DENUDATION OF THE CHALK AND WEALDEN.
Physical geography of certain districts composed of Cretaceous and
Wealden strata--Lines of inland chalk-cliffs on the Seine in
Normandy--Outstanding pillars and needles of chalk--Denudation of the
chalk and Wealden in Surrey, Kent, and Sussex--Chalk once continuous
from the North to the South Downs--Anticlinal axis and parallel
ridges--Longitudinal and transverse valleys--Chalk escarpments--Rise
and denudation of the strata gradual--Ridges formed by harder, valleys
by softer beds--Why no alluvium, or wreck of the chalk, in the central
district of the Weald--At what periods the Weald valley was
denuded--Land has most prevailed where denudation has been
greatest--Elephant bed, Brighton.
All the fossiliferous formations may be studied by the geologist in two
distinct points of view: first, in reference to their position in the
series, their mineral character and fossils; and, secondly, in regard to
their physical geography, or the manner in which they now enter, as mineral
masses, into the external structure of the earth; forming the bed of lakes
and seas, or the surface and foundation of hills and valleys, plains and
table-lands. Some account has already been given on the first head of the
Tertiary, the Cretaceous, and Wealden strata; and we may now proceed to
consider certain features in the physical geography of these groups as they
occur in parts of England and France.
The hills composed of white chalk in the S.E. of England have a smooth
rounded outline, and being usually in the state of sheep pastures, are free
from trees or hedgerows; so that we have an opportunity of observing how
the valleys by which they are drained ramify in all directions, and become
wider and deeper as they descend. Although these valleys are now for the
most part dry, except during heavy rains and the melting of snow, they may
have been due to aqueous denudation, as explained in the sixth chapter;
having been excavated when the chalk emerged gradually from the sea. This
opinion is confirmed by the occasional occurrence of long lines of inland
cliffs, in which the strata are cut off abruptly in steep and often
vertical precipices. The true nature of such escarpments is nowhere more
obvious than in parts of Normandy, where the river Seine and its
tributaries flow through deep winding valleys, hollowed out of chalk
horizontally stratified. Thus, for example, if we follow the Seine for a
distance of about 30 miles from Andelys to Elboeuf, we find the valley
flanked on both sides by a deep slope of chalk, with numerous beds of
flint, the formation being laid open for a thickness of about 250 and 300
feet. Above the chalk is an overlying mass of sand, gravel, and clay, from
30 to 100 feet thick. The two opposite slopes of the hills _a_ and _b_,
where the chalk appears at the surface, are from 2 to 4 miles apart, and
they are often perfectly smooth and even, like the steepest of our downs in
England; but at many points they are broken by one, two, or more ranges of
vertical and even overhanging cliffs of bare white chalk with flints. At
some points detached needles and pinnacles stand in the line of the cliffs,
or in front of them, as at _c_, fig. 245. On the right bank of the Seine,
at Andelys, one range, about 2 miles long, is seen varying from 50 to 100
feet in perpendicular height, and having its continuity broken by a number
of dry valleys or coombs, in one of which occurs a detached rock or needle,
called the Tete d'Homme (see figs. 246, 247.). The top of this rock
presents a precipitous face towards every point of the compass; its
vertical height being more than 20 feet on the side of the downs, and 40
towards the Seine, the average diameter of the pillar being 36 feet. Its
composition is the same as that of the larger cliffs in its neighbourhood,
namely, white chalk, having occasionally a crystalline texture like marble,
with layers of flint in nodules and tabular masses. The flinty beds often
project in relief 4 or 5 feet beyond the white chalk, which is generally in
a state of slow decomposition, either exfoliating or being covered with
white powder, like the chalk cliffs on the English coast; and, as in them,
this superficial powder contains in some places common salt.
[Illustration: Fig. 245. Section across Valley of Seine.]
[Illustration: Fig. 246. View of the Tete d'Homme, Andelys,
seen from above.]
Other cliffs are situated on the right bank of the Seine, opposite
Tournedos, between Andelys and Pont de l'Arche, where the precipices are
from 50 to 80 feet high: several of their summits terminate in pinnacles;
and one of them, in particular, is so completely detached as to present a
perpendicular face 50 feet high towards the sloping down. On these cliffs
several ledges are seen, which mark so many levels at which the waves of
the sea may be supposed to have encroached for a long period. At a still
greater height, immediately above the top of this range, are three much
smaller cliffs, each about 4 feet high, with as many intervening terraces,
which are continued so as to sweep in a semicircular form round an
adjoining coomb, like those in Sicily before described (p. 76.).
[Illustration: Fig. 247. Side view of the Tete d'Homme.
White chalk with flints.]
[Illustration: Fig. 248. Chalk pinnacle at Senneville.]
[Illustration: Fig. 249. Roches d'Orival, Elboeuf.]
If we then descend the river from Vatteville to a place called Senneville,
we meet with a singular needle about 50 feet high, perfectly isolated on
the escarpment of chalk on the right bank of the Seine (see fig. 248.).
