The Ground beneath us, its geological changes and phases. ... Three lectures

Front matter

■ m 4 RpmBtcSh æan THE GROUND BENEATH US, ITS GEOLOGICAL PHASES AND CHANGES : BEING THREE LECTURES ox THE GEOLOGY OF CLAPHAM, AND THE NEIGHBOURHOOD OF LONDON GENERALLY, DELIVERED TO THE MEMBERS OF THE CLAPHAM ATHENÆUM, CLAPHAM COMMON ON 1st MAY, 1854, and 7th and 21st APRIL, 1856. BY JOSEPH PRESTWICH, F.R.S., F.G.S., etc. LONDON: JOHN VAN VOORST, 1 PATERNOSTER ROW. MDCCCXVII. [Price Three Shillings and Sixpence.] THE GROUND BENEATH US, ITS GEOLOGICAL PHASES AND CHANGES : BEING THREE LECTURES ON THE GEOLOGY OF CLAPHAM, AND the NEIGHBOURHOOD OF LONDON GENERALLY, delivered to the MEMBERS OF THE CLAPHAM ATHENÆUM, CLAPHAM COMMON, ON 1st MAY, 1854, and 7th and 21st APRIL, 1856, HY JOSEPH PRESTWICH, F.R.S., F.G.S., etc LONDON: JOHN VAN VOORST, PATERNOSTER ROW. MDCCCLVII. PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET.

Preface

. The following Lectures, given at the instance of some of my friends at Clapham, were not written with a view to publication, as I contemplated the possibility of undertaking at some future period a more special work on the subject. Having, however, been favoured by a request to print them, accompanied by a liberal offer to join me in the expenses, on the part of the Council of the Clapham Athenæum, I have revised them for publication in their original form, with the addition of a few explanatory notes. They do not by any means aim at a complete account of the geology of the district ; for, considering that the subject was probably new to many of the younger Members present, I endeavoured to treat it in a popular manner, and to avoid as much as possible all technical language and local details.

I wished, nevertheless, to point out, however slightly, all those leading geological phænomena which are within the reach of all who care to observe, — to show how to proceed with an inquiry into the causes which produced them by a careful study of effects, — and to give some insight into the chief objects of geological inquiry. At the same time the subject is one of more than local interest, for Clapham stands upon ground which forms part of the London Tertiary district, and consequently the ac count here given of the Drift and of the Tertiary strata is equally applicable to the ground beneath London and to some distance around ; and no separate work having yet been published of the Geology of London and its neighbourhood, and this account, though general, touching upon most of the leading points, a

PREFACE.

. IV besides including a short discussion of some views not yet brought forward elsewhere, especially those respecting the Drift, I trust that this sketch may also be of some use as a guide to those even who have made Geology more particularly their study, and that it may supply for the present the want of a more special treatise. For those who in the mean time wish for fuller information, I would refer to the various papers and notices by Parkinson1, Conybeare and Phillips2, Dr. Buckland3, Dr. Mitchell4, Wetherell5, Morris6, Edwards7, Prof. Owen8, Sir Charles Lyell9, De la Condamine10, Bowerbank11, Mylne12, myself13 and others. The illustrations of these Lectures, although necessarily of the most simple description, are, as far as pos sible, characteristic. Some are copied from other works, and some are drawn from original specimens, any additions to the specimen being marked in dotted lines.

I am much indebted to the artist and geologist, Mr. Samuel Mackie, for so well exhibit ing in mere outlines the essential characters of the fossils. 1 Transactions ofthe Geological Society, vol. i. p. 324. 2 The Geology of England and Wales. 3 Trans. Geol. Soc. vol. iv. p. 277 ; and 2nd ser. vol. ii. p. 119. 4 Proceedings ofthe Geological Society, vol. i. p. 481 ; ii. p. 7. 6 Rid. vol. i. pp. 403, 417 ; ii. p. 93 ; iii. p. 140. Trans. Geol. Soc. 2nd ser. vol. v. p. 133. Philosophical Magazine, 3rd ser. vol. i. p. 233 ; vol. ix. p. 462; vol. xv. p. 540. History and Antiquities of Highgate, p. 133. 6 Ibid. vol. ii. p. 450. Ibid. 3rd ser. vol. xi. p. 104. Magazine of Natural History, 1st ser. vol. viii. p. 356 ; vol. ix. p. 261 ; 2nd ser. vol. ii. p. 539. Quarterly Journal of the Geological Society, vol. vi. p. 201.

