REPRODUCTIVE EFFECTS OF RIVERS--_continued_.
Deltas formed under the influence of tides--Basin and delta of the
Mississippi--Alluvial plain--River-banks and bluffs--Curves of the
river--Natural rafts and snags--New lakes, and effects of
earthquakes--Antiquity of the delta--Delta of the Ganges and
Brahmapootra--Head of the delta and Sunderbunds--Islands formed and
destroyed--Crocodiles--Amount of fluviatile sediment in the
water--Artesian boring at Calcutta--Proofs of subsidence--Age of the
delta--Convergence of deltas--Origin of existing deltas not
contemporaneous--Grouping of strata and stratification in
deltas--Conglomerates--Constant interchange of land and sea.
In the last chapter several examples were given of the deltas of inland
seas, where the influence of the tides is almost imperceptible. We may
next consider those marine or oceanic deltas, where the tides play an
important part in the dispersion of fluviatile sediment, as in the Gulf
of Mexico, where they exert a moderate degree of force, and then in the
Bay of Bengal, where they are extremely powerful. In regard to
estuaries, which Rennel termed "negative deltas," they will be treated
of more properly when our attention is specially turned to the
operations of tides and currents (chapters 20, 21, and 22). In this
case, instead of the land gaining on the sea at the river's mouth, the
tides penetrate far inland beyond the general coast-line.
BASIN AND DELTA OF THE MISSISSIPPI.
_Alluvial plain._--The hydrographical basin of the Mississippi displays,
on the grandest scale, the action of running water on the surface of a
vast continent. This magnificent river rises nearly in the forty-ninth
parallel of north latitude, and flows to the Gulf of Mexico in the
twenty-ninth--a course, including its meanders, of more than three
thousand miles. It passes from a cold climate, where the hunter obtains
his furs and peltries, traverses the temperate latitudes, and discharges
its waters into the sea in the region of rice, the cotton plant, and the
sugar-cane. From near its mouth at the Balize a steamboat may ascend for
2000 miles with scarcely any perceptible difference in the width of the
river. Several of its tributaries, the Red River, the Arkansas, the
Missouri, the Ohio, and others, would be regarded elsewhere as of the
first importance, and, taken together, are navigable for a distance many
times exceeding that of the main stream. No river affords a more
striking illustration of the law before mentioned, that an augmentation
of volume does not occasion a proportional increase of surface, nay, is
even sometimes attended with a narrowing of the channel. The Mississippi
is half a mile wide at its junction with the Missouri, the latter being
also of equal width; yet the united waters have only, from their
confluence to the mouth of the Ohio, a medial width of about half a
mile. The junction of the Ohio seems also to produce no increase, but
rather a decrease, of surface.[356] The St. Francis, White, Arkansas,
and Red rivers are also absorbed by the main stream with scarcely any
apparent increase of its width, although here and there it expands to a
breadth of 1-1/2, or even to 2 miles. On arriving at New Orleans, it is
somewhat less than half a mile wide. Its depth there is very variable,
the greatest at high water being 168 feet. The mean rate at which the
whole body of water flows is variously estimated; according to Mr.
Forshey the mean velocity of the current at the surface, somewhat
exceeds 2-1/4 miles an hour when the water is at a mean height. For 300
miles above New Orleans the distance measured by the winding river is
about twice as great as the distance in a right line. For the first 100
miles from the mouth the rate of fall is 1.80 inch per mile, for the
second hundred 2 inches, for the third 2.30, for the fourth 2.57.
