Transactions of the Geological Society, 1st series, vol. 4/On the Plastic Clay Formation
member of the geological society,
and professor of mineralogy in the university of oxford
[Read 6th January, 1816.]
Description of Beds of the Plastic Clay formation above the Chalk at Reading.
Extent of this formation of Plastic Clay.
Appearance of the same near London.
Sections at Woolwich, Lewisham, Blackheath, Plumsted, and New Cross.
Other localities of this formation in the London Basin.
Appearance of the same on the coast of Sussex.
Sections near Seaford and Newhaven.
Plastic Clay near Arundel.
Connection of beds of the Plastic Clay formation in England with the French beds of the same era.
General character of the Plastic Clay formation in England.
History of the formation of the Marsh Lands immediately below London.
Having an opportunity to visit Reading in July, 1814, I collected a series of specimens numbered as below, which I beg leave to present to the Geological Society. The pits whence they were obtained are at the Catsgrove Hill Brick kilns, distant about half a mile from the town of Reading, on the south-west, where the works have been carried on for more than a century, and at this time present the following section, beginning from the lowest upwards.
|No.||Thickness in Feet.|
|1.||Chalk containing the usual extraneous fossils and black flints|
|2.||Siliceous sand mixed with granular particles of green earth, and containing both rolled and angular chalk-flints, oysters, and many small and nearly cylindrical teeth of fish from a line to an inch in length||3|
|3.||Quartzose sand of a yellowish colour with a few small green particles, and containing no pebbles or organic remains||5|
|5.||White sand used for bricks||4|
|6.||Lowest brick clay of a light grey colour mixed with line sand, and a little iron-shot||5|
|7.||Dark red clay, mottled with blue and occasionally a little iron-shot. It is used for tiles||6|
|8.||Bed called the White vein. A line ash coloured sand mixed with a small portion of clay, and in some parts passing into loose white sand. It is used for bricks||5|
|9.||Fine micaceous sand laminated and partially mixed with clay, and occasionally iron-shot. It is used to make tiles||4|
|10.||Light ash coloured clay, mixed with very fine sand of the same colour. It is used for bricks||7
|11.||Dark red clay partially mottled and mixed with grey clay||4|
|12.||Soft loam, composed in its upper region of line yellow micaceous sand, mixed with Hakes of a delicate ash coloured clay, which become more abundant in the deeper portions of the stratum, and having its lower regions much iron-shot, and occasionally charged with ochreous concretions, and decomposing nodules of iron pyrites. It is used to make soft bricks for arches||11|
|13.||Alluvium composed of clay, sand, and gravel, the gravel chiefly consisting of chalk flints, both rolled and angular, with a few pebbles of quartz, and of brown compact sandstone. This alluvium is covered by vegetable mould|
The oysters of No. 2 are remarkably perfect when first laid open, and seem to have undergone no process of mineralization; they soon fall to pieces by exposure to air and moisture. The chalk flints contained in it are many of them in the state of small rounded pebbles; in others the angles are unbroken. Both varieties are covered with a crust of greenish earth of the same nature with the green particles in the sand. The angular flints appear to have been derived from the partial destruction of the bed of chalk immediately subjacent, of which the upper surface in contact with the sand is considerably decomposed to the depth of about a foot, and its fissures and numerous small tubular cavities (the latter derived apparently from the decay of organic substances,) are filled with granular particles of the green earth and siliceous sand of the incumbent stratum.
An appearance somewhat analogous is noted by M. M. Cuvier and Brongniart (Essai sur la Geog. Min. des Environs de Paris, p. 17,) in a coarse variety of the French plastic clay which immediately covers the chalk at Meudon; where a breccia composed of fragments of chalk imbedded in a kind of argillaceous paste has filled the fissures and irregularities which existed on the surface of the subjacent chalk before the deposition of the plastic clay.
The same thing may be seen on a small scale in the chalk pit at Woolwich, where there are fissures extending some feet downwards into the body of the chalk, varying in breadth from an inch to more than a foot, and sometimes spreading laterally so as to form considerable cavities, which together with the fissures are filled with sand that has been introduced from the incumbent stratum.
At Reading the chalk is quarried below the green sand containing oysters (No. 2) to the depth of about 25 feet, when the workings are stopped by water at a point nearly on a line with the level of the river Kennet, below which there can be no discharge of water from the chalk, through the medium of the neighbouring springs. In this thickness of 25 feet of chalk, there is but one regular and continuous course of flints, and in this they are disposed in tabular masses, for the most part of about two inches in thickness. (This bed is but a few feet above the water). In the chalk that lies above this siliceous stratum, the flints are disposed irregularly with their usual characters and eccentric forms, derived, in many instances, from the organic remains which they envelope. They are collected for the use of the porcelain manufactories. The chalk itself is extracted largely from under the sands and clays, by means of shafts and levels, to be burnt into lime. There are no septaria or concretions in any of the strata above the chalk, nor the smallest traces of animal or vegetable remains, excepting in the green sand, (No. 2.) The same barrenness of organic remains is noticed in the purest beds of the French plastic clay, and by Mr. Webster (Geol. Trans. vol. 2, p. 200), in the plastic clay of the Isle of Wight and Corfe Castle.