(This last work, which contains most of the later papers on the Geology of the London district, is published and sold by Messrs. Longman and Co.) 7 Publications of the Palæontographical Society. 8 Ibid. See also notes to text. 9 Manual of Elementary Geology, 5th Edit., chap. x. xiii. andxvi. 10 Journ. Geol. Soc. vol. vi. p. 440. " The Fossil Fruits of Sheppey. 12 Sections of the Strata beneath London. Geological Map of London. I' Journ. Geol. Soc. vol. iii. pp. 354, 378 ; vol. vi. p. 252 ; vol. viii. p. 235 ; vol. x. pp. 75, 401 ; vol. xi. p. 206 ; and vol. xiii. p. 89. The Water-bearing Strata of London, §§ 2 and 4. 10 Kent Terrace, Regent's Park Road, Oct. 1857. THE GROUND BENEATH US, ITS GEOLOGICAL PHASES AND CHANGES. FIRST LECTURE. POST-PLIOCENE PERIOD.

Introduction

. ON THE ORIGIN OF THE GRAVEL, AND ON THE CONFIGURATION OF THE SURFACE (b, PI. II.). In these Lectures it wili be my object briefly to describe the nature and character of those geological changes ofwhich the immediate vicinity of London, and Clapham in particular, has been the scene*. I will therefore limit my observations to those phænomena only of which the records are preserved or indicated in the strata beneath and near the spot where we are now assembled ; and, while abstaining from all technical descriptions that would be unintelligible without preliminary studies, I will yet endeavour, so far as time will allow, to give you not only the principal results arrived at by geologists, but also a general statement of the mode by which the several problems have been or are being worked outs. * The section in PI.

II. of the ground beneath Clapham corresponds in all its leading features with many sections that might be taken in the central and western parts of London. f Possibly some of these questions may already have been brought before you by a late distinguished geologist, for several years a resident here. I allude to Dr. Mantell, whose felicity of expression and poetical feeling were as remarkable as his zeal was great. He took especial interest in palæon tological1 researches. This inquiry having to a greater extent reference to physical questions, I hope to avoid going over ground with which you have already been made acquainted. Some notice of the geology of this district, by Dr. Mantell, will be found in his ' Sketch of the Geology of Surrey,' inserted in the first volume of Brayley's ' History of Surrey,' 1850. 1 Relating to old extinct animals.

Surface of the Land. We are naturally so much impressed by the beauty of existing nature, by the apparent permanence of the orders of animals and of plants now living, and by their complete possession of, and adaptation to, the present surface of the earth, that we can hardly conceive that beneath an exterior so fully and admirably adorned, there exist the wrecks and ruins of former surfaces of the earth, once probably as rich, teeming, and diversified as the one on which we move. The present terrestrial surface is, in fact, but a film over the solid frame of our planet ; and the com position of the various plants and animals which flourish on it is such, that, let life cease, its whole organic structure would crumble down, and leave scarcely a trace behind. How difficult then would it be after the lapse of a few years to picture the beauty and variety of the present scene !

And so it is now with respect to the past ; entombed in the rocks and ground beneath us are the remains of former surfaces of this planet, but of the then existing plants and animals comparatively few traces are left, — their living forms have passed away, and all that remains of them are a few pieces of the harder skeleton, or a few leaf-impressions and fragments of wood, preserved by chance and protected from decay in the sediment deposited in the beds of former seas and lakes, — sediments now in part hardened into stone, and forming the solid rocks of our hills and valleys. It is in the restoration of these old forms of plants and animals, in the just conception of the part they filled at a former period of the earth's history, and in the study of the changes which led to the successive appearance and disappearance of the many distinct species, that the labours of the geologist in a great measure consist.

