DENUDATION.
Denudation defined--Its amount equal to the entire mass of stratified
deposits in the earth's crust--Horizontal sandstone denuded in
Ross-shire--Levelled surface of countries in which great faults
occur--Coalbrook Dale--Denuding power of the ocean during the
emergence of land--Origin of Valleys--Obliteration of
sea-cliffs--Inland sea-cliffs and terraces in the Morea and
Sicily--Limestone pillars at St. Mihiel, in France--in Canada--in
the Bermudas.
Denudation, which has been occasionally spoken of in the preceding
chapters, is the removal of solid matter by water in motion, whether of
rivers or of the waves and currents of the sea, and the consequent laying
bare of some inferior rock. Geologists have perhaps been seldom in the
habit of reflecting that this operation has exerted an influence on the
structure of the earth's crust as universal and important as sedimentary
deposition itself; for denudation is the inseparable accompaniment of the
production of all new strata of mechanical origin. The formation of every
new deposit by the transport of sediment and pebbles necessarily implies
that there has been, somewhere else, a grinding down of rock into rounded
fragments, sand, or mud, equal in quantity to the new strata. All
deposition, therefore, except in the case of a shower of volcanic ashes, is
the sign of superficial waste going on contemporaneously, and to an equal
amount elsewhere. The gain at one point is no more than sufficient to
balance the loss at some other. Here a lake has grown shallower, there a
ravine has been deepened. The bed of the sea has in one region been raised
by the accumulation of new matter, in another its depth has been augmented
by the abstraction of an equal quantity.
When we see a stone building, we know that somewhere, far or near, a quarry
has been opened. The courses of stone in the building may be compared to
successive strata, the quarry to a ravine or valley which has suffered
denudation. As the strata, like the courses of hewn stone, have been laid
one upon another gradually, so the excavation both of the valley and quarry
have been gradual. To pursue the comparison still farther, the superficial
heaps of mud, sand, and gravel, usually called alluvium, may be likened to
the rubbish of a quarry which has been rejected as useless by the workmen,
or has fallen upon the road between the quarry and the building, so as to
lie scattered at random over the ground.
If, then, the entire mass of stratified deposits in the earth's crust is at
once the monument and measure of the denudation which has taken place, on
how stupendous a scale ought we to find the signs of this removal of
transported materials in past ages! Accordingly, there are different
classes of phenomena, which attest in a most striking manner the vast
spaces left vacant by the erosive power of water. I may allude, first, to
those valleys on both sides of which the same strata are seen following
each other in the same order, and having the same mineral composition and
fossil contents. We may observe, for example, several formations, as Nos.
1, 2, 3, 4, in the accompanying diagram (fig. 89.); No. 1. conglomerate,
No. 2. clay, No. 3. grit, and No. 4. limestone, each repeated in a series
of hills separated by valleys varying in depth. When we examine the
subordinate parts of these four formations, we find, in like manner,
distinct beds in each, corresponding, on the opposite sides of the valleys,
both in composition and order of position. No one can doubt that the strata
were originally continuous, and that some cause has swept away the portions
which once connected the whole series. A torrent on the side of a mountain
produces similar interruptions; and when we make artificial cuts in
lowering roads, we expose, in like manner, corresponding beds on either
side. But in nature, these appearances occur in mountains several thousand
feet high, and separated by intervals of many miles or leagues in extent,
of which a grand exemplification is described by Dr. MacCulloch, on the
north-western coast of Ross-shire, in Scotland.[67-A] The fundamental rock
of that country is gneiss, in disturbed strata, on which beds of nearly
horizontal red sandstone rest unconformably. The latter are often very
thin, forming mere flags, with their surfaces, distinctly ripple-marked.