The alluvial plain of the Mississippi begins to be of great width below
Cape Girardeau, 50 miles above the junction of the Ohio. At this
junction it is about 50 miles broad, south of which it contracts to
about 30 miles at Memphis, expands again to 80 miles at the mouth of the
White River, and then, after various contractions and expansions,
protrudes beyond the general coast-line, in a large delta, about 90
miles in width, from N. E. to S. W. Mr. Forshey estimates the area of
the great plain as above defined at 31,200 square miles, with a
circumference of about 3000 miles, exceeding the area of Ireland. If
that part of this plain which lies below, or to the south of the
branching off of the highest arm, called the Atchafalaya, be termed the
delta, it constitutes less than half of the whole, being 14,000 square
British miles in area. The delta may be said to be bounded on the east,
west, and south by the sea; on the north chiefly by the broad
valley-plain which entirely resembles it in character as in origin. The
east and west boundaries of the alluvial region above the head of the
delta consists of cliffs or bluffs, which on the east side of the
Mississippi are very abrupt, and are undermined by the river at many
points. They consist, from Baton Rouge in Louisiana, where they
commence, as far north as the borders of Kentucky, of geological
formations newer than the cretaceous, the lowest being Eocene, and the
uppermost consisting of loam, resembling the loess of the Rhine, and
containing freshwater and land shells almost all of existing species.
(See fig. 23.) These recent shells are associated with the bones of the
mastodon, elephant, tapir, mylodon, horse, ox, and other quadrupeds,
most of them of extinct species.
I have endeavored to show in my Second Visit to the United States, that
this extensive formation of loam is either an ancient alluvial plain or
a delta of the great river, formed originally at a lower level, and
since upheaved, and partially denuded.
[Illustration: Fig. 23.
VALLEY OF THE MISSISSIPPI.]
The Mississippi in that part of its course which is below the mouth of
the Ohio, frequently washes the eastern bluffs, but never once comes in
contact with the western. These are composed of similar formations; but
I learn from Mr. Forshey that they rise up more gently from the alluvial
plain (as at _a_, fig. 23). It is supposed that the waters are thrown to
the eastern side, because all the large tributary rivers entering from
the west have filled that side of the great valley with their deltas, or
with a sloping mass of clay and sand; so that the opposite bluffs are
undermined, and the Mississippi is slowly but incessantly advancing
eastward.[357]
_Curves of the Mississippi._--The river traverses the plain in a
meandering course, describing immense curves. After sweeping round the
half of a circle, it is carried in a rapid current diagonally across the
ordinary direction of its channel, to another curve of similar shape.
Opposite to each of these, there is always a sand-bar, answering, in the
convexity of its form, to the concavity of "the bend," as it is
called.[358] The river, by continually wearing these curves deep,
returns, like many other streams before described, on its own track, so
that a vessel in some places, after sailing for twenty-five or thirty
miles, is brought round again to within a mile of the place whence it
started. When the waters approach so near to each other, it often
happens at high floods that they burst through the small tongue of
land, and insulate a portion, rushing through what is called the
"cut-off," so that vessels may pass from one point to another in half a
mile to a distance which it previously required a voyage of twenty miles
to reach. As soon as the river has excavated the new passage, bars of
sand and mud are formed at the two points of junction with the old bend,
which is soon entirely separated from the main river by a continuous
mud-bank covered with wood. The old bend then becomes a semicircular
lake of clear water, inhabited by large gar-fish, alligators, and wild
fowl, which the steam-boats have nearly driven away from the main
river. A multitude of such crescent-shaped lakes, scattered far and wide
over the alluvial plain, the greater number of them to the west, but
some of them also eastward of the Mississippi, bear testimony of the
extensive wanderings of the great stream in former ages. For the last
two hundred miles above its mouth the course of the river is much less
winding than above, there being only in the whole of that distance one
great curve, that called the "English Turn." This great straightness of
the stream is ascribed by Mr. Forshey to the superior tenacity of the
banks, which are more clayey in this region.
[Illustration: Fig. 24.
Section of channel, bank, levees (_a_ and _b_), and swamps of
Mississippi river.]