He has, from a few detached facts, to fill up a living picture ; so to identify himself with the past, as to describe, and follow, as though an eye-witness, the different changes which have at various pe riods so greatly modified the surface of the earth. I must premise that rocks, in which term we include not only the hard stony strata*, but also the soft strata of clay and sand, are divided into two classes, viz. those of igneous origin, such as Volcanic Rocks and Granite, and those of sedimentary origin, as the Portland Stone, the Chalk, and the London Clay. This latter class consists of those which have been accumulated * Strata, — the parallel layers of a rock. Varieties of Strata. 7 in the ancient seas and lakes of the earth, and they are either unconsolidated as originally deposited, or else are hardened into stone by chemical agency or by heat*.

In these sedimentary rocks the strata are either fineor coarse-grained, according to the strength of the tides or currents, which swept down from the ancient lands the materials of which they are composed. Consequently some strata have been formed of fine silt or clay, others of sand more or less coarse, and others again, in cases in which the moving power of the sea or rivers has been greater, of rolled fragments of rocks. These latter form con glomerate strata, that is to say, they are full of pebbles or of fragments of rocks more or less worn, and which can gene rally be recognized by their mineral character or their fossils as having been derived from other pre-existing rocks, sometimes adjacent and sometimes at a distance from the spot. There is another class of deposits formed likewise of detritalf materials, but irregularly scattered and loose on the surface, and not stra tified : they constitute superficial deposits of gravel and brick earth, and belong to what geologists formerly termed Diluvium, but which they now more frequently denominate " Drift."

I must also ask you to discard from your minds any feeling of indifference or slight that your familiarity with the materials we deal with might engender. I can understand the difficulty which those who are not geologists may experience, in dis sociating the homeliness and commoness of the substances amongst which we have to work, from the abstract beauty and grandeur of the inquiry itself — in not allowing the rudeness of the tablet to call off attention from the value of the inscriptions. Although dealing with rough rocks and stones, using familiar names, and, in the absence of more perfect specimens, noticing even the smallest fragments that show de fined form and structure J, the true object of our research does not lie in the collecting of a mineral or of a fossil, or in the determination of a certain arrangement of rock masses, what ever interest may attach to these cognate branches separately — these are merely the means, and not the end ; — but, commencing * Either volcanic or central heat. t Broken up.

I Carefully avoiding all mere general resemblances, which are most falla cious. We require identity of some part or other, however small. Object of Research. 8 with the present world as our starting-point, we have to trace back, through bygone ages, the wonderful physical vicissitudes to which this earth has been subject, — to determine with care and reverence the remarkable changes in organic life which have successively taken place on its surface — comparing each creation with that existing at the present day, — to discover its relations therewith, in what respect and to what extent it differs, and the position it occupies in the chain of created things, — a chain of which only a portion of the links remain with us now, and of which the others belong to periods long past by. To do this successfully and properly, we must not overlook either little things or common things, but take all as they offer themselves, — overlook nothing, — give to each its due weight and place, and read off, as best we can, the lesson they impart; for after all our care, and with the closest attention, the reconstruction is but fragmentary, — we cannot follow the history ofthe earth un interruptedly, or always correctly.

Still there are now a certain number of well-established positions, and although geologists may differ as to the mode in which certain phænomena have arisen, we all agree in respect to the main and essential results. With reference to the ground immediately beneath us, I will commence with the surface, and, proceeding downwards*, see what evidence we have of former conditions and of other and older lands having here preceded the present one. The vegetable soil, although now constituting so conspicuous a feature, and performing so important a part, is, geologically speaking, of little moment, as it contains no fossils, and has been formed principally by causes at present in action, such as the disintegration of the surface of the underlying rocks, and the decomposition of vegetable matter. It rarely exceeds a few feet in thickness, and often is not more than a few inches thick.