They end abruptly on the declivities of many insulated mountains, which
rise up at once to the height of about 2000 feet above the gneiss of the
surrounding plain or table land, and to an average elevation of about 3000
feet above the sea, which all their summits generally attain. The base of
gneiss varies in height, so that the lower portions of the sandstone occupy
different levels, and the thickness of the mass is various, sometimes
exceeding 3000 feet. It is impossible to compare these scattered and
detached portions without imagining that the whole country has once been
covered with a great body of sandstone, and that masses from 1000 to more
than 3000 feet in thickness have been removed.
[Illustration: Fig. 89. Valleys of denudation. _a._ alluvium.]
[Illustration: Fig. 90. Denudation of red sandstone on north-west coast of
Ross-shire. (MacCulloch.)]
In the "Survey of Great Britain" (vol. i.), Professor Ramsay has shown
that the missing beds, removed from the summit of the Mendips, must have
been nearly a mile in thickness; and he has pointed out considerable areas
in South Wales and some of the adjacent counties of England, where a series
of palaeozoic strata, not less than 11,000 feet in thickness, have been
stripped off. All these materials have of course been transported to new
regions, and have entered into the composition of more modern formations.
On the other hand, it is shown by observations in the same "Survey," that
the palaeozoic strata are from 20,000 to 30,000 feet thick. It is clear that
such rocks, formed of mud and sand, now for the most part consolidated, are
the monuments of denuding operations, which took place on a grand scale at
a very remote period in the earth's history. For, whatever has been given
to one area must always have been borrowed from another; a truth which,
obvious as it may seem when thus stated, must be repeatedly impressed on
the student's mind, because in many geological speculations it is taken for
granted that the external crust of the earth has been always growing
thicker, in consequence of the accumulation, period after period, of
sedimentary matter, as if the new strata were not always produced at the
expense of pre-existing rocks, stratified or unstratified. By duly
reflecting on the fact, that all deposits of mechanical origin imply the
transportation from some other region, whether contiguous or remote, of an
equal amount of solid matter, we perceive that the stony exterior of the
planet must always have grown thinner in one place whenever, by accessions
of new strata, it was acquiring density in another. No doubt the vacant
space left by the missing rocks, after extensive denudation, is less
imposing to the imagination than a vast thickness of conglomerate or
sandstone, or the bodily presence as it were of a mountain-chain, with all
its inclined and curved strata. But the denuded tracts speak a clear and
emphatic language to our reason, and, like repeated layers of fossil
nummulites, corals or shells, or like numerous seams of coal, each based on
its under clay full of the roots of trees, still remaining in their natural
position, demand an indefinite lapse of time for their elaboration.
No one will maintain that the fossils entombed in these rocks did not
belong to many successive generations of plants and animals. In like
manner, each sedimentary deposit attests a slow and gradual action, and the
strata not only serve as a measure of the amount of denudation
simultaneously effected elsewhere, but are also a correct indication of the
rate at which the denuding operation was carried on.
Perhaps the most convincing evidence of denudation on a magnificent scale
is derived from the levelled surfaces of districts where large faults
occur. I have shown, in fig. 87. p. 63., and in fig. 91., how angular and
protruding masses of rock might naturally have been looked for on the
surface immediately above great faults, although in fact they rarely exist.
This phenomenon may be well studied in those districts where coal has been
extensively worked, for there the former relation of the beds which have
shifted their position may be determined with great accuracy. Thus in the
coal field of Ashby de la Zouch, in Leicestershire (see fig. 91.), a fault
occurs, on one side of which the coal beds _a b c d_ rise to the height of
500 feet above the corresponding beds on the other side. But the uplifted
strata do not stand up 500 feet above the general surface; on the contrary,
the outline of the country, as expressed by the line _z z_, is uniform and
unbroken, and the mass indicated by the dotted outline must have been
washed away.[69-A] There are proofs of this kind in some level countries,
where dense masses of strata have been cleared away from areas several
hundred square miles in extent.
[Illustration: Fig. 91. Faults and denuded coal strata, Ashby de
la Zouch. (Mammat.)]
