REPRODUCTIVE EFFECTS OF TIDES AND CURRENTS.
Estuaries, how formed--Silting up of estuaries does not compensate
the loss of land on the borders of the ocean--Bed of the German
Ocean--Composition and extent of its sand-banks--Strata deposited by
currents in the English channel--On the shores of the
Mediterranean--At the mouths of the Amazon, Orinoco, and
Mississippi--Wide area over which strata may be formed by this
cause.
From the facts enumerated in the last chapter, it appears that on the
borders of the ocean, currents and tides co-operating with the waves of
the sea are most powerful instruments in the destruction and
transportation of rocks; and as numerous tributaries discharge their
alluvial burden into the channel of one great river, so we find that
many rivers deliver their earthy contents to one marine current, to be
borne by it to a distance, and deposited in some deep receptacle of the
ocean. The current, besides receiving this tribute of sedimentary matter
from streams draining the land, acts also itself on the coast, as does a
river on the cliffs which bound a valley. Yet the waste of cliffs by
marine currents constitutes on the whole a very insignificant portion of
the denudation annually effected by aqueous causes, as I shall point out
in the sequel of this chapter (p. 339).
In inland seas, where the tides are insensible, or on those parts of the
borders of the ocean where they are feeble, it is scarcely possible to
prevent a harbor at a river's mouth from silting up; for a bar of sand
or mud is formed at points where the velocity of the turbid river is
checked by the sea, or where the river and a marine current neutralize
each other's force. For the current, as we have seen, may, like the
river, hold in suspension a large quantity of sediment, or, co-operating
with the waves, may cause the progressive motion of a shingle beach in
one direction. I have already alluded to the erection of piers and
groins at certain places on our southern coast, to arrest the course of
the shingle and sand (see p. 318). The immediate effect of these
temporary obstacles is to cause a great accumulation of pebbles on one
side of the barrier, after which the beach still moves on round the end
of the pier at a greater distance from the land. This system, however,
is often attended with a serious evil, for during storms the waves throw
suddenly into the harbor the vast heap of pebbles which have collected
for years behind the groin or pier, as happened during a great gale
(Jan. 1839) at Dover.
The formation and keeping open of large estuaries are due to the
_combined influence_ of tidal currents and rivers; for when the tide
rises, a large body of water suddenly enters the mouth of the river,
where, becoming confined within narrower bounds, while its momentum is
not destroyed, it is urged on, and, having to pass through a contracted
channel, rises and runs with increased velocity, just as a stream when
it reaches the arch of a bridge scarcely large enough to give passage to
its waters, rushes with a steep fall through the arch. During the ascent
of the tide, a body of fresh water, flowing down in an opposite
direction from the higher country, is arrested in its course for several
hours; and thus a large lake of fresh and brackish water is accumulated,
which, when the sea ebbs, is let loose, as on the removal of an
artificial sluice or dam. By the force of this retiring water, the
alluvial sediment both of the river and of the sea is swept away, and
transported to such a distance from the mouth of the estuary, that a
small part only can return with the next tide.
It sometimes happens, that during a violent storm a large bar of sand is
suddenly made to shift its position, so as to prevent the free influx of
the tides, or efflux of river water. Thus about the year 1500 the sands
at Bayonne were suddenly thrown across the mouth of the Adour. That
river, flowing back upon itself, soon forced a passage to the northward
along the sandy plain of Capbreton, till at last it reached the sea at
Boucau, at the distance of _seven leagues_ from the point where it had
formerly entered. It was not till the year 1579 that the celebrated
architect Louis de Foix undertook, at the desire of Henry III., to
reopen the ancient channel, which he at last effected with great
difficulty.[461]
In the estuary of the Thames at London, and in the Gironde, the tide
rises only for five hours and ebbs seven, and in all estuaries the water
requires a longer time to run down than up; so that the preponderating
force is always in the direction which tends to keep open a deep and
broad passage. But for reasons already explained, there is naturally a
tendency in all estuaries to silt up partially, since eddies, and
backwaters, and points where opposing streams meet, are very numerous,
and constantly change their position.
