Fig. 255.View of the chalk escarpment of the SouthDowns. Takenfrom the Devil's Dike, looking towards the west and south-west.a.The town of Steyning is hidden by this point.b.Edburton church.c.Road.d.River Adur.
Fig. 255.
View of the chalk escarpment of the SouthDowns. Takenfrom the Devil's Dike, looking towards the west and south-west.
Although these coincidences may, perhaps, be accidental, it is by no means improbable, as hinted by the author above mentioned, that great amount of elevation towards the centre of the Weald district gave rise to transverse fissures. And as the longitudinal valleys were connected with that linear movement which caused the anticlinal lines running east and west, so the cross fissures might have been occasioned by the intensity of the upheaving force towards the centre of the line.
But before treating of the manner in which the upheaving movement may have acted, I shall endeavour to make the reader more intimately acquainted with the leading geographical features of the district, so far as they are of geological interest.
In whatever direction we travel from the tertiary strata of the basins of London and Hampshire towards the valley of the Weald, we first ascend a slope of white chalk, with flints, and then find ourselves on the summit of a declivityconsisting, for the most part, of different members of the chalk formation; below which the upper greensand, and sometimes, also, the gault, crop out. This steep declivity is the great escarpment of the chalk before mentioned, which overhangs a valley excavated chiefly out of the argillaceous or marly bed, termed Gault (No. 3.). The escarpment is continuous along the southern termination of the North Downs, and may be traced from the sea, at Folkestone, westward to Guildford and the neighbourhood of Petersfield, and from thence to the termination of the South Downs at Beachy Head. In this precipice or steep slope the strata are cut off abruptly, and it is evident that they must originally have extended farther. In the woodcut (fig. 255.p. 245.), part of the escarpment of the South Downs is faithfully represented, where the denudation at the base of the declivity has been somewhat more extensive than usual, in consequence of the upper and lower greensand being formed of very incoherent materials, the upper, indeed, being extremely thin and almost wanting.
Fig. 256.Chalk escarpment, as seen from the hill above Steyning, Sussex. The castle and village of Bramber in the foreground.
Fig. 256.
Chalk escarpment, as seen from the hill above Steyning, Sussex. The castle and village of Bramber in the foreground.
The geologist cannot fail to recognize in this view the exact likeness of a sea cliff; and if he turns and looks in an opposite direction, or eastward, towards Beachy Head (seefig. 256.), he will see the same line of heights prolonged. Even those who are not accustomed to speculate on the former changes which the surface has undergone may fancy the broad and level plain to resemble the flat sands which were laid dry by the receding tide, and the different projecting masses of chalk to be the headlands of a coast which separated the different bays from each other.
In regard to the transverse valleys before mentioned, as intersecting the chalk hills, some idea of them may be derived from the subjoined sketch (fig. 257.), of the gorge of the river Adur, taken from the summit of the chalk downs, at a point in the bridle-way leading from the towns of Bramber and Steyning to Shoreham. If the reader will refer again to the view given in a former woodcut (fig. 255.p. 245.), he will there see the exact point where the gorge of which I am now speaking interrupts the chalk escarpment. A projecting hill, at the pointa, hides the town of Steyning, near which the valley commences where the Adur passes directly to the sea at Old Shoreham. The river flows through a nearly level plain,as do most of the others which intersect the hills of Surrey, Kent, and Sussex; and it is evident that these openings, so far at least as they are due to aqueous erosion, have not been produced by the rivers, many of which, like the Ouse near Lewes, have filled up arms of the sea, instead of deepening the hollows which they traverse.
Fig. 257.Transverse Valley of the Adur in the South Downs.a.Town of Steyning.b.River Adur.c.Old Shoreham.
Fig. 257.
Transverse Valley of the Adur in the South Downs.