In the Newcastle coal district it is ascertained that faults occur in
which the upward or downward movement could not have been less than 140
fathoms, which, had they affected equally the configuration of the
surface to that amount, would produce mountains with precipitous
escarpments nearly 1000 feet high, or chasms of the like depth; yet is
the actual level of the country absolutely uniform--affording no trace
whatever of subterranean movements.[69-B]
The ground from which these materials have been removed is usually
overspread with heaps of sand and gravel, formed out of the ruins of the
very rocks which have disappeared. Thus, in the districts above referred
to, they consist of rounded and angular fragments of hard sandstone,
limestone, and ironstone, with a small quantity of the more destructible
shale, and even rounded pieces of coal.
Allusion has been already made to the shattered state and discordant
position of the carboniferous strata in Coalbrook Dale (p. 62.). The
collier cannot proceed three or four yards without meeting with small
slips, and from time to time he encounters faults of considerable
magnitude, which have thrown the rocks up or down several hundred feet. Yet
the superficial inequalities to which these dislocated masses originally
gave rise are no longer discernible, and the comparative flatness of the
existing surface can only be explained, as Mr. Prestwich has observed, by
supposing the fractured portions to have been removed by water. It is also
clear that strata of red sandstone, more than 1000 feet thick, which once
covered the coal, in the same region, have been carried away from large
areas. That water has, in this case, been the denuding agent, we may infer
from the fact that the rocks have yielded according to their different
degrees of hardness; the hard trap of the Wrekin, for example, and other
hills, having resisted more than the softer shale and sandstone, so as now
to stand out in bold relief.[70-A]
_Origin of valleys._--Many of the earlier geologists, and Dr. Hutton among
them, taught that "rivers have in general hollowed out their valleys." This
is true only of rivulets and torrents which are the feeders of the larger
streams, and which, descending over rapid slopes, are most subject to
temporary increase and diminution in the volume of their waters. The
quantity of mud, sand, and pebbles constituting many a modern delta proves
indisputably that no small part of the inequalities now existing on the
earth's surface are due to fluviatile action; but the principal valleys in
almost every great hydrographical basin in the world, are of a shape and
magnitude which imply that they have been due to other causes besides the
mere excavating power of rivers.
Some geologists have imagined that a deluge, or succession of deluges, may
have been the chief denuding agency, and they have speculated on a series
of enormous waves raised by the instantaneous upthrow of continents or
mountain chains out of the sea. But even were we disposed to grant such
sudden upheavals of the floor of the ocean, and to assume that great waves
would be the consequence of each convulsion, it is not easy to explain the
observed phenomena by the aid of so gratuitous an hypothesis.
On the other hand, a machinery of a totally different kind seems capable of
giving rise to effects of the required magnitude. It has now been
ascertained that the rising and sinking of extensive portions of the
earth's crust, whether insensibly or by a repetition of sudden shocks, is
part of the actual course of nature, and we may easily comprehend how the
land may have been exposed during these movements to abrasion by the waves
of the sea. In the same manner as a mountain mass may, in the course of
ages, be formed by sedimentary deposition, layer after layer, so masses
equally voluminous may in time waste away by inches; as, for example, if
beds of incoherent materials are raised slowly in an open sea where a
strong current prevails. It is well known that some of these oceanic
currents have a breadth of 200 miles, and that they sometimes run for a
thousand miles or more in one direction, retaining a considerable velocity
even at the depth of several hundred feet. Under these circumstances, the
flowing waters may have power to clear away each stratum of incoherent
materials as it rises and approaches the surface, where the waves exert the
greatest force; and in this manner a voluminous deposit may be entirely
swept away, so that, in the absence of faults, no evidence may remain of
the denuding operation. It may indeed be affirmed that the signs of waste
will usually be least obvious where the destruction has been most
complete; for the annihilation may have proceeded so far, that no ruins are
left of the dilapidated rocks.