Immediately under the soil we find in this neighbourhood a mass or bed of ochreous-coloured gravel, varying in thickness from about 3 to 12 feetf. It spreads over the greater part of this Common, a large portion of London stands upon it, and it * On any other occasion I should prefer beginning with the older, and proceeding upwards to the newer strata. t It is not marked in the map : the regular strata only are there given. Cause of Springs in the Gravel. 9 stretches east and west for many miles along the valley of the Thames, reposing through the greater extent of this area, upon a mass of dark-coloured tenacious clay (A, PI. II.). This gravel being permeable by water, a portion of the rain which falls on its surface passes into it, but, on reaching the clay beneath, is stopped by and lodges on the surface of that clay, forming an under-ground sheet of water in the lower part of the gravel.

Consequently when wells are dug in this gravel, on arriving within a few feet of the clay, the stored-up water is everywhere met with, and filters cool and clear into the bottom of these wells. In the same way the water so held in the body of the gravel flows out in natural springs on the slopes of those hills where the gravel is cut off by valleys penetrating into the clay beneath (s, fig. 17, p. 30). It is these gravel-beds which form the great source of water-supply to all the shallow wells and land springs of London and its immediate neighbour hood, — including the spring-well on this Common, the spring on Rush Hill, and all the noted old pumps of the City and Westminster. Let us now study the gravel more closely, look to its past history, and trace its origin by carefully analysing its character and questioning its every feature.

I must, however, first men tion, that the classification of the gravels around London is yet a geological inquiry under discussion. I believe that these beds will have to be referred to more ages than one, for I find that they occur on several different levels, and that organic remains are not dispersed equally through all of them. For instance, at Clapham I am not aware that any have hitherto been found, whereas they have been met with in many parts of London, and at Kew, Brentford, Clapton, Hackney* and else where, often in considerable abundance. Although this is not the place to discuss this special, and at present debateable question, I may give you some of the more general conclusions at which we have arrived. At the same time our present purpose will be answered, and the subject simplified, if we consider the gravels of Clapham and of the neighbourhood of London generally as belonging to one nearly contemporaneous epoch, and * See Quart.

Journ. Geol. Soc. vol. vi. p. 201, and vol. xi. p. 107, for details of their occurrence at some of these localities. Composition of the Gravel. treat of it as a whole. The theoretical questions we have to investigate are, — 1st, Whence did the gravel originally come?, 2nd, by what means was it brought into its present situation ?, and 3rd, at what geological period was it spread over the surfaces where we now find it? With regard to the first question, we must minutely examine the gravel, see what are its component parts, and determine whence they were derived. You will find that it consists of angular stones, with their edges slightly worn (fig. 1), and of pebbles perfectly rounded (fig. 2), imbedded in a coarse sili- Form of Materials from the Gravel of Clapham Common. Fig. 1. Fig. 2. Subangular Flint.

Flint Pebble. ceous sand * mixed with a small proportion of clay, and the whole coloured by the oxides of irons. It may be asked " May not all the parts of this gravel have been formed where found by ordinary direct deposition in a sea or by segregation ?" This cannot be the case, because the mass is neither homo geneous nor simply concretionary, — the pebbles are different from the sand, the sand from the clay, and the colouring matter does not enter into the substance of the pebbles or sand, but merely coats their surface}; each thing is indepen dent, and cannot have been separated out of one common mass. Another reason is, that the stones and pebbles forming the gravel do not contain any remains of the animals which lived at the time the gravel was formed, but, on the contrary, they * As the sand on the shore at Dover or Brighton. f Rust is an oxide of iron.

X As you may see when gravel has been long exposed in walks or else where to the atmosphere how it loses its colour by the washing away of the colouring matter. Origin of the Flints in the Gravel. 11 contain remains of animals of an entirely different and older period ; they must, therefore, have been derived from some older and pre-existing rock or rocks. We further find that, although the component parts of the gravel differ in external appearance, yet that by much the greater portion of it is composed of the same material, and that material is the common black flint. The broken specimens show internally the same texture, although externally the wear and staining to which they have been sub jected have often much changed their appearance. In the in terior of some of these flints, and upon the outer surface of others, you will frequently find casts and impressions of well marked and peculiar fossils * ; and as certain fossils characterize certain rocks, there will be no difficulty, if we can recognize the species of these fossils, to determine the rock-formation from which these flints are derived.