Many writers have declared that the gain on our eastern coast, since the
earliest periods of history, has more than counterbalanced the loss; but
they have been at no pains to calculate the amount of loss, and have
often forgotten that, while the new acquisitions are manifest, there are
rarely any natural monuments to attest the former existence of the land
that has been carried away. They have also taken into their account
those tracts artificially recovered, which are often of great
agricultural importance, and may remain secure, perhaps, for thousands
of years, but which are only a few feet above the mean level of the sea,
and are therefore exposed to be overflowed again by a small proportion
of the force required to move cliffs of considerable height on our
shores. If it were true that the area of land annually abandoned by the
sea in estuaries were equal to that invaded by it, there would still be
no compensation _in kind_.
The tidal current which flows out from the northwest, and bears against
the eastern coast of England, transports, as we have seen, materials of
various kinds. Aided by the waves, it undermines and sweeps away the
granite, gneiss, trap-rocks, and sandstone of Shetland, and removes the
gravel and loam of the cliffs of Holderness, Norfolk, and Suffolk, which
are between twenty and three hundred feet in height, and which waste at
various rates of from one foot to six yards annually. It also bears
away, in co-operation with the Thames and the tides, the strata of
London clay on the coast of Essex and Sheppey. The sea at the same time
consumes the chalk with its flints for many miles continuously on the
shores of Kent and Sussex--commits annual ravages on the freshwater
beds, capped by a thick covering of chalk-flint gravel, in Hampshire,
and continually saps the foundations of the Portland limestone. It
receives, besides, during the rainy months, large supplies of pebbles,
sand, and mud, which numerous streams from the Grampians, Cheviots, and
other chains, send down to the sea. To what regions, then, is all this
matter consigned? It is not retained in mechanical suspension by the
waters of the ocean, nor does it mix with them in a state of chemical
solution--it is deposited _somewhere_, yet certainly not in the
immediate neighborhood of our shores; for, in that case, there would
soon be a cessation of the encroachment of the sea, and large tracts of
low land, like Romney Marsh, would almost everywhere encircle our
island.
As there is now a depth of water exceeding thirty feet, in some spots
where towns like Dunwich flourished but a few centuries ago, it is clear
that the current not only carries far away the materials of the wasted
cliffs, but is capable also of excavating the bed of the sea to a
certain moderate depth.
So great is the quantity of matter held in suspension by the tidal
current on our shores, that the waters are in some places artificially
introduced into certain lands below the level of the sea; and by
repeating this operation, which is called "warping," for two or three
years, considerable tracts have been raised, in the estuary of the
Humber, to the height of about six feet. If a current, charged with such
materials, meets with deep depressions in the bed of the ocean, it must
often fill them up; just as a river, when it meets with a lake in its
course, fills it gradually with sediment.
I have said (p. 337) that the action of the waves and currents on
sea-cliffs, or their power to remove matter from above to below the
sea-level, is insignificant in comparison with the power of rivers to
perform the same task. As an illustration we may take the coast of
Holderness described in the last chapter (p. 304). It is composed, as we
have seen, of very destructible materials, is thirty-six miles long, and
its average height may be taken at forty feet. As it has wasted away at
the rate of two and a quarter yards annually, for a long period, it will
be found on calculation that the quantity of matter thrown down into the
sea every year, and removed by the current, amounts to 51,321,600 cubic
feet. It has been shown that the united Ganges and Brahmapootra carry
down to the Bay of Bengal 40,000,000,000 of cubic feet of solid matter
every year, so that their transporting power is no less than 780 times
greater than that of the sea on the coast above-mentioned; and in order
to produce a result equal to that of the two Indian rivers, we must have
a line of wasting coast, like that of Holderness, nearly 28,000 miles in
length, or longer than the entire circumference of the globe by above
3000 miles. The reason of so great a difference in the results may be
understood when we reflect that the operations of the ocean are limited
to a single line of cliff surrounding a large area, whereas great rivers
with their tributaries, and the mountain torrents which flow into them,
act simultaneously on a length of bank almost indefinite.
Nevertheless we are by no means entitled to infer, that the denuding
force of the great ocean is a geological cause of small efficacy, or
inferior to that of rivers. Its chief influence is exerted at moderate
depths below the surface, on all those areas which are slowly rising, or
are attempting, as it were, to rise above the sea. From data hitherto
obtained respecting subterranean movements, we can scarcely speculate on
an average rate of upheaval of more than two or three feet in a century.
An elevation to this amount is taking place in Scandinavia, and probably
in many submarine areas as vast as those which we know to be sinking
from the proofs derived from circular lagoon islands or coral atolls.
