A Manual of Elementary Geology Part 38

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 ma.s.ses 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. (See fig. 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 a.s.sumed, 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 ma.s.ses of white chalk had been softened and removed by water.

This superficial acc.u.mulation 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 ma.s.s 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 to fig.

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 (see fig. 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.).

[Ill.u.s.tration: 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.]

_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 partic.i.p.ated 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 deposits _a_ may 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 acc.u.mulation of the tertiary strata _a_.

[Ill.u.s.tration: Fig. 264. Cross section.]

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 sh.e.l.ls 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 ma.s.ses 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 sh.o.r.e.

They also indicate the destruction of pre-existing chalk with flints. At the same time fossil sh.e.l.ls of the genera _Melania_, _Cyclas_, and _Unio_, 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 const.i.tuted islands scattered over the s.p.a.ce now occupied by the tertiary basins of the Seine and Thames. The stage of denudation represented in fig. 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, and clayey 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 acc.u.mulate 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 s.p.a.ce 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 s.p.a.ce, 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 even of the last of a series of such acts of removal, but it must be, in the nature of things, almost always impossible to a.s.sign 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 s.p.a.ces 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 sh.o.r.es 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 s.p.a.ce intervening between the North and South Downs.

[Ill.u.s.tration: Fig. 265. Cross section.

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 sh.e.l.ls 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.]

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 Elephant Bed, 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 sh.o.r.es between Brighton and Rottingdean, and another ma.s.s 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 A _a_ was formed, the beach of sand and shingle _b_ having 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 rubble _c_ were acc.u.mulated, so as to cover the ancient beach _b_. Subsequently, the coast was again raised, so that the ancient sh.o.r.e was elevated to a level somewhat higher than its original position.[257-A]

FOOTNOTES:

[241-A] An account of these cliffs was read by the author to the British a.s.soc. at Glasgow, Sept. 1840.

[241-B] Seine-Inferieure, p. 142. and pl. 6. fig. 1.

[243-A] Botley Hill, near G.o.dstone, in Surrey, was found by trigonometrical measurement to be 880 feet above the level of the sea; and Wrotham Hill, near Maidstone, which appears to be next in height of the North Downs, 795 feet.

[243-B] My friend Dr. Mantell has kindly drawn up this scale at my request.

[244-A] Fitton, Geol. of Hastings, p. 55.

[244-B] Conybeare, Outlines of Geol., p. 81.

[245-A] Ibid., p. 145.

[245-B] Geol. of Western Suss.e.x, p. 61.

[247-A] See ill.u.s.trations of this theory by Dr. Fitton, Geol.

Sketch of Hastings.

[248-A] Sir E. Murchison, Geol. Sketch of Suss.e.x, &c., Geol. Trans., Second Series, vol. ii. p. 98.

[248-B] See fig. 94. p. 76.

[251-A] Geol. Soc. Proceed. No. 74. p. 363. 1841, and G. S. Trans.

2 Ser. v. 7.

[251-B] For farther information, see Mantell's Geol. of S. E.

of England, p. 352.

[252-A] Soulevemens Jura.s.siques. Paris, 1832.

[253-A] See above, p. 82.

[257-A] See Mantell's Geol. of S. E. of England, p. 32. After re-examining the elephant bed in 1834, I was no longer in doubt of its having been a regular subaqueous deposit. In 1828, Dr. Mantell discovered in the shingle below the chalk-rubble the jawbone of a whale 12 feet long, which must have belonged to an individual from 60 to 70 feet in length, Medals of Creation, p. 825.

CHAPTER XX.

OOLITE AND LIAS.

Subdivisions of the Oolitic or Jura.s.sic 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--Nerinaean 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 JURa.s.sIC 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,"