The Mississippi has been incorrectly described by some of the earlier
geographers, as a river running along the top of a long hill, or mound
in a plain. In reality it runs in a valley, from 100 to 200 or more feet
in depth, as _a_, _c_, _b_, fig. 24, its banks forming long strips of
land parallel to the course of the main stream, and to the swamps _g_,
_f_, and _d_, _e_, lying on each side. These extensive morasses, which
are commonly well-wooded, though often submerged for months
continuously, are rarely more than fifteen feet below the summit level
of the banks. The banks themselves are occasionally overflowed, but are
usually above water for a breadth of about two miles. They follow all
the curves of the great river, and near New Orleans are raised
artificially by embankments (or levees), _a b_, fig. 24, through which
the river when swollen sometimes cuts a deep channel (or crevasse),
inundating the adjoining low lands and swamps, and not sparing the lower
streets of the great city.
The cause of the uniform upward slope of the river-bank above the
adjoining alluvial plain is this: when the waters charged with sediment
pass over the banks in the flood season, their velocity is checked among
the herbage and reeds, and they throw down at once the coarser and more
sandy matter with which they are charged. But the fine particles of mud
are carried farther on, so that at the distance of about two miles, a
thin film of fine clay only subsides, forming a stiff unctuous black
soil, which gradually envelops the base of trees growing on the borders
of the swamps.
_Waste of the banks._--It has been said of a mountain torrent, that "it
lays down what it will remove, and removes what it has laid down;" and
in like manner the Mississippi, by the continual shifting of its course,
sweeps away, during a great portion of the year, considerable tracts of
alluvium, which were gradually accumulated by the overflow of former
years, and the matter now left during the spring-floods will be at some
future time removed. After the flood season, when the river subsides
within its channel, it acts with destructive force upon the alluvial
banks, softened and diluted by the recent overflow. Several acres at a
time, thickly covered with wood, are precipitated into the stream; and
large portions of the islands are frequently swept away.
"Some years ago," observes Captain Hall, "when the Mississippi was
regularly surveyed, all its islands were numbered, from the confluence
of the Missouri to the sea; but every season makes such revolutions, not
only in the number, but in the magnitude and situation of these islands,
that this enumeration is now almost obsolete. Sometimes large islands
are entirely melted away; at other places they have attached themselves
to the main shore, or, which is the more correct statement, the interval
has been filled up by myriads of logs cemented together by mud and
rubbish."[359]
_Rafts._--One of the most interesting features in the great rivers of
this part of America is the frequent accumulation of what are termed
"rafts," or masses of floating trees, which have been arrested in their
progress by snags, islands, shoals, or other obstructions, and made to
accumulate, so as to form natural bridges, reaching entirely across the
stream. One of the largest of these was called the raft of the
Atchafalaya, an arm of the Mississippi, which was certainly at some
former time the channel of the Red River, when the latter found its way
to the Gulf of Mexico by a separate course. The Atchafalaya being in a
direct line with the general direction of the Mississippi, catches a
large portion of the timber annually brought down from the north; and
the drift-trees collected in about thirty-eight years previous to 1816
formed a continuous raft, no less than ten miles in length, 220 yards
wide, and eight feet deep. The whole rose and fell with the water, yet
was covered with green bushes and trees, and its surface enlivened in
the autumn by a variety of beautiful flowers. It went on increasing till
about 1835, when some of the trees upon it had grown to the height of
about sixty feet. Steps were then taken by the State of Louisiana to
clear away the whole raft, and open the navigation, which was effected,
not without great labor, in the space of four years.
The rafts on Red River are equally remarkable: in some parts of its
course, cedar-trees are heaped up by themselves, and in other places,
pines. On the rise of the waters in summer hundreds of these are seen,
some with their green leaves still upon them, just as they have fallen
from a neighboring bank, others leafless, broken and worn in their
passage from a far distant tributary: wherever they accumulate on the
edge of a sand-bar they arrest the current, and soon become covered with
sediment. On this mud the young willows and the poplars called
cotton-wood spring up, their boughs still farther retarding the stream,
and as the inundation rises, accelerating the deposition of new soil.