Amongst the fossils so found are the remains of sponges, also of various sea-urchins (fig. 3), spines (fig. 4) and shells (figs. 5, 6). Casts of Fossils in Flint from the Gravel of Clapham and London, Fig. 3. Fig. 5. Fig. 6. Fig. 4. 3. Micraster cor-anguinum. 5. Terebratula striatula ? 4. Spine of a C'idaris. 6. Inoceramus Lamarckii. Now all these are fossils common in the Chalk, and belong to species which do not occur in any formation newer than the * Dr. Mantell figures four specimens of chalk fossils in flint from the gravel ofthe Common. — Op. cit. Other Rock Pebbles in the Gravel. 12 chalk, nor in any older than the Cretaceous group (m-n, PI I.). Therefore both the mineral structure (although this alone would not suffice, as varieties of flints are found in other formations than the chalk), and the organic remains, go to show that the great bulk of the pebbles and stones of the gravel of this district are derived from the chalk.

But the gravel reposes here upon a thick bed of clay of a very different age, and no chalk is found near. Consequently it must be from some distant spot that the materials for the gravel have been derived, and you will see in the geological map (PI. I.) that the nearest place where any extent of chalk is exposed is in the range of hills passing by Croydon, Carshalton, Ewell, and Epsom, a distance of six to ten miles southward from Clapham. Yet although flints form by far the larger proportion of the gravel, there are a few other substances showing a different origin. First there are some subangular flattish fragments of a hard, light-coloured, porous stone, and of a brown horny looking flint. Now we have no rocks or minerals like these, either in the tertiary strata around London, or in the chalk ; but beyond the chalk-hills, and rising up from beneath them, are a series of strata known as the Upper and Lower Greensands (Sect. 1, PI.

I.), some of the lower beds of which latter may be well seen in the cutting where the Brighton and Dover Railways diverge at Redhill. This formation, which extends thence to Dorking and Farnham, contains some beds of rock (Ragstone) and of a sort of flint called Chert, and it is worn fragments of these that we here recognize in the gravel. This evidence is thus corroborative, and shows that the gravel of Clapham and London consists essentially of the debris of rocks, transported northward probably from the hills of Surrey and Sussex, and over distances of not less than from six to twentv miles. There is, however, a certain amount of conflicting evidence for we also find a few large pebbles of white quartz, others of a hard, compact, reddish sandstone, of some porphyritic and slate rocks, unlike anything either in the Tertiary or Chalk formations or in the Lower Greensand*.

These pebbles are formed of the Silurian and slate rocks of Wales and the border counties • * Quartz and other pebbles are found in the Lower Greensand, but they are generally very small and rare. Direction from which the Gravel has come. but although derived originally from those formations, they may be more directly referred to the conglomerates of the New Red Sandstone of Worcestershire and Warwickshire, in which these older pebbles were first imbedded*. But even these strata are not found within a distance of eighty to a hundred miles from London. This indicates a still more remarkable extent of trans port than we have hitherto noticed, and one from a different direction. I merely direct your attention to this curious point, which yet remains an open and difficult geological question. Whatever may have been the cause of this exceptional phæno menon, the great and preponderating mass of flint debris from the chalk hills, and of sandstone and chert from the greensand hills of Surrey, leaves no reasonable doubt that the main bulk of the gravel of Clapham and of London has been derived and transported from the Surrey downs and Sussex hills.

Another difficulty may here suggest itself, — What, it may be inquired, has become of the chalk from which all these flints have been derived ? for to form a bed of flint gravel 10 feet thick would probably require a mass of chalk 200 or 300 feet thick, the flints occurring only in thin and distant layers, as you may see by inspecting the cliffs near Brighton and at Dover. The explanation of this is, that flints which consist nearly of pure silica (such as rock-crystal), are insoluble in ordinary surface waters, but as these waters are always impregnated with a certain proportion of carbonic acid gasf, they derive thereby the property of being able to dissolve a small portion of carbonate of lime or chalk. This is so prevalent an action, that, as car bonate of lime is one of the most widely-diffused substances in nature, it is also an ingredient found in almost all waters, whether of spring or river.