Another conspicuous range of inland cliffs is situated about 12 miles below
on the left bank of the Seine, beginning at Elboeuf, and comprehending the
Roches d'Orival (see fig. 249.). Like those before described, it has an
irregular surface, often overhanging, and with beds of flint projecting
several feet. Like them, also, it exhibits a white powdery surface, and
consists entirely of horizontal chalk with flints. It is 40 miles inland,
its height, in some parts, exceeding 200 feet, and its base only a few feet
above the level of the Seine. It is broken, in one place, by a pyramidal
mass or needle, 200 feet high, called the Roche de Pignon, which stands out
about 25 feet in front of the upper portion of the main cliffs, with which
it is united by a narrow ridge about 40 feet lower than its summit (see
fig. 250.). Like the detached rocks before mentioned at Senneville,
Vatteville, and Andelys, it may be compared to those needles of chalk which
occur on the coast of Normandy, as well as in the Isle of Wight and in
Purbeck[241-A] (see fig. 251.).
[Illustration: Fig. 250. View of the Roche de Pignon, seen from the south.]
[Illustration: Fig. 251. Needle and Arch of Etretat, in the chalk cliffs of
Normandy. Height of Arch 100 feet. (Passy.)[241-B]]
The foregoing description and drawings will show, that the evidence of
certain escarpments of the chalk having been originally sea-cliffs, is far
more full and satisfactory in France than in England. If it be asked why,
in the interior of our own country, we meet with no ranges of precipices
equally vertical and overhanging, and no isolated pillars or needles, we
may reply that the greater hardness of the chalk in Normandy may, no doubt,
be the chief cause of this difference. But the frequent absence of all
signs of littoral denudation in the valley of the Seine itself is a
negative fact of a far more striking and perplexing character. The cliffs,
after being almost continuous for miles, are then wholly wanting for much
greater distances, being replaced by a green sloping down, although the
beds remain of the same composition, and are equally horizontal; and
although we may feel assured that the manner of the upheaval of the land,
whether intermittent or not, must have been the same at those intermediate
points where no cliffs exist, as at others where they are so fully
developed. But, in order to explain such apparent anomalies, the reader
must refer again to the theory of denudation, as expounded in the 6th
chapter; where it was shown, first, that the undermining force of the waves
and marine currents varies greatly at different parts of every coast;
secondly, that precipitous rocks have often decomposed and crumbled down;
and thirdly, that many terraces and small cliffs may now lie concealed
beneath a talus of detrital matter.
_Denudation of the Weald Valley._--No district is better fitted to
illustrate the manner in which a great series of strata may have been
upheaved and gradually denuded than the country intervening between the
North and South Downs. This region, of which a ground plan is given in the
accompanying map (fig. 252.), comprises within it the whole of Sussex, and
parts of the counties of Kent, Surrey, and Hampshire. The space in which
the formations older than the White Chalk, or those from the Gault to the
Hastings sand inclusive, crop out, is bounded everywhere by a great
escarpment of chalk, which is continued on the opposite side of the channel
in the Bas Boulonnais in France, where it forms the semicircular boundary
of a tract in which older strata also appear at the surface. The whole of
this district may therefore be considered geologically as one and the same.
[Illustration: Fig. 252. Geological Map of the south-east of England and
part of France, exhibiting the denudation of the Weald.
1. Tertiary.
2. Chalk and upper greensand.
3. Gault.
4. Lower Greensand.
5. Weald clay.
6. Hastings sand.
7. Purbeck beds.
8. Oolite.]
[Illustration: Fig. 253. Section from the London to the Hampshire basin
across the valley of the Weald.
1. Tertiary strata.
2. Chalk and firestone.
3. Gault.
4. Lower greensand.
5. Weald clay.
6. Hastings sands.]
[Illustration: Fig. 254. Highest point of South Downs, 858 feet.
Anticlinal axis of the Weald. Crowborough Hill, 804 feet.
Highest point of North Downs, 880 feet.[243-A]
Section of the country from the confines of the basin of London to
that of Hants, with the principal heights above the level of the sea
on a true scale.[243-B]]
The space here inclosed within the escarpment of the chalk affords an
example of what has been sometimes called a "valley of elevation" (more
properly "of denudation"); where the strata, partially removed by
aqueous excavation, dip away on all sides from a central axis. Thus, it
is supposed that the area now occupied by the Hastings sand (No. 6.) was
once covered by the Weald clay (No. 5.), and this again by the Greensand
(No. 4.), and this by the Gault (No. 3.); and, lastly, that the Chalk
(No. 2.) extended originally over the whole space between the North and
the South Downs. This theory will be better understood by consulting the
annexed diagram (fig. 253.), where the dark lines represent what now
remains, and the fainter ones those portions of rock which are believed
to have been carried away.
At each end of the diagram the tertiary strata (No. 1.) are exhibited
reposing on the chalk. In the middle are seen the Hastings sands (No. 6.)
forming an anticlinal axis, on each side of which the other formations are
arranged with an opposite dip. It has been necessary, however, in order to
give a clear view of the different formations, to exaggerate the
proportional height of each in comparison to its horizontal extent; and a
true scale is therefore subjoined in another diagram (fig. 254.), in order
to correct the erroneous impression which might otherwise be made on the
reader's mind. In this section the distance between the North and South
Downs is represented to exceed forty miles; for the Valley of the Weald is
here intersected in its longest diameter, in the direction of a line
between Lewes and Maidstone.