The bank continuing to enlarge, the channel at length becomes so narrow
that a single long tree may reach from side to side, and the remaining
space is then soon choked up by a quantity of other timber.
"Unfortunately for the navigation of the Mississippi," observes Captain
Hall, "some of the largest trunks, after being cast down from the
position on which they grew, get their roots entangled with the bottom
of the river, where they remain anchored, as it were, in the mud. The
force of the current naturally gives their tops a tendency downwards,
and, by its flowing past, soon strips them of their leaves and branches.
These fixtures, called snags, or planters, are extremely dangerous to
the steam-vessels proceeding up the stream, in which they lie like a
lance in rest, concealed beneath the water, with their sharp ends
pointed directly against the bows of the vessels coming up. For the most
part these formidable snags remain so still that they can be detected
only by a slight ripple above them, not perceptible to inexperienced
eyes. Sometimes, however, they vibrate up and down, alternately showing
their heads above the surface and bathing them beneath it."[360] So
imminent, until lately, was the danger caused by these obstructions,
that almost all the boats on the Mississippi were constructed on a
particular plan, to guard against fatal accidents; but in the last ten
years, by the aid of the power of steam and the machinery of a
snag-boat, as it is called, the greater number of these trunks of trees
have been drawn out of the mud.[361]
The prodigious quantity of wood annually drifted down by the Mississippi
and its tributaries, is a subject of geological interest, not merely as
illustrating the manner in which abundance of vegetable matter becomes,
in the ordinary course of nature, imbedded in submarine and estuary
deposits, but as attesting the constant destruction of soil and
transportation of matter to lower levels by the tendency of rivers to
shift their courses. Each of these trees must have required many years,
some of them centuries, to attain their full size; the soil, therefore,
whereon they grew, after remaining undisturbed for long periods, is
ultimately torn up and swept away.
It is also found in excavating at New Orleans, even at the depth of
several yards below the level of the sea, that the soil of the delta
contains innumerable trunks of trees, layer above layer, some prostrate,
as if drifted, others broken off near the bottom, but remaining still
erect, and with their roots spreading on all sides, as if in their
natural position. In such situations they appeared to me to indicate a
sinking of the ground, as the trees must formerly have grown in marshes
above the sea-level. In the higher parts of the alluvial plain, for many
hundred miles above the head of the delta, similar stools and roots of
trees are also seen buried in stiff clay at different levels, one above
the other, and exposed to view in the banks at low water. They point
clearly to the successive growth of forests in the extensive swamps of
the plain, where the ground was slowly raised, year after year, by the
mud thrown down during inundations. These roots and stools belong
chiefly to the deciduous cypress (_Taxodium distichum_), and other
swamp-trees, and they bear testimony to the constant shifting of the
course of the great river, which is always excavating land originally
formed at some distance from its banks.
_Formation of lakes in Louisiana._.--Another striking feature in the
basin of the Mississippi, illustrative of the changes now in progress,
is the formation by natural causes of great lakes, and the drainage of
others. These are especially frequent in the basin of the Red River in
Louisiana, where the largest of them, called Bistineau, is more than
_thirty miles_ long, and has a medium depth of from _fifteen_ to
_twenty_ feet. In the deepest parts are seen numerous cypress-trees, of
all sizes, now dead, and most of them with their tops broken by the
wind, yet standing erect under water. This tree resists the action of
air and water longer than any other, and, if not submerged throughout
the whole year, will retain life for an extraordinary period. Lake
Bistineau, as well as Black Lake, Cado Lake, Spanish Lake, Natchitoches
Lake, and many others, have been formed, according to Darby, by the
gradual elevation of the bed of Red River, in which the alluvial
accumulations have been so great as to raise its channel, and cause its
waters, during the flood season, to flow up the mouths of many
tributaries, and to convert parts of their courses into lakes. In the
autumn, when the level of Red River is again depressed, the waters rush
back, and some lakes become grassy meadows, with streams meandering
through them.[362] Thus, there is a periodical flux and reflux between
Red River and some of these basins, which are merely reservoirs,
alternately emptied and filled, like our tide estuaries--with this
difference, that in the one case the land is submerged for several
months continuously, and in the other twice in every twenty-four hours.
