river; and many dirt-beds were probably formed in succession, and annihilated in the Wealden, besides those few which now remain.
Fig. 244.Cone from the Isle of Purbeck, resembling theDammaraof theMoluccas. (Fitton.)
Fig. 244.
Cone from the Isle of Purbeck, resembling theDammaraof theMoluccas. (Fitton.)
The plants of the Wealden, so far as our knowledge extends at present, consist chiefly of Ferns, Coniferæ (seefig. 244.), and Cycadeæ, without any exogens; the whole more allied to the Oolitic than to the Cretaceous vegetation, although some of the species seem to be common to the chalk. But the vertebrate and invertebrate animals indicate, in like manner, a relationship to both these periods, though a nearer affinity to the Oolitic. Mr. Brodie has found the remains of beetles and several insects of the homopterous and trichopterous orders, some of which now live on plants, like those of the Wealden, while others hover over the surface of our present rivers. But no bones of mammalia have been met with among those of land-reptiles. Yet, as the reader will learn, in Chapter XX., that the relics of marsupial quadrupeds have been detected in still older beds, and, as it was so long before a single portion of the jaw of an iguanodon was met with in the Tilgate quarries (seep. 228.), we need by no means despair of discovering hereafter some evidence of the existence of warm-blooded quadrupeds at this era. It is, at least, too soon to infer, on mere negative evidence, that the mammalia were foreign to this fauna.
In regard to the geographical extent of the Wealden, it cannot be accurately laid down; because so much of it is concealed beneath the newer marine formations. It has been traced about 200 English miles from west to east, from Lulworth Cove to near Boulogne, in France; and about 220 miles from north-west to south-east, from Whitchurch, in Buckinghamshire, to Beauvais, in France. If the formation be continuous throughout this space, which is very doubtful, it does not follow that the whole was contemporaneous; because, in all likelihood, the physical geography of the region underwent frequent change throughout the whole period, and the estuary mayhave altered its form, and even shifted its place. Dr. Dunker, of Cassel, and H. Von Meyer, in an excellent monograph on the Wealdens of Hanover and Westphalia, have shown that they correspond so closely, not only in their fossils, but also in their mineral characters, with the English series, that we can scarcely hesitate to refer the whole to one great delta. Even then, the magnitude of the deposit may not exceed that of many modern rivers. Thus, the delta of the Quorra or Niger, in Africa, stretches into the interior for more than 170 miles, and occupies, it is supposed, a space of more than 300 miles along the coast, thus forming a surface of more than 25,000 square miles, or equal to about one half of England.[237-A]Besides, we know not, in such cases, how far the fluviatile sediment and organic remains of the river and the land may be carried out from the coast, and spread over the bed of the sea. I have shown, when treating of the Mississippi, that a more ancient delta, including species of shells, such as now inhabit Louisiana, has been upraised, and made to occupy a wide geographical area, while a newer delta is forming[237-B]; and the possibility of such movements, and their effects, must not be lost sight of when we speculate on the origin of the Wealden.
If it be asked where the continent was placed from the ruins of which the Wealden strata were derived, and by the drainage of which a great river was fed, we are half tempted to speculate on the former existence of the Atlantis of Plato. The story of the submergence of an ancient continent, however fabulous in history, must have been true again and again as a geological event.
The real difficulty consists in the persistence of a large hydrographical basin, from whence a great body of fresh water was poured into the sea, precisely at a period when the neighbouring area of the Wealden was gradually going downwards 1000 feet or more perpendicularly. If the adjoining land participated in the movement, how could it escape being submerged, or how could it retain its size and altitude so as to continue to be the source of such an inexhaustible supply of fresh water and sediment? In answer to this question, we are fairly entitled to suggest that the neighbouring land may have been stationary, or may even have undergone a contemporaneous slow upheaval. There may have been an ascending movement in one region, and a descending one in a contiguous parallel zone of country; just as the northern part of Scandinavia is now rising, while the middle portion (that south of Stockholm) is unmoved, and the southern extremity in Scania is sinking, or at least has sunk within the historical period.[237-C]We must, nevertheless, conclude, if we adopt the above hypothesis, that the depression of the land became general throughout a large part of Europe at the close of the Wealden period, a subsidence which brought in the cretaceous ocean.