Through the central portion, then, of the district supposed to be denuded
runs a great anticlinal line, having a direction nearly east and west, on
both sides of which the beds 5, 4, 3, and 2, crop out in succession. But,
although, for the sake of rendering the physical structure of this region
more intelligible, the central line of elevation has alone been introduced,
as in the diagrams of Smith, Mantell, Conybeare, and others, geologists
have always been well aware that numerous minor lines of dislocation and
flexure run parallel to the great central axis.
In the central area of the Hastings sand the strata have undergone the
greatest displacement; one fault being known, where the vertical shift
of a bed of calcareous grit is no less than 60 fathoms.[244-A] Much of
the picturesque scenery of this district arises from the depth of the
narrow valleys and ridges to which the sharp bends and fractures of
the strata have given rise; but it is also in part to be attributed
to the excavating power exerted by water, especially on the
interstratified argillaceous beds.
Besides the series of longitudinal valleys and ridges in the Weald, there
are valleys which run in a transverse direction, passing through the chalk
to the basin of the Thames on the one side, and to the English Channel on
the other. In this manner the chain of the North Downs is broken by the
rivers Wey, Mole, Darent, Medway, and Stour; the South Downs by the Arun,
Adur, Ouse, and Cuckmere.[244-B] If these transverse hollows could be
filled up, all the rivers, observes Mr. Conybeare, would be forced to take
an easterly course, and to empty themselves into the sea by Romney Marsh
and Pevensey Levels.[245-A]
Mr. Martin has suggested that the great cross fractures of the chalk, which
have become river channels, have a remarkable correspondence on each side
of the valley of the Weald; in several instances the gorges in the North
and South Downs appearing to be directly opposed to each other. Thus, for
example, the defiles of the Wey in the North Downs, and of the Arun in the
South, seemed to coincide in direction; and, in like manner, the Ouse
corresponds to the Darent, and the Cuckmere to the Medway.[245-B]
[Illustration: Fig. 255. View of the chalk escarpment of the South Downs.
Taken from the Devil's Dike, looking towards the west and south-west.
_a._ The town of Steyning is hidden by this point.
_b._ Edburton church.
_c._ Road.
_d._ River Adur.]
Although these coincidences may, perhaps, be accidental, it is by no means
improbable, as hinted by the author above mentioned, that great amount of
elevation towards the centre of the Weald district gave rise to transverse
fissures. And as the longitudinal valleys were connected with that linear
movement which caused the anticlinal lines running east and west, so the
cross fissures might have been occasioned by the intensity of the upheaving
force towards the centre of the line.
But before treating of the manner in which the upheaving movement may
have acted, I shall endeavour to make the reader more intimately
acquainted with the leading geographical features of the district, so
far as they are of geological interest.
In whatever direction we travel from the tertiary strata of the basins of
London and Hampshire towards the valley of the Weald, we first ascend a
slope of white chalk, with flints, and then find ourselves on the summit of
a declivity consisting, for the most part, of different members of the
chalk formation; below which the upper greensand, and sometimes, also, the
gault, crop out. This steep declivity is the great escarpment of the chalk
before mentioned, which overhangs a valley excavated chiefly out of the
argillaceous or marly bed, termed Gault (No. 3.). The escarpment is
continuous along the southern termination of the North Downs, and may be
traced from the sea, at Folkestone, westward to Guildford and the
neighbourhood of Petersfield, and from thence to the termination of the
South Downs at Beachy Head. In this precipice or steep slope the strata are
cut off abruptly, and it is evident that they must originally have extended
farther. In the woodcut (fig. 255. p. 245.), part of the escarpment of the
South Downs is faithfully represented, where the denudation at the base of
the declivity has been somewhat more extensive than usual, in consequence
of the upper and lower greensand being formed of very incoherent materials,
the upper, indeed, being extremely thin and almost wanting.
[Illustration: Fig. 256. Chalk escarpment, as seen from the hill above
Steyning, Sussex. The castle and village of Bramber in the foreground.]
The geologist cannot fail to recognize in this view the exact likeness of a
sea cliff; and if he turns and looks in an opposite direction, or eastward,
towards Beachy Head (see fig. 256.), he will see the same line of heights
prolonged. Even those who are not accustomed to speculate on the former
changes which the surface has undergone may fancy the broad and level plain
to resemble the flat sands which were laid dry by the receding tide, and
the different projecting masses of chalk to be the headlands of a coast
which separated the different bays from each other.
In regard to the transverse valleys before mentioned, as intersecting the
chalk hills, some idea of them may be derived from the subjoined sketch
(fig. 257.), of the gorge of the river Adur, taken from the summit of the
chalk downs, at a point in the bridle-way leading from the towns of Bramber
and Steyning to Shoreham. If the reader will refer again to the view given
in a former woodcut (fig. 255. p. 245.), he will there see the exact point
where the gorge of which I am now speaking interrupts the chalk escarpment.