It has happened, in several cases, that a raft of timber or a bar has
been thrown by Red River across some of the openings of these channels,
and then the lakes become, like Bistineau, constant repositories of
water. But, even in these cases, their level is liable to annual
elevation and depression, because the flood of the main river, when at
its height, passes over the bar; just as, where sand-hills close the
entrance of an estuary on the Norfolk or Suffolk coast, the sea, during
some high tide or storm, has often breached the barrier and inundated
again the interior.
I am informed by Mr. Featherstonhaugh that the plains of the Red River
and the Arkansas are so low and flat, that whenever the Mississippi
rises thirty feet above its ordinary level, those great tributaries are
made to flow back, and inundate a region of vast extent. Both the
streams alluded to contain red sediment, derived from the decomposition
of red porphyry; and since 1833, when there was a great inundation in
the Arkansas, an immense swamp has been formed near the Mammelle
mountain, comprising 30,000 acres, with here and there large lagoons,
where the old bed of the river was situated; in which innumerable trees,
for the most part dead, are seen standing, of cypress, cotton-wood, or
poplar, the triple-thorned acacia, and others, which are of great size.
Their trunks appear as if painted red for about fifteen feet from the
ground; at which height a perfectly level line extends through the whole
forest, marking the rise of the waters during the last flood.[363]
But most probably the causes above assigned for the recent origin of
these lakes are not the only ones. Subterranean movements have altered,
so lately as the years 1811-12, the relative levels of various parts of
the basin of the Mississippi, situated 300 miles northeast of Lake
Bistineau. In those years the great valley, from the mouth of the Ohio
to that of the St. Francis, including a tract 300 miles in length, and
exceeding in area the whole basin of the Thames, was convulsed to such a
degree, as to create new islands in the river, and lakes in the alluvial
plain. Some of these were on the left or east bank of the Mississippi,
and were twenty miles in extent; as, for example, those named Reelfoot
and Obion in Tennessee, formed in the channels or valleys of small
streams bearing the same names.[364]
But the largest area affected by the great convulsion lies eight or ten
miles to the westward of the Mississippi, and inland from the town of
New Madrid, in Missouri. It is called "the sunk country," and is said
to extend along the course of the White Water and its tributaries, for
a distance of between seventy and eighty miles north and south, and
thirty miles or more east and west. Throughout this area, innumerable
submerged trees, some standing leafless, others prostrate, are seen; and
so great is the extent of lake and marsh, that an active trade in the
skins of muskrats, mink, otters, and other wild animals, is now carried
on there. In March, 1846, I skirted the borders of the "sunk country"
nearest to New Madrid, passing along the Bayou St. John and Little
Prairie, where dead trees of various kinds, some erect in the water,
others fallen, and strewed in dense masses over the bottom, in the
shallows, and near the shore, were conspicuous. I also beheld countless
rents in the adjoining dry alluvial plains, caused by the movements of
the soil in 1811-12, and still open, though the rains, frost, and river
inundations, have greatly diminished their original depth. I observed,
moreover, numerous circular cavities, called "sunk holes," from ten to
thirty yards wide, and twenty feet or more in depth, which interrupt the
general level of the plain. These were formed by the spouting out of
large quantities of sand and mud during the earthquakes.[365]
That the prevailing changes of level in the delta and alluvial plain of
the Mississippi have been caused by the subsidence, rather than the
upheaval of land, appears to me established by the fact, that there are
no protuberances of upraised alluvial soil, projecting above the level
surface of the great plain. It is true that the gradual elevation of
that plain, by new accessions of matter, would tend to efface every
inequality derived from this source, but we might certainly have
expected to find more broken ground between the opposite bluffs, had
local upthrows of alluvial strata been of repeated occurrence.