Physical geography of certain districts composed of Cretaceous and Wealden strata — Lines of inland chalk-cliffs on the Seine in Normandy — Outstanding pillars and needles of chalk — Denudation of the chalk and Wealden in Surrey, Kent, and Sussex — Chalk once continuous from the North to the South Downs — Anticlinal axis and parallel ridges — Longitudinal and transverse valleys — Chalk escarpments — Rise and denudation of the strata gradual — Ridges formed by harder, valleys by softer beds — Why no alluvium, or wreck of the chalk, in the central district of the Weald — At what periods the Weald valley was denuded — Land has most prevailed where denudation has been greatest — Elephant bed, Brighton.
Physical geography of certain districts composed of Cretaceous and Wealden strata — Lines of inland chalk-cliffs on the Seine in Normandy — Outstanding pillars and needles of chalk — Denudation of the chalk and Wealden in Surrey, Kent, and Sussex — Chalk once continuous from the North to the South Downs — Anticlinal axis and parallel ridges — Longitudinal and transverse valleys — Chalk escarpments — Rise and denudation of the strata gradual — Ridges formed by harder, valleys by softer beds — Why no alluvium, or wreck of the chalk, in the central district of the Weald — At what periods the Weald valley was denuded — Land has most prevailed where denudation has been greatest — Elephant bed, Brighton.
Allthe fossiliferous formations may be studied by the geologist in two distinct points of view: first, in reference to their position in the series, their mineral character and fossils; and, secondly, in regard to their physical geography, or the manner in which they now enter, as mineral masses, into the external structure of the earth; forming the bed of lakes and seas, or the surface and foundation of hills and valleys, plains and table-lands. Some account has already been given on the first head of the Tertiary, the Cretaceous, and Wealden strata; and we may now proceed to consider certain features in the physical geography of these groups as they occur in parts of England and France.
The hills composed of white chalk in the S.E. of England have a smooth rounded outline, and being usually in the state of sheep pastures, are free from trees or hedgerows; so that we have an opportunity of observing how the valleys by which they are drained ramify in all directions, and become wider and deeper as they descend. Although these valleys are now for the most part dry, except during heavy rains and the melting of snow, they may have been due to aqueous denudation, as explained in the sixth chapter; having been excavated when the chalk emerged gradually from the sea. This opinion is confirmed by the occasional occurrence of long lines of inland cliffs, in which the strata are cut off abruptly in steep and often vertical precipices. The true nature of such escarpments is nowhere more obvious than in parts of Normandy, where the river Seine and its tributaries flow through deep winding valleys, hollowed out of chalk horizontally stratified. Thus, for example, if we follow the Seine for a distance of about 30 miles from Andelys to Elbœuf, we find the valley flanked on both sides by a deep slope of chalk, with numerous beds of flint, the formation being laid open for a thickness of about 250 and 300 feet. Above the chalk is an overlying mass of sand, gravel, and clay, from 30 to 100 feet thick. The two opposite slopes of the hillsaandb, where the chalk appears atthe surface, are from 2 to 4 miles apart, and they are often perfectly smooth and even, like the steepest of our downs in England; but at many points they are broken by one, two, or more ranges of vertical and even overhanging cliffs of bare white chalk with flints. At some points detached needles and pinnacles stand in the line of the cliffs, or in front of them, as atc,fig. 245.On the right bank of the Seine, at Andelys, one range, about 2 miles long, is seen varying from 50 to 100 feet in perpendicular height, and having its continuity broken by a number of dry valleys or coombs, in one of which occurs a detached rock or needle, called the Tête d'Homme (seefigs. 246,247.). The top of this rock presents a precipitous face towards every point of the compass; its vertical height being more than 20 feet on the side of the downs, and 40 towards the Seine, the average diameter of the pillar being 36 feet. Its composition is the same as that of the larger cliffs in its neighbourhood, namely, white chalk, having occasionally a crystalline texture like marble, with layers of flint in nodules and tabular masses. The flinty beds often project in relief 4 or 5 feet beyond the white chalk, which is generally in a state of slow decomposition, either exfoliating or being covered with white powder, like the chalk cliffs on the English coast; and, as in them, this superficial powder contains in some places common salt.
Fig. 245.Section across Valley of Seine.
Fig. 245.
Section across Valley of Seine.
Fig. 246.View of the Tête d'Homme, Andelys, seen from above.