Now, in order to account for the manner in which the five groups of strata, 2, 3, 4, 5, 6, represented in the map,fig. 252.and in the sectionfig. 253., may have been brought into their present position, the following hypothesis has been very generally adopted:—Suppose the five formations to lie in horizontal stratification at the bottom of the sea; then let a movement from below press them upwards into the form of a flattened dome, and let the crown of this dome be afterwards cut off, so that the incision should penetrate to the lowest of the five groups. The different beds would then be exposed on the surface, in the manner exhibited in the map, fig. 252.[247-A]
The quantity of denudation or removal by water of stratified masses assumed to have once reached continuously from the North to the South Downs is so enormous, that the reader may at first be startled by the boldness of the hypothesis. But the difficulty vanishes when once sufficient time is allowed for the gradual and successive rise of the strata, during which the waves and currents of the ocean might slowly accomplish an operation, which no sudden diluvial rush of waters could possibly have effected.
Among other proofs of the action of water, it may be stated that the great longitudinal valleys follow the outcrop of the softer andmore incoherent beds, while ridges or lines of cliff usually occur at those points where the strata are composed of harder stone. Thus, for example, the chalk with flints, together with the subjacent upper greensand, which is often used for building, under the provincial name of "firestone," has been cut into a steep cliff on that side on which the sea encroached. This escarpment bounds a deep valley, excavated chiefly out of the soft argillaceous or marly bed, termed gault (No. 3.). In some places the upper greensand is in a loose and incoherent state, and there it has been as much denuded as the gault; as, for example, near Beachy Head; but farther to the westward it is of great thickness, and contains hard beds of blue chert and calcareous sandstone or firestone. Here, accordingly, we find that it produces a corresponding influence on the scenery of the country; for it runs out like a step beyond the foot of the chalk-hills, and constitutes a lower terrace, varying in breadth from a quarter of a mile to three miles, and following the sinuosities of the chalk escarpment.[248-A]
Fig. 258.a.Chalk with flints.b.Chalk without flints.c.Upper greensand, or firestone.d.Gault.
Fig. 258.
It is impossible to desire a more satisfactory proof that the escarpment is due to the excavating power of water during the rise of the strata; for I have shown, in my account of the coast of Sicily, in what manner the encroachments of the sea tend to efface that succession of terraces which must otherwise result from the intermittent upheaval of a coast preyed upon by the waves.[248-B]During the interval between two elevatory movements, the lower terrace will usually be destroyed, wherever it is composed of incoherent materials; whereas the sea will not have time entirely to sweep away another part of the same terrace, or lower platform, which happens to be composed of rocks of a harder texture, and capable of offering a firmer resistance to the erosive action of water. As the yielding clay termed gault would be readily washed away, we find its outcrop marked everywhere by a valley which skirts the base of the chalk hills, and which is usually bounded on the opposite side by the lower greensand; but as the upper beds of this last formation are most commonly loose and incoherent, they also have usually disappeared and increased the breadth of the valley. But in those districts where chert, limestone, and other solid materials enter largely into the composition of this formation (No. 4.), they give rise to a range of hills parallel to the chalk, which sometimes rival the escarpment of the chalk itself in height, or even surpass it, as in Leith Hill, near Dorking. This ridge often presents a steep escarpment towards the soft argillaceous deposit called theWeald clay (No. 5.; see the strong lines infig. 253.p. 243.), which usually forms a broad valley, separating the lower greensand from the Hastings sands or Forest ridge; but where subordinate beds of sandstone of a firmer texture occur, the uniformity of the plain of No. 5. is broken by waving irregularities and hillocks.
It will be easy to show how closely the superficial inequalities agree with those which we might naturally expect to originate during the gradual rise of the Wealden district. Suppose the line of the most energetic movement to have coincided with what is now the central ridge of the Weald valley; in that case the first land which emerged must have been situated where the Forest ridge is now placed. Here many shoals and reefs may first have existed, and islands of chalk devoured in the course of ages by the ocean (seefig. 253.); so that the top of the shattered dome which first appeared above water may have been utterly destroyed, and the masses represented by the fainter lines (fig. 253.) removed.
The dotted lines represent the sea-level.
The dotted lines represent the sea-level.