The section given by Dr. Brewer, in the Phil. Trans. for 1700, differs as little as might be expected from that which is now exposed at Reading. Beginning from the bottom, he gives the following strata:
|2.||Green sand containing oyster shells||2|
|3.||A bluish sort of clay, very hard, brittle, and rugged (called pinney clay); it is of no use||3|
|5.||Clear line white sand||7|
|6.||Stiff red clay used for tiles, the depth of which, he says, could not conveniently be taken from the heighth of the hill, at the top of which, he adds (immediately under 2 feet of common earth) the red clay appears, and is used for tiles.|
The thickness of the beds which Dr. Brewer did not measure was probably made up of those from No. 7 to No. 12 inclusive, in the section I have given, in which also the beds below No. 6 correspond very nearly with his account, which I had not seen till my own was finished as it now stands.
In a hill called David's Hill, west of the town of Reading, on the opposite side of the Kennet to that of the Catsgrove brick kilns, and about one quarter of a mile distant from them, are other large quarries of brick earth, in which many of the subdivisions which have been noted at Catsgrove are not to be recognised, and the entire thickness of some of the pits is made up of the same sands and clays as on the opposite side, but more uniformly disseminated through the whole mass, forming a kind of loam more like No. 12 than any of the other beds that have been there described; ochreous concretions and pyritical nodules abound in it as in No. 12. The total thickness of this deposition at David's Hill above the chalk is about 40 feet. Water occurs in the subjacent chalk, as soon as they sink 30 feet into it. It is separated from the incumbent brick earth by the bed of green sand, with the same oysters as at Catsgrove.
The whole of these beds above the chalk at Reading (those at Catsgrove as well as at David's Hill) appear to be subordinate parts of one formation, the next in order of succession above the chalk, older than the London clay and calcaire grossier of Paris, and contemporaneous with the lowest strata of the plastic clay formation nearest the chalk, the general history of which we propose more fully to consider.
On the north side of the town of Reading these strata do not occur, being cut off by the great valley through which the Thames passes, and which has been excavated to a considerable depth in the subjacent chalk. But they occupy much of the ground between Reading and Newbury, and are seen at Hermitage, on the N.E. of Newbury towards Hamstead Norris, whence a range of low hills composed of them stretches eastwards towards Reading, and westward to Boxford, Wickham, and the neighbourhood of Hungerford, interrupted by vallies, which are often cut down into the subjacent chalk.
The breadth of this deposition on the north and south of Newbury, is from Beedon Hill six miles north on the road to Market Ilsley, to Whitway near Highclere four miles south of Newbury on the road to Whitchurch. On the west of this line, drawn through Newbury north and south, the breadth of the beds on the chalk is gradually contracted till they entirely cease near Hungerford (See Mr. Webster's map, Geol. Trans. vol. 2, pl. 10). On the east side of the said line they occupy the vale of the Kennet till it falls into that of the Thames, near Reading; whence they extend eastward, widening as they advance through Surry and Middlesex, into Kent, Essex, Suffolk, and Norfolk.
In many parts of this great valley or trough of chalk we recognize our Reading beds in their proper place, as the inferior strata of the plastic clay formation; and though with the exception of the lowest bed they do not agree in minute detail with those of the Reading section, as to thickness, or exact order of superposition, nor is the presence of shells or pebbles constant in the beds of clay or sand, yet an attentive examination of the general points of resemblance in the substance of the clays, sands, and pebbles, forming these irregular alterations above the chalk, added to the identity of their organic remains when any occur, leaves no doubt as to their being members of one great series, of nearly contemporaneous depositions, intermediate between the chalk and London clay, and which it may be convenient to associate in a natural family, as members of that formation of which it is one leading feature to contain those peculiar varieties of clay which the French naturalists have characterized by the appellation of plastic clay. As there can be little doubt of the origin of the French beds being contemporaneous with those in England we are now considering, it will assist us in connecting our strata with those of the Continent, to designate them by the appellation appropriated to them in France.
Near London these beds occur with well defined characters; at Blackheath, Lewisham, Charlton, Woolwich, and on the east of Plumsted. In all these places the thin bed next above the chalk, which at Reading contains fishes teeth and oysters, is seen composed of a similar substance of loose green sand mixed with chalk flints, both rolled and angular, and generally coated with a dark green crust; but here they contain no organic remains, and seldom exceed two feet in thickness. Above this thin bed is a thick stratum of fine grained ash coloured sand, destitute of shells or pebbles, and varying in thickness generally from 30 to 40 feet. This stratum is seen to the greatest advantage in the Woolwich sand pits, where is an enormous artificial section, presenting the following order of succession:
(See coloured Section, Pl. 13, No. 1).
|1.||Chalk with beds and nodules of black flint||—|
|2.||Green sand of the Reading oyster bed, containing green coated chalk flints but no organic remains||1|
|3.||Light ash coloured sand without shells or pebbles||35|
|4.||Greenish sand with flint pebbles||1|
|5.||Greenish sand without shells or pebbles||8|
|6.||Iron-shot coarse sand, without shells or pebbles, and containing ochreous concretions disposed in concentric laminæ||9
|7.||Blue and brown clay—striped and full of shells, chiefly cerithia and cythereæ||9|
|8.||Clay striped with brown and red, and containing a few shells of the above species||6|
|9.||Rolled flints mixed with a little sand, occasionally containing shells like those near Bromley; e. g. ostrea, cerithium and cytherea. (These shells occur disseminated in irregular patches)||12|
No. 1 and 2 are not laid open in the great sand pits, but are seen in a chalk pit adjoining to the eastern extremity of the sand pit. The following section at Loam Pit Hill, near Lewisham, about three miles south-west of Woolwich, presents analogies that identify many strata in the two sections, as from the chalk upwards to No. 8; in each inclusively the principal difference consists in the presence of fewer or more pebbles, in beds of sand evidently contemporaneous.