A projecting hill, at the point _a_, hides the town of Steyning, near which
the valley commences where the Adur passes directly to the sea at Old
Shoreham. The river flows through a nearly level plain, as do most of the
others which intersect the hills of Surrey, Kent, and Sussex; and it is
evident that these openings, so far at least as they are due to aqueous
erosion, have not been produced by the rivers, many of which, like the Ouse
near Lewes, have filled up arms of the sea, instead of deepening the
hollows which they traverse.
[Illustration: Fig. 257. Transverse Valley of the Adur in the South Downs.
_a._ Town of Steyning.
_b._ River Adur.
_c._ Old Shoreham.]
Now, in order to account for the manner in which the five groups of
strata, 2, 3, 4, 5, 6, represented in the map, fig. 252. and in the
section fig. 253., may have been brought into their present position,
the following hypothesis has been very generally adopted:--Suppose the
five formations to lie in horizontal stratification at the bottom of the
sea; then let a movement from below press them upwards into the form of
a flattened dome, and let the crown of this dome be afterwards cut off,
so that the incision should penetrate to the lowest of the five groups.
The different beds would then be exposed on the surface, in the manner
exhibited in the map, fig. 252.[247-A]
The quantity of denudation or removal by water of stratified masses assumed
to have once reached continuously from the North to the South Downs is so
enormous, that the reader may at first be startled by the boldness of the
hypothesis. But the difficulty vanishes when once sufficient time is
allowed for the gradual and successive rise of the strata, during which the
waves and currents of the ocean might slowly accomplish an operation, which
no sudden diluvial rush of waters could possibly have effected.
Among other proofs of the action of water, it may be stated that the great
longitudinal valleys follow the outcrop of the softer and more incoherent
beds, while ridges or lines of cliff usually occur at those points where
the strata are composed of harder stone. Thus, for example, the chalk with
flints, together with the subjacent upper greensand, which is often used
for building, under the provincial name of "firestone," has been cut into a
steep cliff on that side on which the sea encroached. This escarpment
bounds a deep valley, excavated chiefly out of the soft argillaceous or
marly bed, termed gault (No. 3.). In some places the upper greensand is in
a loose and incoherent state, and there it has been as much denuded as the
gault; as, for example, near Beachy Head; but farther to the westward it is
of great thickness, and contains hard beds of blue chert and calcareous
sandstone or firestone. Here, accordingly, we find that it produces a
corresponding influence on the scenery of the country; for it runs out like
a step beyond the foot of the chalk-hills, and constitutes a lower terrace,
varying in breadth from a quarter of a mile to three miles, and following
the sinuosities of the chalk escarpment.[248-A]
[Illustration: Fig. 258. Cross section.
_a._ Chalk with flints.
_b._ Chalk without flints.
_c._ Upper greensand, or firestone.
_d._ Gault.]
It is impossible to desire a more satisfactory proof that the escarpment
is due to the excavating power of water during the rise of the strata;
for I have shown, in my account of the coast of Sicily, in what manner
the encroachments of the sea tend to efface that succession of terraces
which must otherwise result from the intermittent upheaval of a coast
preyed upon by the waves.[248-B] During the interval between two
elevatory movements, the lower terrace will usually be destroyed,
wherever it is composed of incoherent materials; whereas the sea will
not have time entirely to sweep away another part of the same terrace,
or lower platform, which happens to be composed of rocks of a harder
texture, and capable of offering a firmer resistance to the erosive
action of water. As the yielding clay termed gault would be readily
washed away, we find its outcrop marked everywhere by a valley which
skirts the base of the chalk hills, and which is usually bounded on the
opposite side by the lower greensand; but as the upper beds of this last
formation are most commonly loose and incoherent, they also have usually
disappeared and increased the breadth of the valley. But in those
districts where chert, limestone, and other solid materials enter
largely into the composition of this formation (No. 4.), they give rise
to a range of hills parallel to the chalk, which sometimes rival the
escarpment of the chalk itself in height, or even surpass it, as in
Leith Hill, near Dorking. This ridge often presents a steep escarpment
towards the soft argillaceous deposit called the Weald clay (No. 5.;
see the strong lines in fig. 253. p. 243.), which usually forms a broad
valley, separating the lower greensand from the Hastings sands or Forest
ridge; but where subordinate beds of sandstone of a firmer texture
occur, the uniformity of the plain of No. 5. is broken by waving
irregularities and hillocks.
It will be easy to show how closely the superficial inequalities agree with
those which we might naturally expect to originate during the gradual rise
of the Wealden district. Suppose the line of the most energetic movement to
have coincided with what is now the central ridge of the Weald valley; in
that case the first land which emerged must have been situated where the
Forest ridge is now placed. Here many shoals and reefs may first have
existed, and islands of chalk devoured in the course of ages by the ocean
(see fig. 253.); so that the top of the shattered dome which first appeared
above water may have been utterly destroyed, and the masses represented by
the fainter lines (fig. 253.) removed.