Although denudation has had a levelling influence on some countries of
shattered and disturbed strata (see fig. 87. p. 63. and fig. 91. p. 69.),
it has more commonly been the cause of superficial inequalities, especially
in regions of horizontal stratification. The general outline of these
regions is that of flat and level platforms, interrupted by valleys often
of considerable depth, and ramifying in various directions. These hollows
may once have formed bays and channels between islands, and the steepest
slope on the sides of each valley may have been a sea-cliff, which was
undermined for ages, as the land emerged gradually from the deep. We may
suppose the position and course of each valley to have been originally
determined by differences in the hardness of the rocks, and by rents and
joints which usually occur even in horizontal strata. In mountain chains,
such as the Jura before described (see fig. 71. p. 55.), we perceive at
once that the principal valleys have not been due to aqueous excavation,
but to those mechanical movements which have bent the rocks into their
present form. Yet even in the Jura there are many valleys, such as C (fig.
71.), which have been hollowed out by water; and it may be stated that in
every part of the globe the unevenness of the surface of the land has been
due to the combined influence of subterranean movements and denudation.
I may now recapitulate a few of the conclusions to which we have
arrived: first, all the mechanical strata have been accumulated
gradually, and the concomitant denudation has been no less gradual:
secondly, the dry land consists in great part of strata formed
originally at the bottom of the sea, and has been made to emerge and
attain its present height by a force acting from beneath: thirdly, no
combination of causes has yet been conceived so capable of producing
extensive and gradual denudation, as the action of the waves and
currents of the ocean upon land slowly rising out of the deep.
Now, if we adopt these conclusions, we shall naturally be led to look
everywhere for marks of the former residence of the sea upon the land,
especially near the coasts from which the last retreat of the waters took
place, and it will be found that such signs are not wanting.
I shall have occasion to speak of ancient sea-cliffs, now far inland, in
the south-east of England, when treating in Chapter XIX. of the denudation
of the chalk in Surrey, Kent, and Sussex. Lines of upraised sea-beaches of
more modern date are traced, at various levels from 20 to 100 feet and
upwards above the present sea-level, for great distances on the east and
west coasts of Scotland, as well as in Devonshire, and other counties in
England. These ancient beach-lines often form terraces of sand and gravel,
including littoral shells, some broken, others entire, and corresponding
with species now living on the adjoining coast. But it would be
unreasonable to expect to meet everywhere with the signs of ancient shores,
since no geologist can have failed to observe how soon all recent marks of
the kind above alluded to are obscured or entirely effaced, wherever, in
consequence of the altered state of the tides and currents, the sea has
receded for a few centuries. We see the cliffs crumble down in a few years
if composed of sand or clay, and soon reduced to a gentle slope. If there
were shells on the beach they decompose, and their materials are washed
away, after which the sand and shingle may resemble any other alluviums
scattered over the interior.
[Illustration: Fig. 92. Section of inland cliff at Abesse, near Dax.
_a._ Sand of the Landes.
_b._ Limestone.
_c._ Clay.]