(See coloured Section, P1. 13, No. 2 )
|1.||Chalk with beds and nodules of flint||──|
|2.||Green sand identical with the Reading oyster bed, and in every respect resembling No. 2 at Woolwich||1
|8.||Ash coloured sand, slightly micaceous, without pebbles or shells||35|
|This bed, though below the general Boor of the middle pit, is sunk into from it by deep shafts.|
|4.||Coarse green sand, containing pebbles||5|
|5.||Thick bed of ferruginous sand, containing flint pebbles||12|
|6.||Loam and sand, in its upper part cream coloured, and containing nodules of friable marl, in its lower part sandy and iron-shot||4|
|7.||Three thin beds of clay, of which the upper and lower contain cytherex, and the middle oysters align="right" valign="bottom"||3|
|8.||Brownish clay containing cytheræ||─|
|This is the lowest bed sunk into in the upper pit, and is not there penetrated to a depth exceeding one foot. The interval between this and No. 7, which occupies the summit of the middle pit is not exactly ascertained, but cannot be considerable; probably||6|
|9.||Lead coloured clay, containing impressions of leaves||2|
|11.||Striped loam and plastic clay, containing a few pyritical casts of shells, and some thin seams of coaly matter||10
|12.||Striped sand, yellow, fine, and iron-shot||10|
At a point still higher on this hill than No. 12, is a thick bed of dark blue clay, without shells, which is used to make tiles and bricks, and which appears to continue upwards trom this brick kiln to the summit of the hill, forming a thick cap over the sands and clays mentioned in the section, and is probably an outlying hummock of the London clay, separated only by a small valley from the extensive mass of that stratum which is found two miles south-west in the Sydenham hills, and being placed between and connecting them with the London clay of Shooter's hill.
A similar section to that at Woolwich and Loam Pit Hill may be traced round the sloping terrace that bounds the north-west and south sides of the plain of Blackheath.
On the east side the beds composing this plain appear to be covered by the clay of Shooter's Hill, an outlying summit of the London clay, like the hills of Sydenham and Highgate, and which probably at one time were all united in a continuous stratum covering the entire series of the plastic clay formation, which is now exposed between the intervals of its remaining fragments. (See map and section, Pl. 13.)
The plain of Blackheath (being a portion of the strata thus laid open,) is covered at the surface with a bed of rounded pebbles, sometimes 20 feet in thickness, which appear to be alluvial, but are of nearly the same substance with the gravel of the neighbouring strata, from which it is therefore matter of great difficulty to distinguish them. Beneath these pebbles is a bed of sand identical with No. 12 at Loam Pit Hill. This sand lies on a bed of plastic clay which supports the water of the well in Mr. Conybeare's garden, and of all the wells on the plain of Blackheath at no great depth; it possesses the same peculiar dark red colour, with the plastic clay of Reading, Corfe Castle, and Paris, and has been used for pottery. Beneath this clay the Woolwich shell beds and subjacent thick ash coloured sand are to be seen in several parts of the sloping terrace that surrounds the Blackheath plain. Under these on the north and west sides appears the chalk, separated from the ash coloured sand by the same thin pebble bed as at Reading. This pebble bed not attaining the thickness of one foot may be seen at the junction displayed by the descent to some ancient subterraneous quarries in chalk, called the caves, on the north side of the road ascending to Blackheath from Deptford; it may be seen also on the south side of the same road in some chalk pits on the slope of the hill: in both places it is covered by the thick ash coloured sand.
In the lane that leads down from the village of Charlton to the Thames, is a good section shewing the Woolwich shell beds incumbent on the ash coloured sand which appears there in great thickness.
The church of Charlton is on the edge of the continuation of the plain of Blackheath, which extends thence eastward to Plumsted Common and Boston Heath. (See map, Pl. 13.)
On the inner edge of this platform at the Plumsted Common brick kilns, which are at the base of the north-east extremity of Shooter's Hill, a large section exposes the London clay, abounding in selenites and septaria. This clay is dug for brick tiles and coarse pottery. In the same field with the clay pits and on the north side of them a shaft is sunk 120 feet to the surface of the subjacent chalk, which has been extracted to the further depth of 24 feet, being the object for which the shaft is made. The upper portion of this shaft is in alluvial gravel, between which and the chalk occur the Woolwich sands. Another shaft was begun in the same fields still nearer to the base of Shooter's Hill, but abandoned from the quantity of water that came in when they were at a depth of which the plastic clay should be found if continued to this point from Woolwich in the same relative position which it there occupies. The same thing happened in an adjoining field, where the shaft for chalk was stopped by the water at the depth of 36 feet.