Fig. 246.
View of the Tête d'Homme, Andelys, seen from above.
Other cliffs are situated on the right bank of the Seine, opposite Tournedos, between Andelys and Pont de l'Arche, where the precipices are from 50 to 80 feet high: several of their summits terminatein pinnacles; and one of them, in particular, is so completely detached as to present a perpendicular face 50 feet high towards the sloping down. On these cliffs several ledges are seen, which mark so many levels at which the waves of the sea may be supposed to have encroached for a long period. At a still greater height, immediately above the top of this range, are three much smaller cliffs, each about 4 feet high, with as many intervening terraces, which are continued so as to sweep in a semicircular form round an adjoining coomb, like those in Sicily before described (p. 76.).
Fig. 247.Side view of the Têted'Homme. Whitechalk with flints.
Fig. 247.
Side view of the Têted'Homme. Whitechalk with flints.
Fig. 248.Chalk pinnacle at Senneville.
Fig. 248.
Chalk pinnacle at Senneville.
Fig. 249.Roches d'Orival, Elbœuf.
Fig. 249.
Roches d'Orival, Elbœuf.
If we then descend the river from Vatteville to a place called Senneville, we meet with a singular needle about 50 feet high, perfectly isolated on the escarpment of chalk on the right bank of the Seine (seefig. 248.). Another conspicuous range of inland cliffs is situated about 12 miles below on the left bank of the Seine, beginning at Elbœuf, and comprehending the Roches d'Orival (seefig. 249.). Like those before described, it has an irregular surface, often overhanging,and with beds of flint projecting several feet. Like them, also, it exhibits a white powdery surface, and consists entirely of horizontal chalk with flints. It is 40 miles inland, its height, in some parts, exceeding 200 feet, and its base only a few feet above the level of the Seine. It is broken, in one place, by a pyramidal mass or needle, 200 feet high, called the Roche de Pignon, which stands out about 25 feet in front of the upper portion of the main cliffs, with which it is united by a narrow ridge about 40 feet lower than its summit (seefig. 250.). Like the detached rocks before mentioned at Senneville, Vatteville, and Andelys, it may be compared to those needles of chalk which occur on the coast of Normandy, as well as in the Isle of Wight and in Purbeck[241-A](seefig. 251.).
Fig. 250.View of the Roche de Pignon, seen from the south.
Fig. 250.
View of the Roche de Pignon, seen from the south.
Fig. 251.Needle and Arch of Etretat, in the chalk cliffs of Normandy. Height of Arch 100feet. (Passy.)[241-B]
Fig. 251.
Needle and Arch of Etretat, in the chalk cliffs of Normandy. Height of Arch 100feet. (Passy.)[241-B]
The foregoing description and drawings will show, that the evidence of certain escarpments of the chalk having been originally sea-cliffs, is far more full and satisfactory in France than in England. If it be asked why, in the interior of our own country, we meet with no ranges of precipices equally vertical and overhanging, and no isolated pillars or needles, we may reply that the greater hardness of the chalk in Normandy may, no doubt, be the chief cause of this difference.But the frequent absence of all signs of littoral denudation in the valley of the Seine itself is a negative fact of a far more striking and perplexing character. The cliffs, after being almost continuous for miles, are then wholly wanting for much greater distances, being replaced by a green sloping down, although the beds remain of the same composition, and are equally horizontal; and although we may feel assured that the manner of the upheaval of the land, whether intermittent or not, must have been the same at those intermediate points where no cliffs exist, as at others where they are so fully developed. But, in order to explain such apparent anomalies, the reader must refer again to the theory of denudation, as expounded in the 6th chapter; where it was shown, first, that the undermining force of the waves and marine currents varies greatly at different parts of every coast; secondly, that precipitous rocks have often decomposed and crumbled down; and thirdly, that many terraces and small cliffs may now lie concealed beneath a talus of detrital matter.
Denudation of the Weald Valley.—No district is better fitted to illustrate the manner in which a great series of strata may have been upheaved and gradually denuded than the country intervening between the North and South Downs. This region, of which a ground plan is given in the accompanying map (fig. 252.), comprises within it the whole of Sussex, and parts of the counties of Kent, Surrey, and Hampshire. The space in which the formations older than the White Chalk, or those from the Gault to the Hastings sand inclusive, crop out, is bounded everywhere by a great escarpment of chalk, which is continued on the opposite side of the channel in the Bas Boulonnais in France, where it forms the semicircular boundary of a tract in which older strata also appear at the surface. The whole of this district may therefore be considered geologically as one and the same.
