The Geological Story of the Isle of Wight Part 3

The deep sea of the White Chalk did not come suddenly. In the oncoming of the period we find much marl--limy clay. As the sea deepened, little reached the bottom but the sh.e.l.ls of foraminifera and other marine organisms. How deep the sea became is uncertain: there is reason to believe that it did not reach a depth such as that of the Atlantic.

It is difficult to draw the line between the Upper Greensand and the Chalk strata. Above the Chert beds is a band a few feet thick known as the Chloritic Marl, which shows a pa.s.sage from sand to calcareous matter. It is named from the abundance of grains of green colouring matter, now recognised as glauconite; so that it would be better called Glauconitic Marl. It is also remarkable for the phosphatic nodules, and for the numerous casts of Ammonites, Turrilites, and other fossils mostly phosphatized, which it contains. This band is one of the richest strata in the Island for fossils. It differs, however, in different localities both in thickness and composition. It is best seen above the Undercliff, and in fallen ma.s.ses along the sh.o.r.e from Ventnor to Niton. It is finely exposed on the top of Gore Cliff, where the flat ledges are covered with fossil Ammonites, Turrilites, Pleurotomaria, and other sh.e.l.ls. The Ammonite (_Schloenbachia varians_) is especially common. The sponge (_Stauronema carteri_) is characteristic of the Glauconitic Marl. As the edge of the cliff is a vertical wall, none should try this locality but those who can be trusted to take proper care on the top of a precipice. When a high wind is blowing the position may be especially dangerous.

[Ill.u.s.tration: PL. III]

(Pecten) Neithea Quinquecostata

Thetironia (Ammonite) Rhynchonella Minor Mantelliceras Mantelli Parvirostris

(Sea Urchins) Micraster Cor-Anguinum Echinocorys Scutatus (Internal cast in flint)

LOWER AND UPPER GREENSAND AND CHALK

The Chloritic Marl is followed by the Chalk Marl, of much greater thickness. This consists of alternations of chalk with bands of Marl, and contains glauconite and also phosphatic nodules in the lower part.

Upwards it merges into the Grey Chalk, a more ma.s.sive rock, coloured grey from admixture of clayey matter. These form the Lower Chalk, the first of the three divisions into which the Chalk is usually divided.

Above this come the Middle and Upper, which together form the White Chalk. They are much purer white than the lower division, which is creamy or grey in colour. The Chalk Marl and Grey Chalk are well seen at the Culver Cliff, and run out in ledges on the sh.o.r.e. The lower part of this division is the most fossiliferous, and contains various species of Ammonities, Turrilites, Nautilus, and other Cephalopoda.

(Of Ammonites _Schloenbachia varians_ is characteristic. Also may be named _S. Coupei_, _Mantelliceras mantelli_, _Metacanthoplites rotomagensis_, _Calycoceras naviculare_, the small Ammonoid Scaphites aequalis; and of Pectens, _aequipecten beaveri_ and _Syncyclonema orbicularis_ may be mentioned). White meandering lines of the sponge _Plocoscyphia labrosa_ are conspicuous in the lower beds. The Chalk Marl is well shown at Gore Cliff, sloping upwards from the flat ledges of the Chloritic Marl. It may be studied well, and fossils found, in the cliff on the Ventnor side of Bonchurch Cove,--which has all slipped down from a higher level.

The uppermost strata of the Lower Chalk are known as the Belemnite Marls. They are dark marly bands, in which a Belemnite, _Actinocamax plenus_, is found. The hard bands known as Melbourn Rock and Chalk Rock, which on the mainland mark the top of the Lower and Middle Chalk respectively, are neither of them well marked in the Isle of Wight. In the Middle Chalk _Inoceramus l.a.b.i.atus_, a large bivalve sh.e.l.l, occurs in great profusion; and in the Upper flinty Chalk are sheets of another species, _I. Cuvieri_. It is hardly ever found perfect, the sh.e.l.ls being of a fibrous structure, with the fibres at right angles to the surface, and so very fragile.