[2 Illustrations: Fig. 259., Fig. 260.
The dotted lines represent the sea-level.]
The upper greensand is represented (fig. 259.) as forming on the left
hand a single precipice with the chalk; while on the right there are two
cliffs, with an intervening terrace, as before described in fig. 258.
Two strips of land would then remain on each side of a channel,
presenting ranges of white cliffs facing each other. A powerful current
might then scoop out a channel in the gault (No. 2.). This softer bed
would yield with ease in proportion as parts of it were brought up from
time to time and exposed to the fury of the waves, so that large spaces
occupied by the harder formation or greensand (No. 3.) would be laid
bare. This last rock, opposing a more effectual resistance, would next
emerge; while the chalk cliffs, at the base of which the gault is
rapidly undermined, would recede farther from each other, after which
four parallel strips of land, or rows of islands, would be caused, which
are represented by the masses which in fig. 260. rise above the dotted
line indicating the sea-level. In this diagram, however, the inclination
of the upper surface of the formations (Nos. 1. and 3.), is exaggerated.
Originally this surface must have been level, like the submarine
terraces produced by denudation, and described before (p. 74. and 77.);
but they were afterwards more and more tilted by that general movement
to which the region of the Weald owes its structure. At length, by the
farther elevation of the dome-shaped mass, the clay (No. 4.) would be
brought within reach of the waves, which would probably gain the more
easy access to the subjacent deposit by the rents which would be caused
in No. 3., and in the central part of the ridge where the uplifting
force had been exerted with the greatest energy. The opposite cliffs, in
which the greensand (No. 3.) terminates, would now begin to recede from
each other, having at their base a yielding stratum of clay (No. 4.).
Lastly, the sea would penetrate to the sand (No. 5.), and then the
state of things indicated in the dark lines of the upper section
(fig. 253.), would be consummated.
[Illustration: Fig. 261. The Coomb, near Lewes.]
It was stated that there are many lines of flexure and dislocation, running
east and west, or parallel to the central axis of the Wealden. They are
numerous in the district of the Hastings sand, and sometimes occur in the
chalk itself. One of the latter kind has given rise to the ravine called
the Coomb, near Lewes, and was first traced out by Dr. Mantell, in whose
company I examined it. This coomb is seen on the eastern side of the valley
of the Ouse, in the suburbs of the town of Lewes. The steep declivities on
each side are covered with green turf, as is the bottom, which is perfectly
dry. No outward signs of disturbance are visible; and the connection of the
hollow with subterranean movements would not have been suspected by the
geologist, had not the evidence of great convulsions been clearly exposed
in the escarpment of the valley of the Ouse, and the numerous chalk pits
worked at the termination of the Coomb. By the aid of these we discover
that the ravine coincides precisely with a line of fault, on one side of
which the chalk with flints (_a_, fig. 262.), appears at the summit of the
hill, while it is thrown down to the bottom on the other.
Mr. Martin, in his work on the geology of Western Sussex, published in
1828, threw much light on the structure of the Wealden by tracing out
continuously for miles the direction of many anticlinal lines and cross
fractures; and the same course of investigation has since been followed
out in greater detail by Mr. Hopkins. The mathematician last-mentioned
has shown that the observed direction of the lines of flexure and
dislocation in the Weald district coincide with those which might have
been anticipated theoretically on mechanical principles, if we assume
certain simple conditions under which the strata were lifted up by an
expansive subterranean force. He finds by calculation that if this force
was applied so as to act uniformly upwards within an elliptic area, the
longitudinal fissures thereby produced would nearly coincide with the
outlines of the ellipse, forming cracks, which are portions of smaller
concentric ellipses, parallel to the margin of the larger one. These
longitudinal fissures would also be intercepted by others running at
right angles to them, and both lines of fracture may have been produced
at the same time.[251-A] In this illustration it is supposed that the
expansive force acted simultaneously and with equal intensity at every
point within the upheaved area, and not with greater energy along the
central axis or region of principal elevation.
[Illustration: Fig. 262. Fault in the cliff hills near Lewes. Mantell.
_a._ Chalk with flints.
_b._ Lower chalk.[251-B]]
The geologist cannot fail to derive great advantage in his speculations
from the mathematical investigation of a problem of this kind, where
results free from all uncertainty are obtained on the assumption of certain
simple conditions. Such results, when once ascertained by mathematical
methods, may serve as standard cases, to which others occurring in nature
of a more complicated kind may be referred. In order that a uniform force
should cause the strata to attain in the centre of the ellipse a height so
far exceeding that which they have reached round the margin, it is
necessary to assume that the mass of upheaved strata offered originally a
very unequal degree of resistance to the subterranean force. This may have
happened either from their being more fractured in one place than in
another, or from being pressed down by a less weight of incumbent strata;
as if we suppose, what is far from improbable, that great denudation had
taken place in the middle of the Wealden before the final and principal
upheaval occurred. It is suggested that the beds may have been acted upon
somewhat in the manner of a carpet spread out loosely on a floor, and
nailed down round the edges, which would swell into the shape of a dome if
pressed up equally at every point by air admitted from beneath. But when we
are reasoning on the particular phenomena of the Weald, we have no
geological data for determining whether it be more probable that originally
the resistance to be overcome was so extremely unequal in different
places, or whether the subterranean force, instead of being everywhere
uniform, was not applied with very different degrees of intensity beneath
distinct portions of the upraised area.