The features of an ancient shore may sometimes be concealed by the
growth of trees and shrubs, or by a covering of blown sand, a good
example of which occurs a few miles west from Dax, near Bordeaux, in the
south of France. About twelve miles inland, a steep bank may be traced
running in a direction nearly north-east and south-west, or parallel to
the contiguous coast. This sudden fall of about 50 feet conducts us from
the higher platform of the Landes to a lower plain which extends to the
sea. The outline of the ground suggested to me, as it would do to every
geologist, the opinion that the bank in question was once a sea-cliff,
when the whole country stood at a lower level. But this is no longer
matter of conjecture, for, in making excavations in 1830 for the
foundation of a building at Abesse, a quantity of loose sand, which
formed the slope _d e_, was removed; and a perpendicular cliff, about 50
feet in height, which had hitherto been protected from the agency of the
elements, was exposed. At the bottom appeared the limestone _b_,
containing tertiary shells and corals, immediately below it the clay
_c_, and above it the usual tertiary sand _a_, of the department of the
Landes. At the base of the precipice were seen large partially rounded
masses of rock, evidently detached from the stratum _b_. The face of the
limestone was hollowed out and weathered into such forms as are seen in
the calcareous cliffs of the adjoining coast, especially at Biaritz,
near Bayonne. It is evident that, when the country was at a somewhat
lower level, the sea advanced along the surface of the argillaceous
stratum _c_, which, from its yielding nature, favoured the waste by
allowing the more solid superincumbent stone _b_ to be readily
undermined. Afterwards, when the country had been elevated, part of the
sand, _a_, fell down, or was drifted by the winds, so as to form the
talus, _d e_, which masked the inland cliff until it was artificially
laid open to view.
When we are considering the various causes which, in the course of ages,
may efface the characters of an ancient sea-coast, earthquakes must not be
forgotten. During violent shocks, steep and overhanging cliffs are often
thrown down and become a heap of ruins. Sometimes unequal movements of
upheaval or depression entirely destroy that horizontality of the base-line
which constitutes the chief peculiarity of an ancient sea-cliff.
It is, however, in countries where hard limestone rocks abound, that inland
cliffs retain faithfully the characters which they acquired when they
constituted the boundary of land and sea. Thus, in the Morea, no less than
three, or even four, ranges of what were once sea-cliffs are well
preserved. These have been described, by MM. Boblaye and Virlet, as rising
one above the other at different distances from the actual shore, the
summit of the highest and oldest occasionally exceeding 1000 feet in
elevation. At the base of each there is usually a terrace, which is in some
places a few yards, in others above 300 yards wide, so that we are
conducted from the high land of the interior to the sea by a succession of
great steps. These inland cliffs are most perfect, and most exactly
resemble those now washed by the waves of the Mediterranean, where they are
formed of calcareous rock, especially if the rock be a hard crystalline
marble. The following are the points of correspondence observed between the
ancient coast lines and the borders of the present sea:--1. A range of
vertical precipices, with a terrace at their base. 2. A weathered state of
the surface of the naked rock, such as the spray of the sea produces. 3. A
line of littoral caverns at the foot of the cliffs. 4. A consolidated beach
or breccia with occasional marine shells, found at the base of the cliffs,
or in the caves. 5. Lithodomous perforations.
In regard to the first of these, it would be superfluous to dwell on the
evidence afforded of the undermining power of waves and currents by
perpendicular precipices. The littoral caves, also, will be familiar to
those who have had opportunities of observing the manner in which the waves
of the sea, when they beat against rocks, have power to scoop out caverns.
As to the breccia, it is composed of pieces of limestone and rolled
fragments of thick solid shell, such as _Strombus_ and _Spondylus_, all
bound together by a crystalline calcareous cement. Similar aggregations are
now forming on the modern beaches of Greece, and in caverns on the
sea-side; and they are only distinguishable in character from those of more
ancient date, by including many pieces of pottery. In regard to the
_lithodomi_ above alluded to, these bivalve mollusks are well known to have
the power of excavating holes in the hardest limestones, the size of the
cavity keeping pace with the growth of the shell. When living they require
to be always covered by salt water, but similar pear-shaped hollows,
containing the dead shells of these creatures, are found at different
heights on the face of the inland cliffs above mentioned. Thus, for
example, they have been observed near Modon and Navarino on cliffs in the
interior 125 feet high above the Mediterranean. As to the weathered surface
of the calcareous rocks, all limestones are known to suffer chemical
decomposition when moistened by the spray of the salt water, and are
corroded still more deeply at points lower down where they are just reached
by the breakers. By this action the stone acquires a wrinkled and furrowed
outline, and very near the sea it becomes rough and branching, as if
covered with corals. Such effects are traced not only on the present shore,
but at the base of the ancient cliffs far in the interior. Lastly, it
remains only to speak of the terraces, which extend with a gentle slope
from the base of almost all the inland cliffs, and are for the most part
narrow where the rock is hard, but sometimes half a mile or more in breadth
where it is soft. They are the effects of the encroachment of the ancient
sea upon the shore at those levels at which the land remained for a long
time stationary. The justness of this view is apparent on examining the
shape of the modern shore wherever the sea is advancing upon the land, and
removing annually small portions of undermined rock. By this agency a
submarine platform is produced on which we may walk for some distance from
the beach in shallow water, the increase of depth being very gradual, until
we reach a point where the bottom plunges down suddenly. This platform is
widened with more or less rapidity according to the hardness of the rocks,
and when upraised it constitutes an inland terrace.