There is a striking difference between the Middle and Upper Chalk, which all will observe. It consists in the numerous bands of dark flints which run through the Upper Chalk parallel to the strata. The Lower Chalk is entirely, and the Middle Chalk nearly, devoid of flint.

Though the line at which the commencement of the Upper Chalk is taken is rather below the first flint band of the Upper Chalk, and a few flints occur in the highest beds of the Middle Chalk; yet, speaking generally, the great distinction between the Middle and Upper Chalk, the two divisions of the White Chalk, may be considered to be that of flintless chalk and chalk with flints.

Early in our studies we noticed the great curves into which the upheaved strata have been thrown, and that on the northern side of the anticline the strata are in places vertical. This can be well observed in the Culver Cliffs and Brading Down, where the strata of the Upper Chalk are marked by the lines of black flints. In the large quarry on Brading Down the vertical lines of flint can be clearly seen; and by walking at low tide at Whitecliff Bay round the corner of the cliff, or by observing the cliff from a boat, we may see a beautiful section of the flinty chalk, the lines of black flints sloping at a high angle. The flints in general form round or oval ma.s.ses, but of irregular shape with many projections, and the ma.s.ses lie in regular bands parallel to the stratification. The tremendous earth movement which has bent the strata into a great curve has compressed the vertical portion into about half its original thickness, and has made the chalk of our downs extremely hard. It has also shattered the flints in the chalk into fragments. The rounded ma.s.ses retain their form, but when pulled out of the chalk fall into sharp angular fragments, and we find they are shattered through and through.

[Ill.u.s.tration: _Photo by J. Milman Brown, Shanklin._]

CULVER CLIFFS--HIGHLY INCLINED CHALK STRATA

Now, what are flints, and how were they formed? Flints are a form of silica, a purer form than chert, as the chalk in which they are embedded was formed in the deep sea, and so we have no admixture of sand. Flints, as we find them in the chalk, are generally black translucent nodules, with a white coating, the result of a chemical action which has affected the outside after they were formed. Flint is very hard,--harder than steel. You cannot scratch it with a knife, though you may leave a streak of steel on the surface of the flint.

This hardness is a property of other forms of silica, as quartz and chalcedony. The question how the flints were formed is a difficult one. As to this much still remains obscure. The sea contains mineral substances in solution. Calcium sulphate and chloride, and a small amount of calcium carbonate (carbonate of lime) are in solution in the sea. From these salts is derived the calcium deposited as calcium carbonate to form the sh.e.l.ls of the Foraminifera and the larger sh.e.l.ls in the Chalk. There is also silica in small quant.i.ty in sea water. From this the skeletons of radiolaria and diatoms and the spicules of sponges are formed. Now, many flints contain fossil sponges, and when broken show a section of the sponge clearly marked.

Especially well can this be seen in flints which have lain some time in a gravel bed formed of flints worn out of the chalk by denudation.

Hard as a flint seems, it is penetrated by numerous fine pores. The gravel beds are usually stained yellow by water containing iron, and this has penetrated by the pores through the substance of the flints, staining them brown and orange. Many of the stained flints show beautifully the sponge markings,--a wide central ca.n.a.l with fine thread-like ca.n.a.ls leading into it from all sides.

The Chalk Sea evidently abounded in siliceous organisms, and it cannot be doubted that it is from such organisms that the silica was derived, which has formed the ma.s.ses of flint. Silica occurs in two forms--in a crystalline form as quartz or rock crystal, and as amorphous, _i.e._, formless or uncrystalline (also called opaline) silica. The siliceous skeletons of marine organisms are formed of amorphous silica. Flint consists of innumerable fine crystalline grains, closely packed together. Amorphous silica is less stable than crystalline, and is capable of being dissolved in alkaline water, _i.e._, water containing carbonate of sodium or pota.s.sium in solution. If the silica so dissolved be deposited again, it is generally in the crystalline form.

It seems probable, therefore, that the amorphous silica of the skeletal parts of marine organisms has been dissolved by alkaline water percolating through the strata, and re-deposited as flint.