In a ravine at the east end of Plumsted Common that falls towards the Thames, the plastic clay that upholds the water of these wells and shafts, is laid open on each side of the hollow way, and throws out a line of springs at its junction with an incumbent stratum that is identical with the bed covering the plastic clay at Blackheath and Woolwich. On the east of this ravine in a deeper hollow called the King's Highway we recognise the sand and gravel beds below this plastic clay corresponding with Nos. 4,5,6, of the Woolwich Pit, and 4, 5, 6, of the Loam Pit Hill section; beneath these is the ash coloured sand No. 3 of Woolwich. The King's Highway descends into a still deeper valley (through which runs the road leading from Plumsted to Wickham); this valley is cut to a considerable depth in the chalk. The north-east side of it is steep, and has at its base a large chalk pit, the top of which displays the Reading oyster bed one foot thick between the chalk and incumbent ash coloured Woolwich sand.
The plain at the summit of this bank is Boston Heath, where a well has recently been sunk about 200 feet; through gravel 65 feet, sandy beds 65, chalk 70. The water stands five feet deep in the chalk. I could get no accurate detail of the sinkings, but learnt that in descending they came to water far above the chalk though not in quantity sufficient to supply the well. The upper gravel in this well, and in the shafts at Plumsted, appears to be alluvial, though like that at Blackheath composed almost wholly of pebbles of rolled chalk flints, such as the neighbouring strata of the plastic clay formation contain abundantly, and from which they were probably derived.
The thickness of the alluvium in this district is exceedingly irregular, swelling suddenly, and as suddenly disappearing. It covers, however nearly the whole surface of the under table land extending from Blackheath to Plumsted Common and Boston Heath, and is found also on the upper table of the summit of Shooter's Hill, as well as on many parts of the slope of its sides. The slopes that fall from the under table to the valley of the Thames are so frequently and so completely covered by this alluvium that except in places where they are laid open by artificial sections, it is difficult to discover the existence of any strata of the plastic clay formation. A striking example of this fact may be seen in the Park at Greenwich, where, nearly all traces of the subjacent beds are concealed by a mass of alluvium along the steep slope where we might expect to see them exposed, and where there can be no doubt of their existence from the strength and regularity in which they appear at Charlton and Woolwich on the east, and near Deptford on the west of Greenwich Park along the continuation of the same escarpment.
In the banks of the Croydon canal at New Cross near Deptford is another section that confirms the place that has been assigned to the Woolwich beds below the London clay, and connected with the plastic. The section does not penetrate so deep as the thick ash coloured sand of Woolwich; but in the canal bank above the bridge we have the following beds laid open, though not sufficiently to ascertain their exact thickness, it does not however vary much from that of the upper beds in the Woolwich pits.
Section at the Canal in New Crors, beginning from the lowest bed.
- Plastic clay abundantly charged with the same shells as in the Woolwich pits.
- Bed of small pebbles chiefly of rolled chalk flints.
- Sandy loam and plastic clay.
- Blue clay full of small selenites, probably the London clay.
The blue clay, No. 4, probably owes its selenites to the decomposition of its shells and iron pyrites; at present no shells are visible near the surface. Its juxta position to the London clay of the Sydenham Hills, of which it seems to be the continuation at their north-east extremity, goes far to identify it with that formation. The plastic clay, No. 3, is used for bricks and coarse pottery in a field adjoining this canal called Counter Hill, close to the New Cross on the east; and the Woolwich shell beds may be seen again at a lock of the canal about a mile above New Cross towards Croydon, in the plain that lies under the east side of the Sydenham Hills. At this lock Mr. Warburton pointed out to me the following shells. Ancilla buccinoides, cerithium denticulatum, cyclas deperdita, a small buccinum, and a small nerite.
It is mentioned by Woodward that the Woolwich shells are found at Camberwell and Beckenham, on the north-west and south-east sides of the Sydenham Hills. I have other authority for their occurrence at the following places on the south side of the Thames, Camberwell, Redriffe Tunnel, New Cross, Lewisham, Blackheath, Woolwich, Plumsted, Beckenham, Bromley, Chislehurst, Bexley, Cockleshell Bank, two miles south of South Fleet, Windmill Hill near Gravesend, and Higham on the Thames and Medway canal. They are found also at Rungewell Hill near Epsom, and at Headley between Epsom and Dorking.
These localities seem sufficient to warrant us in concluding that the formation of plastic clay extends over a large space in the south portion of the valley of the Thames from Reading to Gravesend.
Woodward mentions oysters as being found on the north side of the Thames in a stratum of sand that covers the chalk near Hertford; this probably is one of the oyster beds of the plastic clay formation.
I remember that in 1806, fire bricks were burnt from some beds of fine sand and clay in the Park at Bulstrode, by the late Duke of Portland, and that moulds for refining sugar were (and are still) made within a mile of it, at some clay pits on the north side of the London road about two miles east of Beaconsfield. These with other extensive beds of the same era which occur between Bulstrode and Windsor, are in almost immediate contact above the chalk, and appear to belong to the formation of plastic clay. The Windsor fire bricks and soft sandy bricks for arches, are probably also made from beds of this same formation. Mr. Warburton has been told that at Clewer near Windsor, the Thames cuts through a bed of shells which he suspects to be the same as are found at Woolwich.