_Antiquity of the delta._--The vast size of the alluvial plain both
above and below the head of the delta, or the branching off of the
uppermost arm of the Atchafalaya, has been already alluded to. Its
superficial dimensions, according to Mr. Forshey, exceed 30,000 square
miles, nearly half of which belong to the true delta. The deposits
consist partly of sand originally formed upon or near the banks of the
river, and its tributaries, partly of gravel, swept down the main
channel, of which the position has continually shifted, and partly of
fine mud slowly accumulated in the swamps. The farther we descend the
river towards its mouth, the finer becomes the texture of the sediment.
The whole alluvial formation, from the base of the delta upwards, slopes
with a very gentle inclination, rising about three inches in a mile from
the level of the sea at the Balize, to the height of about 200 feet in a
distance of about 800 miles.
That a large portion of this fluviatile deposit, together with the
fluvio-marine strata now in progress near the Balize, consists of mud
and sand with much vegetable matter intermixed, may be inferred from
what has been said of the abundance of drift trees floated down every
summer. These are seen matted together into a net-work around the
extensive mud banks at the extreme mouths of the river. Every one
acquainted with the geography of Louisiana is aware that the most
southern part of the delta forms a long narrow tongue of land protruding
for 50 miles into the Gulf of Mexico, at the end of which are numerous
channels of discharge. This singular promontory consists simply of the
river and its two low, flat banks, covered with reeds, young willows,
and poplars. Its appearance answers precisely to that of the banks far
in the interior, when nothing appears above water during inundations but
the higher part of the sloping glacis or bank. In the one case we have
the swamps or an expanse of freshwater with the tops of trees appearing
above, in the other the bluish green surface of the Gulf of Mexico. An
opinion has very commonly prevailed that this narrow promontory, the
newest product of the river, has gained very rapidly upon the sea, since
the foundation of New Orleans; but after visiting the Balize in 1846, in
company with Dr. Carpenter, and making many inquiries of the pilots, and
comparing the present outline of the coast with the excellent Spanish
chart, published by Charlevoix 120 years before, we came to a different
conclusion. The rate of permanent advance of the new land has been very
slow, not exceeding perhaps one mile in a century. The gain may have
been somewhat more rapid in former years, when the new strip of soil
projected less far into the gulf, since it is now much more exposed to
the action of a strong marine current. The tides also, when the waters
of the river are low, enter into each opening, and scour them out,
destroying the banks of mud and the sand-bars newly formed during the
flood season.
An observation of Darby, in regard to the strata composing part of this
delta, deserves attention. In the steep banks of the Atchafalaya, before
alluded to, the following section, he says, is observable at low
water:--first an upper stratum, consisting invariably of bluish clay,
common to the banks of the Mississippi; below this a stratum of red
ochreous earth, peculiar to Red River, under which the blue clay of the
Mississippi again appears; and this arrangement is constant, proving, as
that geographer remarks, that the waters of the Mississippi and the Red
River occupied alternately, at some former periods, considerable tracts
below their present point of union.[366] Such alternations are probably
common in submarine spaces situated between two converging deltas; for,
before the two rivers unite, there must almost always be a certain
period when an intermediate tract will by turns be occupied and
abandoned by the waters of each stream; since it can rarely happen that
the season of highest flood will precisely correspond in each. In the
case of the Red River and Mississippi, which carry off the waters from
countries placed under widely distant latitudes, an exact coincidence in
the time of greatest inundation is very improbable.
The antiquity of the delta, or length of the period which has been
occupied in the deposition of so vast a mass of alluvial matter, is a
question which may well excite the curiosity of every geologist.
Sufficient data have not yet been obtained to afford a full and
satisfactory answer to the inquiry, but some approximation may already
be made to the minimum of time required.