The opinion that both the longitudinal and transverse lines of fracture
may have been produced simultaneously, accords well with that expressed
by M. Thurmann, in his work on the anticlinal ridges and valleys of
elevation of the Bernese Jura.[252-A] For the accuracy of his map and
sections I can vouch, from personal examination, in 1835, of part of the
region surveyed by him. Among other results, at which this author
arrived, it appears that the breadth of all the numerous anticlinal
ridges and dome-shaped masses in the Jura is invariably great in
proportion to the number of the formations exposed to view; or, in other
words, to the depth to which the superimposed groups of secondary strata
have been laid open. (See fig. 71. p. 55. for structure of Jura.) He
also remarks, that the anticlinal lines are occasionally oblique and
cross each other, in which case the greatest dislocation of the beds
takes place. Some of the cross fractures are imagined by him to have
been contemporaneous, others subsequent to the longitudinal ones.
I have assumed, in the former part of this chapter, that the rise of the
Weald was gradual, whereas many geologists have attributed its elevation to
a single effort of subterranean violence. There appears to them such a
unity of effect in this and other lines of deranged strata in the
south-east of England, such as that of the Isle of Wight, as is
inconsistent with the supposition of a great number of separate movements
recurring after long intervals of time. But we know that earthquakes are
repeated throughout a long series of ages in the same spots, like volcanic
eruptions. The oldest lavas of Etna were poured out many thousands, perhaps
myriads of years before the newest, and yet they, and the movements
accompanying their emission, have produced a symmetrical mountain; and if
rivers of melted matter thus continue to flow in the same direction, and
towards the same point, for an indefinite lapse of ages, what difficulty is
there in conceiving that the subterranean volcanic force, occasioning the
rise or fall of certain parts of the earth's crust, may, by reiterated
movements, produce the most perfect unity of result?
_Alluvium of the Weald._--Our next inquiry may be directed to the alluvium
strewed over the surface of the supposed area of denudation. Has any wreck
been left behind of the strata removed? To this we may answer, that the
chalk downs even on their summits are covered every where with gravel
composed of unrounded and partially rounded chalk flints, such as might
remain after masses of white chalk had been softened and removed by water.
This superficial accumulation of the hard or siliceous materials of the
disintegrated strata may be due in some degree to pluvial action; for
during extraordinary rains a rush of water charged with calcareous matter,
of a milk-white colour, may be seen to descend even gently sloping hills of
chalk. If a layer no thicker than the tenth of an inch be removed once in a
century, a considerable mass may in the course of indefinite ages melt
away, leaving nothing save a layer of flinty nodules to attest its former
existence. These unrolled flints may remain mixed with others more or less
rounded, which the waves left originally on the surface of the chalk, when
it first emerged from the sea. A stratum of fine clay sometimes covers the
surface of slight depressions and the bottom of valleys in the white chalk,
which may represent the aluminous residue of the rock, after the pure
carbonate of lime has been dissolved by rain water, charged with excess of
carbonic acid derived from decayed vegetable matter.[253-A]
Although flint gravel is so abundant on the chalk itself, it is usually
wanting in the deep longitudinal valleys at the foot of the chalk
escarpment, although, in some few instances, the detritus of the chalk has
been traced in patches over the gault, and even the lower greensand, for a
distance of several miles from the escarpment of the North and South Downs.
But no vestige of the chalk and its flints has been seen on the central
ridge of the Weald or the Hastings sands, but merely gravel derived from
the rocks immediately subjacent. This distribution of alluvium, and
especially the absence of chalk detritus in the central district, agrees
well with the theory of denudation before set forth; for to return to fig.
259., if the chalk (No. 1.) were once continuous and covered every where
with flint gravel, this superficial covering would be the first to be
carried away from the highest part of the dome long before any of the gault
(No. 2.) was laid bare. Now if some ruins of the chalk remain at first on
the gault, these would be, in a great degree, cleared away before any part
of the lower greensand (No. 3.) is denuded. Thus in proportion to the
number and thickness of the groups removed in succession, is the
probability lessened of our finding any remnants of the highest group
strewed over the bared surface of the lowest.
As an exception to the general rule of the small distance to which any
wreck of the chalk can be traced from the escarpments of the North and
South Downs, I may mention a thick bed of chalk flints which occurs near
Barcombe, about three miles to the north of Lewes (see fig. 263.), a place
which I visited with Dr. Mantell, to whom I am indebted for the
accompanying section. Even here it will be seen that the gravel reaches no
farther than the Weald Clay. The same section shows one of the minor east
and west anticlinal lines before alluded to (p. 244.).
[Illustration: Fig. 263. Section from the north escarpment of the South
Downs to Barcombe.