We will now leave the beds of the plastic clay formation in the London basin, to trace them in the same relative position on the coast of Sussex.
A similar deposition of sand to that of Reading containing a breccia of chalk flints as its lowest stratum, (about three feet thick) was noticed by the Honourable H. G. Bennet and myself in July, 1814, between Newhaven and Beachy Head, in the cliff at Chimting Castle half a mile on the east side of Seaford. The sand here is fawn coloured passing into olive with flakes of mica almost a line in diameter, and occasionally contains irregular veins and masses of tubular concretions of iron-stone. Its greatest thickness is under 50 feet. Mr. Warburton informs me that he has seen similar concretions in the same stratum of sand at Sudbury in Suffolk, in immediate contact above the chalk. Under this sand at Chimting the breccia of the lowest bed forms an ochreous pudding stone composed of sand and chalk flints, (the latter both rolled and angular) the whole being strongly united by a ferruginous cement, and the flints covered externally with a green coating like those in the oyster bed at Reading. Specimens of this breccia have been presented to the Society by the Hon. H. G. Bennet. At Chimting Castle there is but a small insulated portion of these strata immediately incumbent on the chalk. This chalk rises suddenly to a lofty cliff on the east side of the flat ground that lies between Newhaven and Seaford, dividing the beds of the plastic clay formation at Newhaven from their outlying fragment at Chimting, with which they probably were connected before the excavation of the valley of the Ouse.
The upper surface of the chalk at Chimting, as seen in the cliffs, dips at an angle of about 20° to the west. The dip of the incumbent beds of breccia and sand is conformable with it. These last beds are soon lost in ascending the hill eastward from the Castle; first the sand ceases, and afterwards the breccia having formed a thin cap on the chalk for a short distance disappears a little below the Signal House about one mile east of Seaford. Hence the chalk extends forming a cliff to Cuckmere Haven, where on the heights composed of it on the west of the Cuckmere river, we sought in vain for the stratified sand and breccia, finding nothing but an alluvial cap of sand and gravel; and as far as the eye could judge, looking eastward from this point, there was no appearance of superior beds on any summits of the chalk which forms the entire substance of those magnificent cliffs that extend from Cuckmere Haven to Beachy Head.
At Newhaven, in the lowest part of the Castle Hill close to the mouth of the Ouse on the west side, we again found the breccia that has been described at Chimting Castle, nearly of the same thickness and in the same state and relative position between the upper surface of the chalk and the incumbent beds of sand; it differs from it only in being less firmly cemented, and appears equally identical with the oyster bed at Reading. The greater number of its flints are not much rolled.
The state of the tide, and their elevated position, prevented us from examining the hollows on the surface of the chalk in which Mr. Webster discovered the pure alumine; we found however a large loose block containing this substance which filled cavities and veins in a mass of singular structure, composed of irregularly concentric thin layers of gypsum alternating with still thinner laminæ of ochreous iron ore. There were also small crystals of selenite in the same cavities with the alumine. The whole mass had strong marks of stalactitic origin, and was probably introduced into one of the cavities on the surface of the chalk by infiltration from the incumbent beds of marl, which abound in shells and iron pyrites, and contain all the elements from which the alumine, iron, and gypsum might be derived.
In this cliff of the Castle Hill at Newhaven the following section is presented, shewing beds of the plastic clay formation above the chalk.
|1.||Chalk, containing alumine in hollows on its surface||50|
|2.||Brescia of green sand and chalk flints, the latter covered with a ferruginous crust||1|
|3.||Sand, varying from yellow to green and ash colour||20|
|4.||Series of clay beds containing coaly matter, selenites and fibrous gypsum, also leaves of plants, and sulphur coloured clay||20|
|5.||Foliated blue clay containing cerithia, and cyclades, and a few oysters||10|
|In this clay is a seam of iron pyrites about an inch thick with pyritical casts of cyclades and cerithia.
|6.||Consolidated argillaceous rock full of oysters, with a few cyclades and cerithia||5|
|7.||Alluvium full of broken chalk flints mixt with sand||10|
On comparing this section with those we have given in the London Basin we shall find a correspondence very striking. (See coloured sections, Pl. 13, No. 1 & 2.) On the same chalk No. 1. is the Reading Oyster bed, No. 2. which though inconsiderable in thickness seems constantly to form the next stratum above the chalk, though organic remains have been noticed in it only at Reading. No. 3, at Newhaven, is the ash coloured sand of Woolwich in diminished thickness. Nos. 4, 5, and 6, appear to be an enlarged condition of the plastic clay bed No. 7 and 8 of Woolwich, and from 7 to 11 inclusive at Loam Pit Hill.
We again observed localities of the red variety of plastic clay in a small valley at the village of Binstead, three miles west of Arundel, and again on the declivity of the hill by which the Binstead and Chichester road descends into Arundel.
These insulated portions of strata of the plastic clay formation that have been noticed at Seaford and Newhaven, and other places at the south base of the chalk hills of the South Downs of Sussex, appear to be outlying fragments at the eastern extremity of the great series of depositions above the chalk in the south of England, which Mr. Webster describes as extending from near Dorchester by the Trough of Pool and the New Forest to Portsmouth, Chichester, and the flat coast on the south-east of Arundel. (See Mr. Webster's Map, vol. ii. Geo. Trans. Pl. 10.*) Here they enter the English Channel, and just touching the coast with their outlying fragments at Newhaven and Chimting Castle, appear again on the opposite shores of France in the same relative position.