Thus, when clear, the water from the Thames, that from the spring on the Common, and distilled water, are to the eye exactly alike, and look equally pure and limpid ; but if we add to them a small quantity of oxalate of ammonia}, the Thames water becomes milky and gives a very * Like the flint-pebbles (fig. 2, p. 10) of the gravel which are referred to the Lower Tertiaries, although derived originally from the Chalk : see p. 16. t This gas is everywhere present in the atmosphere in the proportion of about JL part in 1000, and is absorbed by rain, spring, and river waters. + Formed of oxalic acid and ammonia. When a solution of this salt is Mode of Transport. 14 visible precipitate, the spring water deposits a still stronger white precipitate, whilst the distilled water remains perfectly clear and without any precipitate.

This property of water may, therefore, in the course of time, or else under some unusual con ditions, have led to the dissolving away of a large body of chalk, leaving untouched the harder and insoluble flints. We now come to the second question, — as to the means by which the gravel was formed, or brought into its present position at and around London. We have shown that it is not formed from any rocks on the spot, but that it is derived almost entirely from strata situated at a distance of from six to twenty miles. Therefore it has been transported that distance by some means or other. It is evident that no such operation is at present going on, or has gone on during the historical period, and con sequently that the cause, whatever it was, belonged to some past order of things. Geologists agree that such a transport could only have been effected whilst the surface, now covered with gravel, was under water ; but, although of one opinion on this starting-point, they differ much as to the mode of operation. 1st. Was it by the sudden and transient action of a body of water, passing rapidly over the surface and sweeping down with irresistible force the debris of the hills of Surrey upon the plain of the valley of the Thames ? 2ndly.

Or was it by the action of a large river, flowing through Sussex and Surrey and then into the Thames Valley, and bringing down debris or frag ments of the rocks over which it passed? 3rdly. Could the ordinary action of sea-currents have scattered the gravel over the surface? 4thly. Or, finally, was the destruction of the Chalk and the strewing of the flints over the surface of the adjacent London Clay the result of ice action and transport by coast-ice and icebergs*? All these views have their advocates. added to these waters, the ammonia unites with the carbonic acid of the lime, forming carbonate of ammonia, which is soluble and remains dissolved in the water, whilst the oxalic acid unites with the lime, forming an insoluble oxalate of lime, which falls down as a white sediment. This is a common test for hard calcareous waters. * The reasons for believing this great cold to have prevailed here will be given presently (see p. 19).

Extent to which the Gravel is worn. 15 I will endeavour to put you in possession of the leading facts of the case, so as to enable you to judge of the relative probabilities of the different hypotheses. To arrive at a correct conclusion we must well examine the physical evidence afforded by the gravel itself. In the first place we must look to see to what amount of wear the stones forming it have been subjected. The bulk of the gravel you will find to consist of pieces of flint stained brown (figs. 2, 4, 5, 6) ; these, as before shown, are derived from the chalk ; but in the chalk they occur in large masses, black inside and white outside ; in the gravel, on the contrary, we find them broken and more or less discoloured. You will observe also that they have all lost the sharpness of a newly-fractured flint, and that the edges are worn ; still these fragments are worn to a certain moderate extent only, — none of them present any approach to the form of a rounded pebble, like those you find formed by the action of a sea on a beach, such as that at Brighton and Dover.

But besides these subangular fragments of flint, there are found with them black flint-pebbles (fig. 1), which, unlike the former, are most perfectly rounded — not one angle remaining : their form is so regular, their surface so smooth, that it could only have been produced by a very long and constant action of a sea on a shore. Now, it is not possible, that the action which left the greater number of the flints in their rough subangular form, could have formed these round pebbles, or that the action which produced the latter, could have left the other flints so slightly worn*. Whence it is evident, that the two sets of flints could not have received their present form from one and the same action, for if these rounded pebbles had received their form at the gravel period, the whole mass of the gravel must necessarily have been subjected to the same or nearly the same amount * It may be urged, that on the present shore of a chalk district there are flints in all stages of wear, those just fallen from the cliff being sharp and angular, and those long on the shore being worn down into pebbles.