1. Gravel composed of partially rounded chalk flints.
2. Chalk with and without flints.
3. Lowest chalk or chalk marl (upper greensand wanting).
4. Gault.
5. Lower greensand.
6. Weald clay.]
_At what period the Weald Valley was denuded._--If we inquire at what
geological period the denudation of the Weald was effected, we shall
immediately perceive that the question is limited to this point, whether it
took place during or subsequent to the deposition of the Eocene strata of
the south of England. For in the basins of London and Hampshire the Eocene
strata are conformable to the chalk, being horizontal where the beds of
chalk are horizontal, and vertical where they are vertical, so that both
series of rocks appear to have participated in nearly the same movements.
At the eastern extremity of the Isle of Wight, some beds even of the
freshwater series have been thrown on their edges, like those of the London
clay. Nevertheless we can by no means infer that all the tertiary deposits
of the London and Hampshire basins once extended like the chalk over the
entire valley of the Weald, because the denudation of the chalk and
greensand may have been going on in the centre of that area, while
contiguous parts of the sea were sufficiently deep to receive and retain
the matter derived from that waste. Thus while the waves and currents were
excavating the longitudinal valleys D and C (fig. 264.), the deposits _a_
may have been thrown down to the bottom of the contiguous deep water E, the
sediment being drifted through transverse fissures, as before explained. In
this case, the rise of the formations Nos. 1, 2, 3, 4, 5, may have been
going on contemporaneously with the excavation of the valleys C and D, and
with the accumulation of the tertiary strata _a_.
[Illustration: Fig. 264. Cross section.]
This idea receives some countenance from the fact of the tertiary strata,
near their junction with the chalk of the London and Hampshire basins,
often consisting of dense beds of sand and shingle, as at Blackheath and in
the Addington Hills near Croydon. They also contain occasionally freshwater
shells and the remains of land animals and plants, which indicate the
former presence of land at no great distance, some part of which may have
occupied the centre of the Weald.
Such masses of well-rolled pebbles occurring in the lowest Eocene
strata, or those called "the plastic clay and sands" before described
(No. 3. _b_, Tab. p. 197.), imply the neighbourhood of an ancient shore.
They also indicate the destruction of pre-existing chalk with flints. At
the same time fossil shells of the genera _Melania_, _Cyclas_, and
_Unio_, appearing here and there in beds of the same age, together with
plants and the bones of land animals, bear testimony to contiguous land,
which probably constituted islands scattered over the space now occupied
by the tertiary basins of the Seine and Thames. The stage of denudation
represented in fig. 259., p. 249., may explain the state of things
prevailing at points where such islands existed. By the alternate rising
and sinking of the white chalk and older beds, a large area may have
become overspread with gravelly sandy, and clayey beds of fluvio-marine
and shallow-water origin, before any of the London clay proper (or
Calcaire grossier in France) were superimposed. This may account for the
fact that patches of "plastic clay and sand" (No. 3. _b_, Tab. p. 197.),
are scattered over the surface of the chalk, reaching in some places to
great heights, and approaching even the edges of the escarpments. We
must suppose that subsequently a gradual subsidence took place in
certain areas, which allowed the London clay proper to accumulate over
the Lower Eocene sands and clays, in a deep sea. During this sinking
down (the vertical amount of which equalled 800, and in parts of the
Isle of Wight, according to Mr. Prestwich, 1800 feet), the work of
denudation would be unceasing, being always however confined to those
areas where land or islands existed. At length, when the Bagshot sand
had been in its turn thrown down on the London clay, the space covered
by these two formations was again upraised from the sea to about the
height which it has since retained. During this upheaval, the waves
would again exert their power, not only on the white chalk and lower
cretaceous and Wealden strata, but also on the Eocene formations of
the London basin, excavating valleys and undermining cliffs as the
strata emerged from the deep.
There are grounds, as before stated (p. 205.), for presuming that the
tertiary area of London was converted into land before that of
Hampshire, and for this reason it contains no marine Eocene deposits so
modern as those of Barton Cliff, or the still newer freshwater and
fluvio-marine beds of Hordwell and the Isle of Wight. These last seem
unequivocally to demonstrate the local inequality of the upheaving and
depressing movements of the period alluded to; for we find, in spite of
the evidence afforded in Alum and White Cliff Bays, of continued
depression to the extent of 1800 or 2000 feet, that at the close of the
Eocene period a dense formation of freshwater strata was produced. The
fossils of these strata bear testimony to rivers draining adjacent
lands, and the existence of numerous quadrupeds on those lands.
Instead of such phenomena, the signs of an open sea might naturally
have been expected as the consequence of so much subsidence, had
not the depression been accompanied or followed by upheaval in a
region immediately adjoining.
When we attempt to speculate on the geographical changes which took place
in the earlier part of the Eocene epoch, and to restore in imagination the
former state of the physical geography of the south-east of England, we
shall do well to bear in mind that wherever there are proofs of great
denudation, there also the greatest area of land has probably existed. In
the same space, moreover, the oscillations of level, and the alternate
submergence and emergence of coasts, may be presumed to have been most
frequent; for these fluctuations facilitate the wasting and removing power
of waves, currents, and rivers.