(See chap. 50.) Suppose strata as destructible as those of the Wealden,
or the lower and upper cretaceous formation, or the tertiary deposits of
the British Isles to be thus slowly upheaved, how readily might they all
be swept away by waves and currents in an open sea! How entirely might
each stratum disappear as it was brought up successively and exposed to
the breakers! Shoals of wide extent might be produced, but it is
difficult to conceive how any continent could ever be formed under such
circumstances. Were it not indeed for the hardness and toughness of the
crystalline and volcanic rocks, which are often capable of resisting the
action of the waves, few lands might ever emerge from the midst of an
open sea.
_Supposed filling up of the German Ocean._--The German Ocean is deepest
on the Norwegian side, where the soundings give 190 fathoms; but the
mean depth of the whole basin may be stated at no more than thirty-one
fathoms.[462] The bed of this sea is traversed by several enormous
banks, the greatest of which is the Dogger Bank, extending for upwards
of 354 miles from north to south. The whole superficies of these shoals
is equal to about one-third of the whole extent of England and Scotland.
The average height of the banks measures, according to Mr. Stevenson,
about seventy-eight feet; the upper portion of them consisting of fine
and coarse siliceous sand, mixed with comminuted corals and shells.[463]
It had been supposed that these vast submarine hills were made up bodily
of loose materials supplied from the waste of the English, Dutch, and
other coasts; but the survey of the North Sea, conducted by Captain
Hewett, affords ground for suspecting this opinion to be erroneous. If
such immense mounds of sand and mud had been accumulated under the
influence of currents, the same causes ought nearly to have reduced to
one level the entire bottom of the German Ocean; instead of which some
long narrow ravines are found to intersect the banks. One of these
varies from seventeen to forty-four fathoms in depth, and has very
precipitous sides; in one part, called the "Inner Silver Pits," it is
fifty-five fathoms deep. The shallowest parts of the Dogger Bank were
found to be forty-two feet under water, except in one place, where the
wreck of a ship had caused a shoal. Such uniformity in the minimum depth
of water seems to imply that the currents, which vary in their velocity
from a mile to two miles and a half per hour, have power to prevent the
accumulation of drift matter in places of less depth.
_Strata deposited by currents._--It appears extraordinary, that in some
tracts of the sea, adjoining the coast of England, where we know that
currents are not only sweeping along rocky masses, thrown down, from
time to time, from the high cliffs, but also occasionally scooping out
channels in the regular strata, there should exist fragile shells and
tender zoophytes in abundance, which live uninjured by these violent
movements. The ocean, however, is in this respect a counterpart of the
land; and as, on the continents, rivers may undermine their banks,
uproot trees, and roll along sand and gravel, while their waters are
inhabited by testacea and fish, and their alluvial plains are adorned
with rich vegetation and forests, so the sea may be traversed by rapid
currents, and its bed may here and there suffer great local derangement,
without any interruption of the general order and tranquillity. It has
been ascertained by soundings in all parts of the world, that where new
deposits are taking place in the sea, coarse sand and small pebbles
commonly occur near the shore, while farther from land, and in deeper
water, finer sand and broken shells are spread out over the bottom.
Still farther out, the finest mud and ooze are alone met with. Mr.
Austen observes that this rule holds good in every part of the English
Channel examined by him. He also informs us, that where the tidal
current runs rapidly in what are called "races," where surface
undulations are perceived in the calmest weather, over deep banks, the
discoloration of the water does not arise from the power of such a
current to disturb the bottom at a depth of 40 or 80 fathoms, as some
have supposed. In these cases, a column of water sometimes 500 feet in
height, is moving onwards with the tide clear and transparent above,
while the lower portion holds fine sediment in suspension (a fact
ascertained by soundings), when suddenly it impinges upon a bank, and
its height is reduced to 300 feet. It is thus made to boil up and flow
off at the surface, a process which forces up the lower strata of water
charged with fine particles of mud, which in their passage from the
coast had gradually sunk to a depth of 300 feet or more.[464]
One important character in the formations produced by currents is, the
immense extent over which they may be the means of diffusing homogeneous
mixtures, for these are often coextensive with a great line of coast;
and, by comparison with their deposits, the deltas of rivers must shrink
into significance. In the Mediterranean, the same current which is
rapidly destroying many parts of the African coast, between the Straits
of Gibraltar and the Nile, checks also the growth of the delta of the
Nile, and drifts the sediment of that great river to the eastward. To
this source may be attributed the rapid accretions of land on parts of
the Syrian shores where rivers do not enter.