I am indebted to the kindness of M. Brongniart for the following section near Dieppe, which forms a valuable link connecting the formations above the chalk in France with those of the same era in the south of England. He has observed the following strata in the perpendicular cliff under the light-house of St. Margaret, on the west of Dieppe, counting upwards from the lowest stratum:
- Sand and sandstone in thick beds containing concretions of the same substance.
- Strata of plastic clay, impure and containing lignite much charged with iron pyrites, also oysters and cerithia, both in beds and irregularly disseminated.
These strata M. Brongniart considers as identical with beds of the plastic clay formation in many other parts of France, particularly at Marly, and in the Soissonnois, where the same organic remains occupy strata similarly disposed and identical with those near Dieppe.
I shall add a few more circumstances of resemblance in the French and English formations of plastic clay.
It is noted by M. M. Cuvier and Brongniart, that in the basin of Paris the sand between the chalk and plastic clay, though very pure, is often coloured red or bluish grey. In the latter state it occurs at Woolwich, Lewisham, and Newhaven. We have already (p. 280.) stated the analogy which the Reading oyster bed bears to the brecciated bed next above the chalk at Meudon. Of the plastic clay it is also stated by the same authority that it often consists of two beds separated from each other by a stratum of sand. The lowest of these two being properly the pure plastic clay, while the upper is coarse, sandy, and blackish. The sand dividing them is also said to vary in colour like the plastic clay itself, from white, grey, and yellow, to grey mixed with red and pure red. The English beds of plastic clay and the sands attending them at Reading, Corfe Castle, and Alum Bay, exhibit analogous variations in colour and consistency; indeed at the latter place they run through almost every possible combination in the scale of colours. Between the upper plastic clay and calcaire grossier of Paris, there is also stated to be sometimes found a bed of sand of irregular thickness, which they are doubtful whether to consider as belonging to the formation of plastic clay or calcaire grossier, but are rather inclined to attribute it to the former. It contains organic remains in very few places. Is it not improbable that this bed is contemporaneous with some of the upper strata of the plastic clay formation which we find at Loam Pit Hill, at Blackheath, and in the Isle of Wight, at which last place the beds belonging to this series are accumulated to a thickness far greater than has been yet noticed in any other spot, amounting at Alum Bay, according to Mr. Webster, to 1131 feet, interposed between the chalk and London clay.
Viewing it on the great scale then we may consider this formation, which has been characterized by the title of plastic clay, as composed of an indefinite number of sand, clay, and pebble beds, irregularly alternating. Of these, the sand forms in England, the most extensive deposition, in which the clay and pebbles are interposed subordinately and at irregular intervals.
Again, the occurrence of organic remains in the different beds of this formation, is like the alternation of the strata composing it, exceedingly irregular: sometimes they occupy the clay, at other times the sand or pebbles, and very frequently are wanting in them all.
A good example of shells occurring, mixed with large pebbles, may be seen at Sundridge Park, near Bromley, in Kent, where we find an immense deposit of shells, peculiar to the plastic clay formation, accumulated confusedly in a bed of loose sand and pebbles. Of these shells some are broken and others entire, and delicately preserved. They are also sometimes fixed together by a calcareous cement (derived apparently from the substance of the shells themselves) forming a hard breccia with the siliceous pebbles and sand in which they are imbedded. A similar breccia was sunk into in the workings of the Redriffe tunnel.
I have from this bed at Bromley a specimen, in which five oyster shells are so affixed to the opposite sides of a large kidney-shaped pebble, that they seem to have commenced their first growth on it, and to have been attached to it through life, without injury by friction from the neighbouring pebbles. We cannot but infer then that these pebbles received their form during a long period of agitation, which was succeeded by a period of repose; in which latter they were in a state of sufficient tranquillity for the shells in question to live and die undisturbed in the midst of them.
The enormous quantity of these completely rolled and rounded chalk flint pebbles  that occur in the English plastic clay formation on the south of London, corroborate the arguments adduced by M. M. Cuvier and Brongniart, from the irregular projections and furrowed surface of the French chalk, and from the fragments of chalk forming a breccia with the plastic clay at Meudon, to prove the consolidation of the chalk to have been completed before that partial destruction of its upper strata by the force of water, to which they justly attribute these furrows and the Meudon breccia. These English beds of chalk flint pebbles (the wreck of strata thus destroyed) afford additional evidence of the immense scale on which this aqueous destruction was carried on, and confirm also the conjecture (which by them is chiefly grounded on the total difference of the organic remains in the two formations) that a long period of time has probably intervened between the deposition of the chalk and the plastic clay.
It may not be uninteresting to insert here the following notices, illustrative of the formation of the marsh lands immediately below London, which, though not directly connected with our subject, the plastic clay, yet forms a prominent feature in the physical history of that part of the neighbourhood of the metropolis which we have been describing.