But the four principal lines of cliff observed in the Morea do not imply,
as some have imagined, four great eras of sudden upheaval; they simply
indicate the intermittence of the upheaving force. Had the rise of the land
been continuous and uninterrupted, there would have been no one prominent
line of cliff; for every portion of the surface having been, in its turn,
and for an equal period of time, a sea-shore, would have presented a nearly
similar aspect. But if pauses occur in the process of upheaval, the waves
and currents have time to sap, throw down, and clear away considerable
masses of rock, and to shape out at certain levels lofty ranges of cliffs
with broad terraces at their base.
There are some levelled spaces, however, both ancient and modern, in the
Morea, which are not due to denudation, although resembling in outline
the terraces above described. They may be called Terraces of Deposition,
since they have resulted from the gain of land upon the sea where rivers
and torrents have produced deltas. If the sedimentary matter has filled
up a bay or gulf surrounded by steep mountains, a flat plain is formed
skirting the inland precipices; and if these deposits are upraised,
they form a feature in the landscape very similar to the areas of
denudation before described.
In the island of Sicily I have examined many inland cliffs like those of
the Morea; as, for example, near Palermo, where a precipice is seen
consisting of limestone at the base of which are numerous caves. One of
these called San Ciro, about 2 miles distant from Palermo, is about 20 feet
high, 10 wide, and 180 above the sea. Within it is found an ancient beach
(_b_, fig. 93.), formed of pebbles of various rocks, many of which must
have come from places far remote. Broken pieces of coral and shell,
especially of oysters and pectens, are seen intermingled with the pebbles.
Immediately above the level of this beach, _serpulae_ are still found
adhering to the face of the rock, and the limestone is perforated by
_lithodomi_. Within the grotto, also, at the same level, similar
perforations occur; and so numerous are the holes, that the rock is
compared by Hoffmann to a target pierced by musket balls. But in order to
expose to view these marks of boring-shells in the interior of the cave, it
was necessary first to remove a mass of breccia, which consisted of
numerous fragments of rock and an immense quantity of bones of the mammoth,
hippopotamus, and other quadrupeds, imbedded in a dark brown calcareous
marl. Many of the bones were rolled as if partially subjected to the action
of the waves. Below this breccia, which is about 20 feet thick, was found a
bed of sand filled with sea-shells of recent species; and underneath the
sand, again, is the secondary limestone of Monte Grifone. The state of the
surface of the limestone in the cave above the level of the marine sand is
very different from that below it. _Above_, the rock is jagged and uneven,
as is usual in the roofs and sides of limestone caverns; _below_, the
surface is smooth and polished, as if by the attrition of the waves.
[Illustration: Fig. 93. Cross section.
_a._ Monte Grifone.
_b._ Cave of San Ciro.[75-A]
_c._ Plain of Palermo, in which are Newer Pliocene strata of
limestone and sand.
_d._ Bay of Palermo.]
The platform indicated at _c_, fig. 93., is formed by a tertiary deposit
containing marine shells almost all of living species, and it affords an
illustration of the terrace of deposition, or the last of the two kinds
before mentioned (p. 74.).