The upper greensand is represented (fig. 259.) as forming on the left hand a single precipice with the chalk; while on the right there are two cliffs, with an intervening terrace, as before described infig. 258.Two strips of land would then remain on each side of a channel, presenting ranges of white cliffs facing each other. A powerful current might then scoop out a channel in the gault (No. 2.). This softer bed would yield with ease in proportion as parts of it were brought up from time to time and exposed to the fury of the waves, so that large spaces occupied by the harder formation or greensand (No. 3.) would be laid bare. This last rock, opposing a more effectual resistance, would next emerge; while the chalk cliffs, at the base of which the gault is rapidly undermined, would recede farther from each other, after which four parallel strips of land, or rows of islands, would be caused, which are represented by the masses which infig. 260.rise above the dotted line indicating the sea-level. In this diagram, however, the inclination of the upper surface of the formations (Nos. 1. and 3.), is exaggerated. Originally this surface must have been level, like the submarine terraces produced by denudation, and described before (p. 74.and77.); but they were afterwards more and more tilted by that general movement to which the region of the Weald owes its structure. At length, by the farther elevation of the dome-shaped mass, the clay (No. 4.) would be broughtwithin reach of the waves, which would probably gain the more easy access to the subjacent deposit by the rents which would be caused in No. 3., and in the central part of the ridge where the uplifting force had been exerted with the greatest energy. The opposite cliffs, in which the greensand (No. 3.) terminates, would now begin to recede from each other, having at their base a yielding stratum of clay (No. 4.). Lastly, the sea would penetrate to the sand (No. 5.), and then the state of things indicated in the dark lines of the upper section (fig. 253.), would be consummated.
Fig. 261.The Coomb, near Lewes.
Fig. 261.
The Coomb, near Lewes.
It was stated that there are many lines of flexure and dislocation, running east and west, or parallel to the central axis of the Wealden. They are numerous in the district of the Hastings sand, and sometimes occur in the chalk itself. One of the latter kind has given rise to the ravine called the Coomb, near Lewes, and was first traced out by Dr. Mantell, in whose company I examined it. This coomb is seen on the eastern side of the valley of the Ouse, in the suburbs of the town of Lewes. The steep declivities on each side are covered with green turf, as is the bottom, which is perfectly dry. No outward signs of disturbance are visible; and the connection of the hollow with subterranean movements would not have been suspected by the geologist, had not the evidence of great convulsions been clearly exposed in the escarpment of the valley of the Ouse, and the numerous chalk pits worked at the termination of the Coomb. By the aid of these we discover that the ravine coincides precisely with a line of fault, on one side of which the chalk with flints (a,fig. 262.), appears at the summit of the hill, while it is thrown down to the bottom on the other.
Mr. Martin, in his work on the geology of Western Sussex, published in 1828, threw much light on the structure of the Wealden by tracing out continuously for miles the direction of many anticlinallines and cross fractures; and the same course of investigation has since been followed out in greater detail by Mr. Hopkins. The mathematician last-mentioned has shown that the observed direction of the lines of flexure and dislocation in the Weald district coincide with those which might have been anticipated theoretically on mechanical principles, if we assume certain simple conditions under which the strata were lifted up by an expansive subterranean force. He finds by calculation that if this force was applied so as to act uniformly upwards within an elliptic area, the longitudinal fissures thereby produced would nearly coincide with the outlines of the ellipse, forming cracks, which are portions of smaller concentric ellipses, parallel to the margin of the larger one. These longitudinal fissures would also be intercepted by others running at right angles to them, and both lines of fracture may have been produced at the same time.[251-A]In this illustration it is supposed that the expansive force acted simultaneously and with equal intensity at every point within the upheaved area, and not with greater energy along the central axis or region of principal elevation.
Fig. 262.Fault in the cliff hills nearLewes. Mantell.a.Chalk with flints.b.Lower chalk.[251-B]
Fig. 262.
Fault in the cliff hills nearLewes. Mantell.