Fig. 252.Geological Map of the south-east of England and part of France, exhibiting the denudation of the Weald.
Fig. 252.
Geological Map of the south-east of England and part of France, exhibiting the denudation of the Weald.
Fig. 253.Section from the London to the Hampshire basin across the valley of the Weald.1. Tertiary strata.2. Chalk and firestone.3. Gault.4. Lower greensand.5. Weald clay.6. Hastings sands.
Fig. 253.
Section from the London to the Hampshire basin across the valley of the Weald.
Fig. 254.Highest point of North Downs, 880 feet.[243-A]Section of the country from the confines of the basin of London to that of Hants, with the principal heights above the level of the sea on a true scale.[243-B]
Fig. 254.
Highest point of North Downs, 880 feet.[243-A]
Section of the country from the confines of the basin of London to that of Hants, with the principal heights above the level of the sea on a true scale.[243-B]
The space here inclosed within the escarpment of the chalk affords an example of what has been sometimes called a "valley of elevation" (more properly "of denudation"); where the strata, partially removed by aqueous excavation, dip away on all sides from a centralaxis. Thus, it is supposed that the area now occupied by the Hastings sand (No. 6.) was once covered by the Weald clay (No. 5.), and this again by the Greensand (No. 4.), and this by the Gault (No. 3.); and, lastly, that the Chalk (No. 2.) extended originally over the whole space between the North and the South Downs. This theory will be better understood by consulting the annexed diagram (fig. 253.), where the dark lines represent what now remains, and the fainter ones those portions of rock which are believed to have been carried away.
At each end of the diagram the tertiary strata (No. 1.) are exhibited reposing on the chalk. In the middle are seen the Hastings sands (No. 6.) forming an anticlinal axis, on each side of which the other formations are arranged with an opposite dip. It has been necessary, however, in order to give a clear view of the different formations, to exaggerate the proportional height of each in comparison to its horizontal extent; and a true scale is therefore subjoined in another diagram (fig. 254.), in order to correct the erroneous impression which might otherwise be made on the reader's mind. In this section the distance between the North and South Downs is represented to exceed forty miles; for the Valley of the Weald is here intersected in its longest diameter, in the direction of a line between Lewes and Maidstone.
Through the central portion, then, of the district supposed to be denuded runs a great anticlinal line, having a direction nearly east and west, on both sides of which the beds 5, 4, 3, and 2, crop out in succession. But, although, for the sake of rendering the physical structure of this region more intelligible, the central line of elevation has alone been introduced, as in the diagrams of Smith, Mantell, Conybeare, and others, geologists have always been well aware that numerous minor lines of dislocation and flexure run parallel to the great central axis.
In the central area of the Hastings sand the strata have undergone the greatest displacement; one fault being known, where the vertical shift of a bed of calcareous grit is no less than 60 fathoms.[244-A]Much of the picturesque scenery of this district arises from the depth of the narrow valleys and ridges to which the sharp bends and fractures of the strata have given rise; but it is also in part to be attributed to the excavating power exerted by water, especially on the interstratified argillaceous beds.
Besides the series of longitudinal valleys and ridges in the Weald, there are valleys which run in a transverse direction, passing through the chalk to the basin of the Thames on the one side, and to the English Channel on the other. In this manner the chain of the North Downs is broken by the rivers Wey, Mole, Darent, Medway, and Stour; the South Downs by the Arun, Adur, Ouse, and Cuckmere.[244-B]If these transverse hollows could be filled up, all the rivers, observes Mr. Conybeare, would be forced to take an easterly course, and toempty themselves into the sea by Romney Marsh and Pevensey Levels.[245-A]
Mr. Martin has suggested that the great cross fractures of the chalk, which have become river channels, have a remarkable correspondence on each side of the valley of the Weald; in several instances the gorges in the North and South Downs appearing to be directly opposed to each other. Thus, for example, the defiles of the Wey in the North Downs, and of the Arun in the South, seemed to coincide in direction; and, in like manner, the Ouse corresponds to the Darent, and the Cuckmere to the Medway.[245-B]