There are also numerous instances in Sicily of terraces of denudation. One
of these occurs on the east coast to the north of Syracuse, and the same is
resumed to the south beyond the town of Noto, where it may be traced
forming a continuous and lofty precipice, _a b_, fig. 94., facing towards
the sea, and constituting the abrupt termination of a calcareous formation,
which extends in horizontal strata far inland. This precipice varies in
height from 500 to 700 feet, and between its base and the sea is an
inferior platform, _c b_, consisting of similar white limestone. All the
beds dip towards the sea, but are usually inclined at a very slight angle:
they are seen to extend uninterruptedly from the base of the escarpment
into the platform, showing distinctly that the lofty cliff was not produced
by a fault or vertical shift of the beds, but by the removal of a
considerable mass of rock. Hence we may conclude that the sea, which is now
undermining the cliffs of the Sicilian coast, reached at some former
period the base of the precipice _a b_, at which time the surface of the
terrace _c b_ must have been covered by the Mediterranean. There was a
pause, therefore, in the upward movement, when the waves of the sea had
time to carve out the platform _c b_; but there may have been many other
stationary periods of minor duration. Suppose, for example, that a series
of escarpments _e_, _f_, _g_, _h_, once existed, and that the sea, during a
long interval free from subterranean movements, advances along the line _c
b_, all preceding cliffs must have been swept away one after the other, and
reduced to the single precipice _a b_.
[Illustration: Fig. 94. Cross section.]
[Illustration: Fig. 95. Valley called Gozzo degli Martiri, below
Melilli, Val di Noto.]
That such a series of smaller cliffs, as those represented at _e_, _f_,
_g_, _h_, fig. 94., did really once exist at intermediate heights in place
of the single precipice _a b_, is rendered highly probable by the fact,
that in certain bays and inland valleys opening towards the east coast of
Sicily, and not far from the section given in fig. 94., the solid limestone
is shaped out into a great succession of ledges, separated from each other
by small vertical cliffs. These are sometimes so numerous, one above the
other, that where there is a bend at the head of a valley, they produce an
effect singularly resembling the seats of a Roman amphitheatre. A good
example of this configuration occurs near the town of Melilli, as seen in
the annexed view (fig. 95.). In the south of the island, near Spaccaforno,
Scicli, and Modica, precipitous rocks of white limestone, ascending to the
height of 500 feet, have been carved out into similar forms.
[Illustration: Fig. 96. Cross section.]
This appearance of a range of marble seats circling round the head of a
valley, or of great flights of steps descending from the top to the bottom,
on the opposite sides of a gorge, may be accounted for, as already hinted,
by supposing the sea to have stood successively at many different levels,
as at _a a_, _b b_, _c c_, in the accompanying fig. 96. But the causes of
the gradual contraction of the valley from above downwards may still be
matter of speculation. Such contraction may be due to the greater force
exerted by the waves when the land at its first emergence was smaller in
quantity, and more exposed to denudation in an open sea; whereas the wear
and tear of the rocks might diminish in proportion as this action became
confined within bays or channels closed in on two or three sides. Or,
secondly, the separate movements of elevation may have followed each other
more rapidly as the land continued to rise, so that the times of those
pauses, during which the greatest denudation was accomplished at certain
levels, were always growing shorter. It should be remarked, that the cliffs
and small terraces are rarely found on the opposite sides of the Sicilian
valleys at heights so precisely answering to each other as those given in
fig. 96., and this might have been expected, to whichever of the two
hypotheses above explained we incline; for, according to the direction of
the prevailing winds and currents, the waves may beat with unequal force on
different parts of the shore, so that while no impression is made on one
side of a bay, the sea may encroach so far on the other as to unite several
smaller cliffs into one.
