THE CHALK
As we have traced the world's history written in the rocks we have
seen an old continent gradually submerged, a deepening sea flowing
over this part of the earth's surface. Now we shall find evidence of
the deepening of the sea to something like an ocean depth. We are
coming to the great period of the Chalk, the time when the material
was made which forms the undulating downs of the south-east of England,
and of which the line of white cliffs consists, which with sundry
breaks half encircles our shores, from Flamborough Head in Yorkshire,
by Dover and the Isle of Wight, to Bere in Devon. Across the Channel
white cliffs of chalk face those of England, and the chalk stretches
inland into the Continent. Its extent was formerly greater still.
Fragments of chalk and flint are preserved in Mull under basalt, an
old lava flow, and flints from the chalk are found in more recent
deposits (Boulder Clay) on the East of Scotland, pointing to a former
great extension northward, which has been nearly all removed by
denudation. In the Isle of Wight the chalk cliffs of Freshwater and
the Culvers are the grandest features of the Island; while all the
Island is dominated by the long lines of chalk downs running through
it from east to west. Now what is the chalk? And how was it made? The
microscope must tell us. It is found that this great mass of chalk is
made up principally of tiny microscopic shells called Foraminifera,
whole and in crushed fragments. There are plenty of foraminifera in
the seas to-day; and we need not go far to find similar shells. On the
shore near Shanklin you will often see streaks of what look like tiny
bits of broken shell washed into depressions in the sand. These,
however, often consist almost entirely of complete microscopic shells,
some of them of great beauty. The creature that lives in one of these
shells is only like a drop of formless jelly, and yet around itself it
forms a complex shell of surprising beauty. The shells are pierced
with a number of holes, hence their name (fr. Lat. _foramen_, a hole,
and _ferre_, to bear). Through these holes the animal puts out a
number of feelers like threads of jelly, and in these entangles
particles of food, and draws them into itself. Now, do we anywhere
to-day find these tiny shells in such masses as to build up rocks? We
do. The sounding apparatus, with which we measure the depths of the
sea, is so constructed as to bring up a specimen of the sea bottom.
This has been used in the Atlantic, and it is found that the really
deep sea bottom, too far out for rivers and currents to bring sand and
mud from the land, is covered with a white mud or ooze. And the
microscope shows this to be made up of an unnumerable multitude of the
tiny shells of foraminifera. As the little creatures die in the sea,
their shells accumulate on the bottom, and in time will be pressed
into a hard mass like chalk, the whole being cemented together by
carbonate of lime, in the way we explained in describing the making of
limestones. So we find chalk still forming at the present day. But
what ages it must take to form strata of solid rock of such tiny
shells! And what a vast period of time it must have required to build
up our chalk cliffs and downs, composed in large part of tiny
microscopic shells! With the foraminifera the microscope shows in the
chalk a multitude of crushed fragments, largely the prisms which
compose bivalve shells, flakes of shells of Terebratula and
Rhynchonella, and minute fragments of corals and Bryozoa. Scattered in
the chalk we shall also find larger shells and other remains of the
life of the ancient sea. The base of the cliffs and fallen blocks on
the shore are the best places to find fossils. Much of the base of the
cliffs is inaccessible except by boat. The lower strata may be
examined in Sandown and Compton Bays, and the upper in Whitecliff Bay.
A watch should always be kept on the tide. The quarries along the
downs are not as a rule good for collecting, as the chalk does not
become so much sculptured by weathering.
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 shells 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 passage 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 masses along the shore 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 shells. 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.
[Illustration: 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 massive 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 shore. 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 labiatus_, a large bivalve shell, occurs
in great profusion; and in the Upper flinty Chalk are sheets of
another species, _I. Cuvieri_. It is hardly ever found perfect, the
shells 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 masses, but of
irregular shape with many projections, and the masses 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 masses retain their
form, but when pulled out of the chalk fall into sharp angular
fragments, and we find they are shattered through and through.
[Illustration: _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 shells of the Foraminifera and the larger
shells in the Chalk. There is also silica in small quantity 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 canal with fine
thread-like canals 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 masses 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 potassium 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
masses 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 shells, 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
constituents 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 interspaces 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 masses 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
masses 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 mass 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 shore 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 shores is
from west to east; and so there is a continual passage of pebbles on
the shore 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.
[Illustration: _Photo by J. Milman Brown, Shanklin._]
SCRATCHELL'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 planus.
Middle { Terebratulina lata.
Chalk. { Inoceramus labiatus.
{ 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 mass 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.]