We should also remember that there is always a tendency in the last
denuding operations, to efface all signs of preceding denudation, or at
least all those marks of waste from which alone a geologist can ascertain
the date of the removal of the missing strata within the denuded area. It
may often be difficult to settle the chronology even of the last of a
series of such acts of removal, but it must be, in the nature of things,
almost always impossible to assign a date to each of the antecedent
denudations. If we wish to determine the times of the destruction of rocks,
we must look any where rather than to the spaces once occupied by the
missing rocks. We must inquire to what regions the ruins of the white
chalk, greensand, Wealden, and other strata which have disappeared were
transported. We are then led at once to the examination of all the deposits
newer than the chalk, and first to the oldest of these, the Lower Eocene,
and its sand, shingle, and clay. In them, so largely developed in the
immediate neighbourhood of the denuded area, we discover the wreck we are
in search of, regularly stratified, and inclosing, in some of its layers,
organic remains of a littoral, and sometimes fluviatile character. What
more can we desire? The shores must have consisted of chalk, greensand, and
Wealden, since these were the only superficial rocks in the south-east of
England, at the commencement of the Eocene epoch. The waves of the sea,
therefore, and the rivers were grinding down chalk-flints and chert from
the greensand into shingle and sand, or were washing away calcareous and
argillaceous matter from the cretaceous and Wealden beds, during the whole
of the Eocene period. Thus we obtain the date of a great part at least of
that enormous amount of denudation of which we have such striking monuments
in the space intervening between the North and South Downs.
[Illustration: Fig. 265. Cross section.
A. Chalk with layers of flint dipping slightly to the south.
_b._ Ancient beach, consisting of fine sand, from one to four feet thick,
covered by shingle from five to eight feet thick of pebbles of
chalk-flint, granite, and other rocks, with broken shells of recent
marine species, and bones of cetacea.
_c._ Elephant bed, about fifty feet thick, consisting of layers of white
chalk rubble, with broken chalk-flints, in which deposit are found
bones of ox, deer, horse, and mammoth.
_d._ Sand and shingle of modern beach.]
There have been some movements of land on a smaller scale since the Eocene
period in the south-east of England. One of the latest of these happened in
the Pleistocene, or even perhaps as late as the Post-Pliocene period. The
formation called by Dr. Mantell the Elephant Bed, at the foot of the chalk
cliffs at Brighton, is not merely a talus of calcareous rubble collected at
the base of an inland cliff, but exhibits every appearance of having been
spread out in successive horizontal layers by water in motion.
The deposit alluded to skirts the shores between Brighton and Rottingdean,
and another mass apparently of the same age occurs at Dover. The phenomena
appear to me to suggest the following conclusions:--First, the
south-eastern part of England had acquired its actual configuration when
the ancient chalk cliff A _a_ was formed, the beach of sand and shingle _b_
having then been thrown up at the base of the cliff. Afterwards the whole
coast, or at least that part of it where the elephant bed now extends,
subsided to the depth of 50 or 60 feet; and during the period of
submergence successive layers of white calcareous rubble _c_ were
accumulated, so as to cover the ancient beach _b_. Subsequently, the coast
was again raised, so that the ancient shore was elevated to a level
somewhat higher than its original position.[257-A]
FOOTNOTES:
[241-A] An account of these cliffs was read by the author to the British
Assoc. at Glasgow, Sept. 1840.
[241-B] Seine-Inferieure, p. 142. and pl. 6. fig. 1.
[243-A] Botley Hill, near Godstone, in Surrey, was found by
trigonometrical measurement to be 880 feet above the level of the sea;
and Wrotham Hill, near Maidstone, which appears to be next in height of
the North Downs, 795 feet.
[243-B] My friend Dr. Mantell has kindly drawn up this scale at my request.
[244-A] Fitton, Geol. of Hastings, p. 55.
[244-B] Conybeare, Outlines of Geol., p. 81.
[245-A] Ibid., p. 145.
[245-B] Geol. of Western Sussex, p. 61.
[247-A] See illustrations of this theory by Dr. Fitton, Geol.
Sketch of Hastings.
[248-A] Sir E. Murchison, Geol. Sketch of Sussex, &c., Geol. Trans., Second
Series, vol. ii. p. 98.
[248-B] See fig. 94. p. 76.
[251-A] Geol. Soc. Proceed. No. 74. p. 363. 1841, and G. S. Trans.
2 Ser. v. 7.
[251-B] For farther information, see Mantell's Geol. of S. E.
of England, p. 352.
[252-A] Soulevemens Jurassiques. Paris, 1832.
[253-A] See above, p. 82.
[257-A] See Mantell's Geol. of S. E. of England, p. 32. After
re-examining the elephant bed in 1834, I was no longer in doubt of its
having been a regular subaqueous deposit. In 1828, Dr. Mantell
discovered in the shingle below the chalk-rubble the jawbone of a whale
12 feet long, which must have belonged to an individual from 60 to 70
feet in length, Medals of Creation, p. 825.