It is well known that at this time the waters of the Thames from London to the sea are upheld by dykes or sea walls. Within these dykes the river by its daily sediment of mud has so raised its bed, that even in ordinary tides the water is above the level of the meadows, as far up as Woolwich and the Isle of Dogs. This elevation of its bed is precisely analogous to what has happened near the mouths of the Po, the Rhine, and other large rivers, which have been upheld for many centuries by embankments. The following facts tend to illustrate the process that was going on before the period at which these embankments were made.
In the account given by Capt. J. Perry, about 100 years ago, of the stopping of the breach made in the sea wall at Dagenham, about twelve miles below London, that able engineer particularly describes what he calls moor log. This, he says, was composed of vegetable matter heaped together, but chiefly of brushwood, among which there appeared to be a considerable quantity of hazel trees; hazel nuts were also found in the mass, but were easily crushed, the kernel being entirely perished. There were also trunks of other trees, of which the yews were the least decayed; some of them measured 15 or 16 inches in diameter. There were also willows two feet and upwards in diameter; they retained a whitish colour like touch wood, and were softer than the adjacent earth or moor log. The moor log appeared at about three or four feet under the marsh ground, and differed in thickness at different parts; at Deptford it was six feet thick; at Woolwich Reach, opposite the ballast wharf, it was between seven and eight feet thick; its thickness as well as its breadth gradually increasing down the river. Beneath the moor log was a stratum of blue clay, and under this gravel and sand. Stags horns were likewise found in different places, a little above the vein of moor log.
Mr. Derham's account of the Dagenham marsh land (Phil. Trans. 1710, p. 478), affords the following particulars in addition to those given by Capt. Perry.
The stumps and roots of many trees were found in the same posture in which they grew, situated in a soil consisting of a black oozy earth, full of the roots of reeds; the tops of these stumps were so worn that it could not be ascertained whether the bodies had been cut off by the ax, or broken by natural violence. The bodies themselves lay horizontally on the surface of the oozy earth, in confusion, but a northerly direction seemed most prevalent. They appeared almost all of them to have been alder, though at first they were supposed to be yew. Over the trees lay a covering of grey mould, of the same nature with the sediment of the Thames at this day, varying in thickness from seven to twelve feet. Mr. Derham mentions the names of the following places in which he noticed traces of this subterranean forest. Dagenham, Havering, Rainham, Wennington, Purfleet, West Thorrock.
It happened a few years ago that in cutting the canal and basins in the Isle of Dogs, a subterranean forest containing hazel nuts, with hazel and other trees, was discovered to lie under the bed of indurated mud, that forms the surface of that peninsula.
These data throw much light on the natural operations that were going on, between the period of the last retreat of the diluvian waters, and that at which sea walls began to be erected against the rising waters of the Thames. The substratum of clay and gravel mentioned by Capt. Perry, formed the first surface of the valley uncovered by mud or water, and lying at a small elevation above the then existing high water level of the river. The sediments of the river gradually raised its bed, and caused its waters to spread laterally over the adjacent low lands; first converting to marsh by inundations at high tides, and at length completely burying, by its daily sediment of mud, those tracts which in the early periods of the rise of the bed of Thames, had been quite dry and covered with extensive forests. The horns of stags that inhabited them lie on the surface of the moor log, which appears to be the wreck of these ancient forests, first converted to swamps as the water began to reach their level, at length wholly destroyed by the constant inundation of the ground on which they grew, and still affording evidence of their position and extent, in the roots and trunks that lie buried on the surface and in the mass of the moor log, and over which a bed of mud has subsequently been deposited by those gradually rising waters which caused the destruction of the forest.
- The measures in this and all the following sections were taken by the eye and do not pretend to extreme accuracy.
- Although a Section of these pits is already before the Society, it seemed necessary to insert that I am now giving, as it differs from Mr. Webster's (Geol. Trans. vol. 2, p. 195.) in a few minute particulars, and was the result of a careful examination by the Rev. Wm. Conybeare and myself, in April, 1815. I have also the sanction of Mr. Conybeare's authority, and am indebted to his observations, confirmed subsequently by my own, for the Notices, Map, and Sections, which I have given of the neighbourhood of Blackheath.
- These beds cannot all be observed at one section, but may be traced along the sloping surface of the hill, at three successive apertures near each other, in which the upper stratum of each lower pit is dug into, and forms the floor of the one next above it.
In the section No. 2, the intermediate spaces are unnaturally contracted, and expressed by two narrow caps of alluvium.
- In this alluvium four large and entire tusks of elephants were discovered a few years ago, in a garden opposite the chalk pit, at the base of Loam Pit Hill, and on the north side of the turnpike road; they soon perished by exposure to the air, but were for some time in the possession of Mr. Lee, the owner of the extensive brick works on Loam Pit Hill, to whom I am indebted for this information.
- Mr.Webster mentions (Geol. Trans. v. 2, p. 235) that rounded flints are found in the sand strata, at the bottom of the blue or London clay, in several parts of the London basin. And again (p. 185), that the abundant supply of water which is constantly found in boring through the same clay, indicates an extensive deposition of three beds of sand. The sandy strata containing pebbles, and the watery sand thus alluded to appear to be the continuation of the upper strata of the plastic clay formation, and connected with those of Loam Pit Hill.