Before quitting the subject of ancient sea-cliffs, carved out of
limestone, I shall mention the range of precipitous rocks, composed of a
white marble of the Oolitic period, which I have seen near the northern
gate of St. Mihiel in France. They are situated on the right bank of the
Meuse, at a distance of 200 miles from the nearest sea, and they present
on the precipice facing the river three or four horizontal grooves, one
above the other, precisely resembling those which are scooped out by the
undermining waves. The summits of several of these masses are detached
from the adjoining hill, in which case the grooves pass all round them,
facing towards all points of the compass, as if they had once formed
rocky islets near the shore.[78-A]
Captain Bayfield, in his survey of the Gulf of St. Lawrence, discovered in
several places, especially in the Mingan islands, a counterpart of the
inland cliffs of St. Mihiel, and traced a succession of shingle beaches,
one above the other, which agreed in their level with some of the principal
grooves scooped out of the limestone pillars. These beaches consisted of
calcareous shingle, with shells of recent species, the farthest from the
shore being 60 feet above the level of the highest tides. In addition to
the drawings of the pillars called the flower-pots, which he has
published[78-B], I have been favoured with other views of rocks on the same
coast, drawn by Lieut. A. Bowen, R. N. (See fig. 97.)
[Illustration: Fig. 97. Limestone columns in Niapisca Island, in the Gulf
of St. Lawrence. Height of the second column on the left, 60 feet.]
In the North-American beaches above mentioned rounded fragments of
limestone have been found perforated by _lithodomi_; and holes drilled
by the same mollusks have been detected in the columnar rocks or
"flower-pots," showing that there has been no great amount of
atmospheric decomposition on the surface, or the cavities alluded
to would have disappeared.
[Illustration: Fig. 98. The North Rocks, Bermuda, lying outside the
great coral reef. A. 16 feet high, and B. 12 feet. _c._ _c._ Hollows
worn by the sea.]
We have an opportunity of seeing in the Bermuda islands the manner in
which the waves of the Atlantic have worn, and are now wearing out, deep
smooth hollows on every side of projecting masses of hard limestone. In the
annexed drawing, communicated to me by Lieut. Nelson, the excavations _c_,
_c_, _c_, have been scooped out by the waves in a stone of very modern
date, which, although extremely hard, is full of recent corals and shells,
some of which retain their colour.
When the forms of these horizontal grooves, of which the surface is
sometimes smooth and almost polished, and the roofs of which often overhang
to the extent of 5 feet or more, have been carefully studied by geologists,
they will serve to testify the former action of the waves at innumerable
points far in the interior of the continents. But we must learn to
distinguish the indentations due to the original action of the sea, and
those caused by subsequent chemical decomposition of calcareous rocks, to
which they are liable in the atmosphere.
Notwithstanding the enduring nature of the marks left by littoral action on
calcareous rocks, we can by no means detect sea-beaches and inland cliffs
everywhere, even in Sicily and the Morea. On the contrary, they are, upon
the whole, extremely partial, and are often entirely wanting in districts
composed of argillaceous and sandy formations, which must, nevertheless,
have been upheaved at the same time, and by the same intermittent
movements, as the adjoining calcareous rocks.
FOOTNOTES:
[67-A] Western Islands, vol. ii. p. 93. pl. 31. fig. 4.
[69-A] See Mammat's Geological Facts, &c. p. 90. and plate.
[69-B] Conybeare's Report to Brit. Assoc. 1842, p. 381.
[70-A] Prestwich, Geol. Trans. second series, vol. v. pp. 452. 473.
[75-A] Section given by Dr. Christie, Edin. New Phil. Journ. No.
xxiii., called by mistake the Cave of Mardolce, by the late M. Hoffmann.
See account by Mr. S. P. Pratt, F. G. S. Proceedings of Geol. Soc.
No. 32. 1833.
[78-A] I was directed by M. Deshayes to this spot, which I visited
in June, 1833.
[78-B] See Trans. of Geol. Soc., second series, vol. v. plate v.