The geologist cannot fail to derive great advantage in his speculations from the mathematical investigation of a problem of this kind, where results free from all uncertainty are obtained on the assumption of certain simple conditions. Such results, when once ascertained by mathematical methods, may serve as standard cases, to which others occurring in nature of a more complicated kind may be referred. In order that a uniform force should cause the strata to attain in the centre of the ellipse a height so far exceeding that which they have reached round the margin, it is necessary to assume that the mass of upheaved strata offered originally a very unequal degree of resistance to the subterranean force. This may have happened either from their being more fractured in one place than in another, or from being pressed down by a less weight of incumbent strata; as if we suppose, what is far from improbable, that great denudation had taken place in the middle of the Wealden before the final and principal upheaval occurred. It is suggested that the beds may have been acted upon somewhat in the manner of a carpet spread out loosely on a floor, and nailed down round the edges, which would swell into the shape of a dome if pressed up equally at every point by air admitted from beneath. But when we are reasoning on the particular phenomena of the Weald, we have no geological data for determining whether it be more probable that originally the resistance to be overcome wasso extremely unequal in different places, or whether the subterranean force, instead of being everywhere uniform, was not applied with very different degrees of intensity beneath distinct portions of the upraised area.
The opinion that both the longitudinal and transverse lines of fracture may have been produced simultaneously, accords well with that expressed by M. Thurmann, in his work on the anticlinal ridges and valleys of elevation of the Bernese Jura.[252-A]For the accuracy of his map and sections I can vouch, from personal examination, in 1835, of part of the region surveyed by him. Among other results, at which this author arrived, it appears that the breadth of all the numerous anticlinal ridges and dome-shaped masses in the Jura is invariably great in proportion to the number of the formations exposed to view; or, in other words, to the depth to which the superimposed groups of secondary strata have been laid open. (Seefig. 71.p. 55.for structure of Jura.) He also remarks, that the anticlinal lines are occasionally oblique and cross each other, in which case the greatest dislocation of the beds takes place. Some of the cross fractures are imagined by him to have been contemporaneous, others subsequent to the longitudinal ones.
I have assumed, in the former part of this chapter, that the rise of the Weald was gradual, whereas many geologists have attributed its elevation to a single effort of subterranean violence. There appears to them such a unity of effect in this and other lines of deranged strata in the south-east of England, such as that of the Isle of Wight, as is inconsistent with the supposition of a great number of separate movements recurring after long intervals of time. But we know that earthquakes are repeated throughout a long series of ages in the same spots, like volcanic eruptions. The oldest lavas of Etna were poured out many thousands, perhaps myriads of years before the newest, and yet they, and the movements accompanying their emission, have produced a symmetrical mountain; and if rivers of melted matter thus continue to flow in the same direction, and towards the same point, for an indefinite lapse of ages, what difficulty is there in conceiving that the subterranean volcanic force, occasioning the rise or fall of certain parts of the earth's crust, may, by reiterated movements, produce the most perfect unity of result?
Alluvium of the Weald.—Our next inquiry may be directed to the alluvium strewed over the surface of the supposed area of denudation. Has any wreck been left behind of the strata removed? To this we may answer, that the chalk downs even on their summits are covered every where with gravel composed of unrounded and partially rounded chalk flints, such as might remain after masses of white chalk had been softened and removed by water. This superficial accumulation of the hard or siliceous materials of the disintegrated strata may be due in some degree to pluvial action; for during extraordinary rains a rush of water charged with calcareous matter, of a milk-white colour, may be seen to descend even gently sloping hills of chalk. If a layer no thicker than the tenth of an inch be removed once in a century,a considerable mass may in the course of indefinite ages melt away, leaving nothing save a layer of flinty nodules to attest its former existence. These unrolled flints may remain mixed with others more or less rounded, which the waves left originally on the surface of the chalk, when it first emerged from the sea. A stratum of fine clay sometimes covers the surface of slight depressions and the bottom of valleys in the white chalk, which may represent the aluminous residue of the rock, after the pure carbonate of lime has been dissolved by rain water, charged with excess of carbonic acid derived from decayed vegetable matter.[253-A]
Although flint gravel is so abundant on the chalk itself, it is usually wanting in the deep longitudinal valleys at the foot of the chalk escarpment, although, in some few instances, the detritus of the chalk has been traced in patches over the gault, and even the lower greensand, for a distance of several miles from the escarpment of the North and South Downs. But no vestige of the chalk and its flints has been seen on the central ridge of the Weald or the Hastings sands, but merely gravel derived from the rocks immediately subjacent. This distribution of alluvium, and especially the absence of chalk detritus in the central district, agrees well with the theory of denudation before set forth; for to return tofig. 259., if the chalk (No. 1.) were once continuous and covered every where with flint gravel, this superficial covering would be the first to be carried away from the highest part of the dome long before any of the gault (No. 2.) was laid bare. Now if some ruins of the chalk remain at first on the gault, these would be, in a great degree, cleared away before any part of the lower greensand (No. 3.) is denuded. Thus in proportion to the number and thickness of the groups removed in succession, is the probability lessened of our finding any remnants of the highest group strewed over the bared surface of the lowest.