In the shaft at the northern extremity of the tunnel under the Thames, near Rotherhithe, these same beds were found, covered by more than 30 feet of London clay, although from their rapid rise under the bed of the Thames towards the south, the shaft on the south side (of which Mr. Webster has given a section, p. 197) exhibits only nine feet of this clay incumbent on the watery gravel and subjacent beds of the plastic clay formation.
A curious section is preserved in Sir C. Wren's Parentalla (p. 285), obtained in preparing the foundations of the present cathedral church of St. Paul, In London.
The Surveyor observed that the foundations of the old church stood upon a layer of very clan hard pot earth, which he therefore judged firm enough to support the new building; and on digging wells in several places he found this pot earth to be about six feet thick and more, on the north side of the church yard, but thinner and thinner towards the south, till it was scarce four feet upon the declivity of the hull. Below this he found noting but dry sand, mixed sometimes unequally, but loose, so that it would run through the fingers. He went on till he came to water and sand mixed with perriwincles and other sea shells; these were about the level of low water mark. He continued boring till he came to natural hard clay.
The upper stratum of pot earth had been used at a Roman pottery, near the N.E. angle of the present church, where they found urns, sacrificing vessels, and other pottery in great abundance, and were interrupted in digging the foundation of the N.E. angle of the church, by the quarry from which the pot earth had been extracted: the subjacent sand and gravel beds being considered too loose to support the weight of the intended building, it was thought necessary to secure this part of the foundation by erecting it upon an arch, The outer or N.E. pier of this arch stands in the old clay pit, in a shaft sunk to receive it more than 40 feet below the stratum of pot earth that had been removed, and descending through the beds of sand and gravel above mentioned, to the subjacent stratum of hard clay.
- It is probable that the plastic clay contains at Blackheath as at Corfe Castle, Alum Bay and Loam Pit Hill, the remains of vegetable matter in a state approaching to coal; and that this circumstance has given origin to the erroneous opinion so prevalent, that there is good coal at Blackheath if Government would allow it to be worked.
The very high improbability of finding good coal above the chalk is acknowledged by all who have even the smallest acquaintance with the geological relations of the English coal mines. The presence of black vegetable matter in a state approaching charcoal in almost all our secondary argillaceous strata, has caused endless vain attempts to search for useful coal in formations where the discovery of that substance would be contrary to all experience in this country. No good coal has I believe been yet found in England in any stratum more recent than the new red sandstone, or red rock marl. That of the Cleveland Moors in Yorkshire, being above lias and in the oolitc formation, is of so bad a quality as scarcely to form an exception to this position.
- In Chislehurst, at the north-west angle of the park at Camden Place, the section of a chalk pit displays a great thickness of the ash coloured Woolwich sand, separated from the chalk by the thin pebble bed as at Reading.
The thick Woolwich sand (No. 3,) occurs also at Bexley, where (as is the case in many of the woods about Dartford) shafts 40 or 50 feet in depth have been sunk through it at an early period for the purpose of extracting the subjacent chalk, as is now done at Reading, and Plumsted brick kiln. Mr. Heated, in his History of Kent, conjectures that many of these quarries were excavated by the Saxons, as places of retreat in times of danger. He states that some of them are 20 fathoms in depth, and that they are to be found also near Feversham, and at Fritwood on the south of Marston Passage near Milton. The explanation that is suggested by the geological position of all these places appears to be much more satisfactory.
- It may be observed of these pebbles occurring in the plastic clay formation, that they are never calcareous, but composed almost entirely of oval or roundish and rather flat chalk flints, completely rolled down and slightly altered, sometimes to the centre, by decomposition; which beginning from without has produced, in some cases, a number of concentric zones, disposed in agate like rings, nearly parallel to the outer surface of the pebble, and resembling an agate in colour though inferior in purity. The fact that in these pebbles we occasionally find fragments of organic remains peculiar to the chalk formation, shews that they were not formed like agates in empty cavities. And the do composition of their iron commencing from the outer surface, is fully adequate to produce the concentric structure which they present; as may be seen in similar concentric zones resulting from the same cause in pebbles of sandstone, and many other rocks, of which the substance is compact and tolerably uniform in texture.
More frequently the pebbles are clouded with tints of red and yellow, presenting an indefinite variety of beautiful modifications, and assuming the irregular arrangement of the colours in an Egyptian pebble. The finest varieties of these colours are displayed to the best advantage in polished specimens of the Hertfordshire pudding-stone, so common in cabinets and ornamental jewellery. The pebbles of this pudding-stone appear to be no other than altered chalk flints of the same era with those found at Blackheath, and differing only in the accident of their being firmly united by a strong siliceous cement. Many of the purest varieties of the Blackheath pebbles if polished, are exactly similar to those of the Hertfordshire pudding-stone.
Large blocks of a coarse variety of the same siliceous pudding-stone are not uncommon on the surface of the chalk in the south of England. I have seen them at Bradenham, near High Wycombe, at Nettlebed, at Portesham, near Abbotsbury, and in Devonshire, lying insulated on the bare chalk. They have not yet I believe been found imbedded in their native stratum, which seems to have been destroyed extensively above the English chalk, and to have been a member of that series of irregular alternations of beds of clay, sand, and gravel, either separate or mixed together, which for reasons already stated, has been designated by the appellation of the plastic clay formation.