As the silica was deposited, chalk was removed. The large irregular ma.s.ses of flint lying in the Chalk strata have clearly taken the place of chalk which has been removed. Water charged with silica soaking through the strata has deposited silica, and at the same time dissolved out so much carbonate of lime. Bivalve sh.e.l.ls, originally carbonate of lime, are often replaced, and filled up by flint, and casts of sea urchins in solid flint are common, and often beautiful fossils. This process of change took place after the foraminiferal ooze had been compacted into chalk strata; and to some extent at any rate, there has been deposition of silica after the chalk had become hard and solid; for we find flat sheets, called tabular flint, lying along the strata, or filling cracks cutting through the strata at right angles. But in all probability the re-arrangement of the const.i.tuents of the strata took place in the main during the first consolidation, as the strata rose above the sea-level, and the sea-water drained out. A suggestion has been made by R. E. Liesegang, of Dresden, to explain the occurrence of the flints in the bands with clear inters.p.a.ces between, which are such a marked feature of the Upper Chalk. He has shown how "a solution diffusing outward and encountering something with which it reacts and forms a precipitate, moves on into this medium until a concentration sufficient to cause precipitation of the particular salt occurs. A zone of precipitation is thus formed, through which the first solution penetrates until the conditions are repeated, and a second zone of precipitate is thrown down. Zone after zone may thus arise as diffusion goes on." He suggests that the zones of flint may be similar phenomena, water diffusing through the ma.s.ses of chalk taking up silica till such concentration is reached that precipitation takes place, the water then percolating further and repeating the process.[8]

The precipitation of silica and replacement of the chalk occurs irregularly along the zone of precipitation, forming great irregular ma.s.ses of flint, which enclose the sponges and other marine organisms that lay in the chalk strata. Where a deposit of silica has begun, it will probably have determined the precipitation of more silica, in the manner constantly seen in chemical precipitation; and it would seem that siliceous organisms as sponges have to some extent served as centres around which silica has been precipitated, for flints are very commonly found, having the evident external form of sponges.

It will be well to say something here of the history of the flints as the chalk which contains them is gradually denuded away. Rain water containing carbonic dioxide has a great effect in eating away all limestone rocks, chalk included. A vast extent of chalk, which formerly covered much of England has thus disappeared. The arch of chalk connecting our two ranges of downs has been cut through, and from the top of the downs themselves a great thickness of chalk has been removed. The chalk in the downs above Ventnor and Bonchurch is nearly horizontal. It consists of Lower and Middle Chalk; and probably a small bit of the Upper occurs. But the top of St. Boniface Down is covered with a great ma.s.s of angular flint gravel, which must have come from the Upper Chalk. The gravel is of considerable thickness, perhaps 20 ft., and on the spurs of the down falls over to a lower level like a table-cloth. It is worked in many pits for road metal.

This flint gravel represents the insoluble residue which has been left when the Chalk was dissolved away.

On the top of the cliffs between Ventnor and Bonchurch, at a point called Highport, is a stratum of flint gravel carried down from the top of the down. The sh.o.r.e here is strewn with large flints fallen from the gravel. The substance of many of the flints has undergone a remarkable change. Instead of black or dull grey flint it has become translucent agate, of splendid orange and purple colours, or has been changed into clear translucent chalcedony. In the agate the forms of fossil sponges can often be beautifully seen. The colours are due to iron-charged water percolating into the flint in the gravel bed, but further structural changes have altered the form of the silica; chalcedony having a structure of close crystalline fibres, revealed by polarized light: when variously stained and coloured, it is usually called agate. Many of these flints, when cut through and polished, are of great beauty. The main force of the tides along these sh.o.r.es is from west to east; and so there is a continual pa.s.sage of pebbles on the sh.o.r.e in that direction. The flints in Sandown Bay have in the main travelled round from here; and towards the Culvers small handy specimens of agates and chalcedonies rounded by the waves may be collected.

[Ill.u.s.tration: _Photo by J. Milman Brown, Shanklin._]

SCRATCh.e.l.l'S BAY--HIGHLY INCLINED CHALK STRATA

The extensive downs in the centre of the Island are largely overspread with angular flint gravel similarly formed to that of St. Boniface. Of other beds of gravel, which have been washed down to a lower level by rivers or other agency we shall have more to say later.