As an exception to the general rule of the small distance to which any wreck of the chalk can be traced from the escarpments of the North and South Downs, I may mention a thick bed of chalk flints which occurs near Barcombe, about three miles to the north of Lewes (seefig. 263.), a place which I visited with Dr. Mantell, to whom I am indebted for the accompanying section. Even here it will be seen that the gravel reaches no farther than the Weald Clay. The same section shows one of the minor east and west anticlinal lines before alluded to (p. 244.).
Fig. 263.Section from the north escarpment of the South Downs to Barcombe.1. Gravel composed of partially rounded chalk flints.2. Chalk with and without flints.3. Lowest chalk or chalk marl (upper greensand wanting).4. Gault.5. Lower greensand.6. Weald clay.
Fig. 263.
Section from the north escarpment of the South Downs to Barcombe.
At what period the Weald Valley was denuded.—If we inquire at what geological period the denudation of the Weald was effected, we shall immediately perceive that the question is limited to this point, whether it took place during or subsequent to the deposition of the Eocene strata of the south of England. For in the basins of London and Hampshire the Eocene strata are conformable to the chalk, being horizontal where the beds of chalk are horizontal, and vertical where they are vertical, so that both series of rocks appear to have participated in nearly the same movements. At the eastern extremity of the Isle of Wight, some beds even of the freshwater series have been thrown on their edges, like those of the London clay. Nevertheless we can by no means infer that all the tertiary deposits of the London and Hampshire basins once extended like the chalk over the entire valley of the Weald, because the denudation of the chalk and greensand may have been going on in the centre of that area, while contiguous parts of the sea were sufficiently deep to receive and retain the matter derived from that waste. Thus while the waves and currents were excavating the longitudinal valleys D and C (fig. 264.), the depositsamay have been thrown down to the bottom of the contiguous deep water E, the sediment being drifted through transverse fissures, as before explained. In this case, the rise of the formations Nos. 1, 2, 3, 4, 5, may have been going on contemporaneously with the excavation of the valleys C and D, and with the accumulation of the tertiary strataa.
Fig. 264.
Fig. 264.
This idea receives some countenance from the fact of the tertiary strata, near their junction with the chalk of the London and Hampshire basins, often consisting of dense beds of sand and shingle, as at Blackheath and in the Addington Hills near Croydon. They also contain occasionally freshwater shells and the remains of land animals and plants, which indicate the former presence of land at no great distance, some part of which may have occupied the centre of the Weald.