Among the greatest deposits now in progress, and of which the
distribution is chiefly determined by currents, we may class those
between the mouths of the Amazon and the southern coast of North
America. Captain Sabine found that the equatorial current before
mentioned (p. 292) was running with the rapidity of four miles an hour
where it crosses the stream of the Amazon, which river preserves part of
its original impulse, and has its waters not wholly mingled with those
of the ocean at the distance of 300 miles from its mouth.[465] The
sediment of the Amazon is thus constantly carried to the northwest as
far as to the mouths of the Orinoco, and an immense tract of swamp is
formed along the coast of Guiana, with a long range of muddy shoals
bordering the marshes, and becoming converted into land.[466] The
sediment of the Orinoco is partly detained, and settles near its mouth,
causing the shores of Trinidad to extend rapidly, and is partly swept
away into the Carribean Sea by the Guinea current. According to
Humboldt, much sediment is carried again out of the Carribean Sea into
the Gulf of Mexico.
It should not be overlooked that marine currents, even on coasts where
there are no large rivers, may still be the agents of spreading not only
sand and pebbles, but the finest mud, far and wide over the bottom of
the ocean. _For several thousand miles_ along the western coast of South
America, comprising the larger parts of Peru and Chili, there is a
perpetual rolling of shingle along the shore, part of which, as Mr.
Darwin has shown, are incessantly reduced to the finest mud by the
waves, and swept into the depths of the Pacific by the tides and
currents. The same author however has remarked that, notwithstanding the
great force of the waves on that shore, all rocks 60 feet under water
are covered by sea-weed, showing that the bed of the sea is not denuded
at that depth, the effects of the winds being comparatively superficial.
In regard to the distribution of sediment by currents it may be
observed, that the rate of subsidence of the finer mud carried down by
every great river into the ocean, or of that caused by the rolling of
the waves upon a shore, must be extremely slow; for the more minute the
separate particles of mud, the slower will they sink to the bottom, and
the sooner will they acquire what is called their terminal velocity. It
is well known that a solid body, descending through a resisting medium,
falls by the force of gravity, which is constant, but its motion is
resisted by the medium more and more as its velocity increases, until
the resistance becomes sufficient to counteract the farther increase of
velocity. For example, a leaden ball, one inch diameter, falling through
air of density as at the earth's surface, will never acquire greater
velocity than 260 feet per second, and, in water, its greatest velocity
will be 8 feet 6 inches per second. If the diameter of the ball were
1/100 of an inch, the terminal velocities in air would be 26 feet, and
in water .86 of a foot per second.
Now, every chemist is familiar with the fact, that minute particles
descend with extreme slowness through water, the extent of their surface
being very great in proportion to their weight, and the resistance of
the fluid depending on the amount of surface. A precipitate of sulphate
of baryta, for example, will sometimes require more than five or six
hours to subside one inch;[467] while oxalate and phosphate of lime
require nearly an hour to subside about an inch and a half and two
inches respectively,[468] so exceedingly small are the particles of
which these substances consist.
When we recollect that the depth of the ocean is supposed frequently to
exceed three miles, and that currents run through different parts of
that ocean at the rate of four miles an hour, and when at the same time
we consider that some fine mud carried away from the mouths of rivers
and from sea-beaches, where there is a heavy surf, as well as the
impalpable powder showered down by volcanoes, may subside at the rate of
only an inch per hour, we shall be prepared to find examples of the
transportation of sediment over areas of indefinite extent.
It is not uncommon for the emery powder used in polishing glass to take
more than an hour to sink one foot. Suppose mud composed of coarser
particles to fall at the rate of two feet per hour, and these to be
discharged into that part of the Gulf Stream which preserves a mean
velocity of three miles an hour for a distance of two thousand miles; in
twenty-eight days these particles will be carried 2016 miles, and will
have fallen only to a depth of 224 fathoms.
In this example, however, it is assumed that the current retains its
superficial velocity at the depth of 224 fathoms, for which we have as
yet no data, although we have seen that the motion of a current may
continue at the depth of 100 fathoms. (See above, p. 28.) Experiments
should be made to ascertain the rate of currents at considerable
distances from the surface, and the time taken by the finest sediment to
settle in sea-water of a given depth, and then the geologist may
determine the area over which homogeneous mixtures may be simultaneously
distributed in certain seas.