The Chalk strata in the Isle of Wight are of great thickness. In the Culver Cliff there are some 400 feet of flintless Chalk (Lower and Middle Chalk), and then some 1,000 feet of chalk with flints. There is some variation in the thickness of the strata in different parts of the Island, and the amount of the Upper strata, which has been removed by denudation, varies considerably. The average thickness of the white chalk in the Island is about 1,350 feet.[9] Including the Lower Chalk, the maximum thickness of the Chalk strata is 1,630 ft.

The divisions of the chalk we have so far considered depend on the character of the rock: we must say a word about another way of dividing the strata. It is found that in the chalk, as in other strata, fossils change with every few feet of deposit. We may make a zoological division of the chalk by seeing how the fossils are distributed. The Chalk was first studied from this point of view by the great French geologist, M. Barrois, who divided it into zones, according to the nature of the animal life, the zones being called by the name of some fossil specially characteristic of a particular zone.

More recently Dr. A. W. Rowe has made a very careful study of the zones of the White Chalk, and is now our chief authority on the subject. The strata have been grouped into zones as follows:--

Zones. Sub-Zones.

{ Belemnitella mucronata.

{ Actinocamax quadratus.

{ { Offaster pilula.

Upper { Offaster pilula. { Echinocorys depressus.

Chalk. { { Marsupites { Marsupites.

{ testudinarius. { Uintacrinus.

{ Micraster cor-anguinum.

{ Micraster cor-testudinarium.

{ Holaster pla.n.u.s.

Middle { Terebratulina lata.

Chalk. { Inoceramus l.a.b.i.atus.

{ Holaster subglobosus. { Actinocamax Lower { { plenus (at top).

Chalk. { Schloenbachia varians.{ Stauronema { { carteri (at base).

The method of study according to zoological zones is of great interest. The period of the White Chalk was of long duration, and the physical conditions remained very uniform. So that by studying the succession of life during this period we may learn much about the gradual change of life on the earth, and the evolution of living things.

We have seen that the whole ma.s.s of the chalk is made up mainly of the remains of living things,--mostly of the microscopic foraminifera. We have seen that sponges were very plentiful in that ancient sea. Of other fossils we find brachiopods--different species of Terebratula and Rhynchonella--a large bivalve _Inoceramus_ sometimes very common; the very beautiful bivalve, _Spondylus spinosus_, belemnites, serpulae; and different species of sea-urchin are very common. A pretty heart-shaped one, _Micraster cor-anguinum_, marks a zone of the higher chalk, which runs along the top of our northern downs. Other common sea urchins are various species of _Cidaris_, of a form like a turban (Gk. _cidaris_, a Persian head-dress); _Cyphosoma_, another circular form; the oval _Echinocorys scutatus_, which, with varieties of the same and allied species, abounds in the Upper Chalk, and the more conical _Conulus conicus_. The topmost zone, that of _B. Macronata_, would yield a record of exuberant life, were the chalk soft and horizontal. There was a rich development of echinoderms (sea urchins and star fishes), but nothing is perfect, owing to the hardness of the rock (Dr. Rowe). The general difference in the life of the Chalk period is the great development of Ammonites and other Cephalopods in the Lower Chalk, and of sea urchins and other echinoderms in the Upper, while the Middle Chalk is wanting in the one and the other.

Shark's teeth tell of the larger inhabitants of the ocean that flowed above the chalky bottom.

Many quarries have been opened on the flanks of the Chalk Downs, of which a large number are now disused. They occur just where they are needed for chalk to lay on the land, the pure chalk on the north of the Downs to break up the heavy Tertiary clays, which largely cover the north of the Island; the more clayey beds of the Grey Chalk on the south of the downs to stiffen the light loams of the Greensand.[10]

[Footnote 8: See _Common Stones_, by Grenville A. J. Cole, F.R.S. 1921.]

[Footnote 9: 1,472 ft. at the western end of the Island, 1,213 ft. at the eastern.--Dr. Rowe's measurements.]

[Footnote 10: Dr. A. W. Rowe.]

Chapter VIII