Such masses of well-rolled pebbles occurring in the lowest Eocene strata, or those called "the plastic clay and sands" before described (No. 3.b, Tab.p. 197.), imply the neighbourhood of an ancient shore. They also indicate the destruction of pre-existing chalk with flints. At the same time fossil shells of the generaMelania,Cyclas, andUnio, appearing here and there in beds of the same age, together with plants and the bones of land animals, bear testimony to contiguous land, which probably constituted islands scattered over the space now occupied by the tertiary basins of the Seine and Thames. The stage of denudation represented infig. 259.,p. 249., may explain the state of things prevailing at points where such islands existed. By the alternate rising and sinking of the white chalk and older beds, a large area may have become overspread with gravelly sandy, andclayey beds of fluvio-marine and shallow-water origin, before any of the London clay proper (or Calcaire grossier in France) were superimposed. This may account for the fact that patches of "plastic clay and sand" (No. 3.b, Tab.p. 197.), are scattered over the surface of the chalk, reaching in some places to great heights, and approaching even the edges of the escarpments. We must suppose that subsequently a gradual subsidence took place in certain areas, which allowed the London clay proper to accumulate over the Lower Eocene sands and clays, in a deep sea. During this sinking down (the vertical amount of which equalled 800, and in parts of the Isle of Wight, according to Mr. Prestwich, 1800 feet), the work of denudation would be unceasing, being always however confined to those areas where land or islands existed. At length, when the Bagshot sand had been in its turn thrown down on the London clay, the space covered by these two formations was again upraised from the sea to about the height which it has since retained. During this upheaval, the waves would again exert their power, not only on the white chalk and lower cretaceous and Wealden strata, but also on the Eocene formations of the London basin, excavating valleys and undermining cliffs as the strata emerged from the deep.
There are grounds, as before stated (p. 205.), for presuming that the tertiary area of London was converted into land before that of Hampshire, and for this reason it contains no marine Eocene deposits so modern as those of Barton Cliff, or the still newer freshwater and fluvio-marine beds of Hordwell and the Isle of Wight. These last seem unequivocally to demonstrate the local inequality of the upheaving and depressing movements of the period alluded to; for we find, in spite of the evidence afforded in Alum and White Cliff Bays, of continued depression to the extent of 1800 or 2000 feet, that at the close of the Eocene period a dense formation of freshwater strata was produced. The fossils of these strata bear testimony to rivers draining adjacent lands, and the existence of numerous quadrupeds on those lands. Instead of such phenomena, the signs of an open sea might naturally have been expected as the consequence of so much subsidence, had not the depression been accompanied or followed by upheaval in a region immediately adjoining.
When we attempt to speculate on the geographical changes which took place in the earlier part of the Eocene epoch, and to restore in imagination the former state of the physical geography of the south-east of England, we shall do well to bear in mind that wherever there are proofs of great denudation, there also the greatest area of land has probably existed. In the same space, moreover, the oscillations of level, and the alternate submergence and emergence of coasts, may be presumed to have been most frequent; for these fluctuations facilitate the wasting and removing power of waves, currents, and rivers.
We should also remember that there is always a tendency in the last denuding operations, to efface all signs of preceding denudation, or at least all those marks of waste from which alone a geologist can ascertain the date of the removal of the missing strata within the denuded area. It may often be difficult to settle the chronology evenof the last of a series of such acts of removal, but it must be, in the nature of things, almost always impossible to assign a date to each of the antecedent denudations. If we wish to determine the times of the destruction of rocks, we must look any where rather than to the spaces once occupied by the missing rocks. We must inquire to what regions the ruins of the white chalk, greensand, Wealden, and other strata which have disappeared were transported. We are then led at once to the examination of all the deposits newer than the chalk, and first to the oldest of these, the Lower Eocene, and its sand, shingle, and clay. In them, so largely developed in the immediate neighbourhood of the denuded area, we discover the wreck we are in search of, regularly stratified, and inclosing, in some of its layers, organic remains of a littoral, and sometimes fluviatile character. What more can we desire? The shores must have consisted of chalk, greensand, and Wealden, since these were the only superficial rocks in the south-east of England, at the commencement of the Eocene epoch. The waves of the sea, therefore, and the rivers were grinding down chalk-flints and chert from the greensand into shingle and sand, or were washing away calcareous and argillaceous matter from the cretaceous and Wealden beds, during the whole of the Eocene period. Thus we obtain the date of a great part at least of that enormous amount of denudation of which we have such striking monuments in the space intervening between the North and South Downs.
Fig. 265.A. Chalk with layers of flint dipping slightly to the south.b.Ancient beach, consisting of fine sand, from one to four feet thick, covered by shingle from five to eight feet thick of pebbles of chalk-flint, granite, and other rocks, with broken shells of recent marine species, and bones of cetacea.c.Elephant bed, about fifty feet thick, consisting of layers of white chalk rubble, with broken chalk-flints, in which deposit are found bones of ox, deer, horse, and mammoth.d.Sand and shingle of modern beach.
Fig. 265.
There have been some movements of land on a smaller scale since the Eocene period in the south-east of England. One of the latest of these happened in the Pleistocene, or even perhaps as late as the Post-Pliocene period. The formation called by Dr. Mantell the ElephantBed, at the foot of the chalk cliffs at Brighton, is not merely a talus of calcareous rubble collected at the base of an inland cliff, but exhibits every appearance of having been spread out in successive horizontal layers by water in motion.
The deposit alluded to skirts the shores between Brighton and Rottingdean, and another mass apparently of the same age occurs at Dover. The phenomena appear to me to suggest the following conclusions:—First, the south-eastern part of England had acquired its actual configuration when the ancient chalk cliff Aawas formed, the beach of sand and shinglebhaving then been thrown up at the base of the cliff. Afterwards the whole coast, or at least that part of it where the elephant bed now extends, subsided to the depth of 50 or 60 feet; and during the period of submergence successive layers of white calcareous rubblecwere accumulated, so as to cover the ancient beachb. Subsequently, the coast was again raised, so that the ancient shore was elevated to a level somewhat higher than its original position.[257-A]
Subdivisions of the Oolitic or Jurassic group — Physical geography of the Oolite in England and France — Upper Oolite — Portland stone and fossils — Lithographic stone of Solenhofen — Middle Oolite, coral rag — Zoophytes — Nerinæan limestone — Diceras limestone — Oxford clay, Ammonites and Belemnites — Lower Oolite, Crinoideans — Great Oolite and Bradford clay — Stonesfield slate — Fossil mammalia, placental and marsupial — Resemblance to an Australian fauna — Doctrine of progressive development — Collyweston slates — Yorkshire Oolitic coal-field — Brora coal — Inferior Oolite and fossils.
Subdivisions of the Oolitic or Jurassic group — Physical geography of the Oolite in England and France — Upper Oolite — Portland stone and fossils — Lithographic stone of Solenhofen — Middle Oolite, coral rag — Zoophytes — Nerinæan limestone — Diceras limestone — Oxford clay, Ammonites and Belemnites — Lower Oolite, Crinoideans — Great Oolite and Bradford clay — Stonesfield slate — Fossil mammalia, placental and marsupial — Resemblance to an Australian fauna — Doctrine of progressive development — Collyweston slates — Yorkshire Oolitic coal-field — Brora coal — Inferior Oolite and fossils.
Oolitic or Jurassic Group.—Below the freshwater group called the Wealden, or, where this is wanting, immediately beneath the Cretaceous formation, a great series of marine strata, commonly called "the Oolite," occurs in England and many other parts of Europe. This group has been so named, because, in the countries where it was first examined, the limestones belonging to it had an oolitic structure (seep. 12.). These rocks occupy in England a zone which is nearly 30 miles in average breadth, and extends across the island, from Yorkshire in the north-east, to Dorsetshire in the south-west. Their mineral characters are not uniform throughout thisregion; but the following are the names of the principal subdivisions observed in the central and south-eastern parts of England:—
The Upper oolitic system of the above table has usually the Kimmeridge clay for its base; the Middle oolitic system, the Oxford clay. The Lower system reposes on the Lias, an argillo-calcareous formation, which some include in the Lower Oolite, but which will be treated of separately in the next chapter. Many of these subdivisions are distinguished by peculiar organic remains; and though varying in thickness, may be traced in certain directions for great distances, especially if we compare the part of England to which the above-mentioned type refers with the north-east of France, and the Jura mountains adjoining. In that country, distant above 400 geographical miles, the analogy to the English type, notwithstanding the thinness, or occasional absence of the clays, is more perfect than in Yorkshire or Normandy.
Physical geography.—The alternation, on a grand scale, of distinct formations of clay and limestone, has caused the oolitic and liassic series to give rise to some marked features in the physical outline of parts of England and France. Wide valleys can usually be traced throughout the long bounds of country where the argillaceous strata crop out; and between these valleys the limestones are observed, composing ranges of hills, or more elevated grounds. These ranges terminate abruptly on the side on which the several clays rise up from beneath the calcareous strata.
Fig. 266.
Fig. 266.
The annexed diagram will give the reader an idea of the configuration of the surface now alluded to, such as may be seen in passing from London to Cheltenham, or in other parallel lines, from east to west, in the southern part of England. It has been necessary, however, in this drawing, greatly to exaggerate the inclination of the beds, and the height of the several formations, as compared to their horizontal extent. It will be remarked, that the lines of cliff, or escarpment, face towards the west in the great calcareous eminences formed by the Chalk and the Upper, Middle, and Lower Oolites; and at the base of which we have respectively the Gault, Kimmeridge clay, Oxford clay, and Lias. This last forms, generally, a broad valeat the foot of the escarpment of inferior oolite, but where it acquires considerable thickness, and contains solid beds of marlstone, it occupies the lower part of the escarpment.
The external outline of the country which the geologist observes in travelling eastward from Paris to Metz is precisely analogous, and is caused by a similar succession of rocks intervening between the tertiary strata and the Lias; with this difference, however, that the escarpments of Chalk, Upper, Middle, and Lower Oolites, face towards the east instead of the west.
The Chalk crops out from beneath the tertiary sands and clays of the Paris basin, near Epernay, and the Gault from beneath the Chalk and Upper Greensand at Clermont-en-Argonne; and passing from this place by Verdun and Etain to Metz, we find two limestone ranges, with intervening vales of clay, precisely resembling those of southern and central England, until we reach the great plain of Lias at the base of the Inferior Oolite at Metz.
It is evident, therefore, that the denuding causes have acted similarly over an area several hundred miles in diameter, sweeping away the softer clays more extensively than the limestones, and undermining these last so as to cause them to form steep cliffs wherever the harder calcareous rock was based upon a more yielding and destructible clay. This denudation probably occurred while the land was slowly rising out of the sea.[259-A]
The Portland stone has already been mentioned as forming in Dorsetshire the foundation on which the freshwater limestone of the Lower Purbeck reposes (seep. 232.). It supplies the well-known building stone of which St. Paul's and so many of the principal edifices of London are constructed. This upper member, characterized by peculiar marine fossils, rests on a dense bed of sand, called the Portland sand, below which is the Kimmeridge clay. In England these Upper Oolite formations are almost wholly confined to the southern counties. Corals are rare in them, although one species is found plentifully at Tisbury, in Wiltshire, in the Portland sand converted into flint and chert, the original calcareous matter being replaced by silex (fig. 267.).
Fig. 267.Columnaria oblonga, Blainv.As seen on a polished slab of chert from the sand of the Upper Oolite, Tisbury.
Fig. 267.
Columnaria oblonga, Blainv.
As seen on a polished slab of chert from the sand of the Upper Oolite, Tisbury.
Among the characteristic fossils of the Upper Oolite, may be mentioned theOstrea deltoidea(fig. 269.), found in the Kimmeridge clay throughout England and the north of France, and also in Scotland, near Brora. TheGryphæa virgula(fig. 268.), also met with in the same clay near Oxford, is so abundant in the Upper Oolite of parts of France as to have caused the deposit to be termed "marnes à gryphées virgules." NearClermont, in Argonne, a few leagues from St. Menehould, where these indurated marls crop out from beneath the gault, I have seen them, on decomposing, leave the surface of every ploughed field literally strewed over with this fossil oyster.