When I visited New Orleans, in February, 1846, I found that Dr. Riddell
had made numerous experiments to ascertain the proportion of sediment
contained in the waters of the Mississippi; and he concluded that the
mean annual amount of solid matter was to the water as 1/1245 in weight,
or about 1/3000 in volume.[367] From the observations of the same
gentleman, and those of Dr. Carpenter and Mr. Forshey, an eminent
engineer, to whom I have before alluded, the average width, depth, and
velocity of the Mississippi, and thence the mean annual discharge of
water were deduced. I assumed 528 feet, or the tenth of a mile, as the
probable thickness of the deposit of mud and sand in the delta; founding
my conjecture chiefly on the depth of the Gulf of Mexico, between the
southern point of Florida and the Balize, which equals on an average 100
fathoms, and partly on some borings 600 feet deep in the delta, near
Lake Pontchartrain, north of New Orleans, in which the bottom of the
alluvial matter is said not to have been reached. The area of the delta
being about 13,600 square statute miles, and the quantity of solid
matter annually brought down by the river 3,702,758,400 cubic feet, it
must have taken 67,000 years for the formation of the whole; and if the
alluvial matter of the plain above be 264 feet deep, or half that of the
delta,[368] it must have required 33,500 more years for its
accumulation, even if its area be estimated as only equal to that of the
delta, whereas it is in fact larger. If some deduction be made from the
time here stated, in consequence of the effect of the drift-wood, which
must have aided in filling up more rapidly the space above alluded to, a
far more important allowance must be made on the other hand, for the
loss of matter, owing to the finer particles of mud not settling at the
mouths of the river, but being swept out far to sea during the
predominant action of the tides, and the waves in the winter months,
when the current of fresh water is feeble. Yet however vast the time
during which the Mississippi has been transporting its earthy burden to
the ocean, the whole period, though far exceeding, perhaps, 100,000
years, must be insignificant in a geological point of view, since the
bluffs or cliffs, bounding the great valley, and therefore older in
date, and which are from 50 to 250 feet in perpendicular height, consist
in great part of loam containing land, fluviatile, and lacustrine shells
of species still inhabiting the same country. (See fig. 23, p. 265.)
Before we take leave of the great delta, we may derive an instructive
lesson from the reflection that the new deposits already formed, or now
accumulating, whether marine or freshwater, must greatly resemble in
composition, and the general character of their organic remains, many
ancient strata, which enter largely into the earth's structure. Yet
there is no sudden revolution in progress, whether on the land or in the
waters, whether in the animate or the inanimate world. Notwithstanding
the excessive destruction of soil and uprooting of trees, the region
which yields a never-failing supply of drift-wood is densely clothed
with noble forests, and is almost unrivalled in its power of supporting
animal and vegetable life. In spite of the undermining of many a lofty
bluff, and the encroachments of the delta on the sea--in spite of the
earthquake, which rends and fissures the soil, or causes areas more than
sixty miles in length to sink down several yards in a few months, the
general features of the district remain unaltered, or are merely
undergoing a slow and insensible change. Herds of wild deer graze on the
pastures, or browse upon the trees; and if they diminish in number, it
is only where they give way to man and the domestic animals which follow
in his train. The bear, the wolf, the fox, the panther, and the
wild-cat, still maintain themselves in the fastnesses of the forests of
cypress and gum-tree. The racoon and the opossum are everywhere
abundant, while the musk-rat, otter, and mink still frequent the rivers
and lakes, and a few beavers and buffaloes have not yet been driven from
their ancient haunts. The waters teem with alligators, tortoises, and
fish, and their surface is covered with millions of migratory waterfowl,
which perform their annual voyage between the Canadian lakes and the
shores of the Mexican Gulf. The power of man begins to be sensibly felt,
and many parts of the wilderness to be replaced by towns, orchards, and
gardens. The gilded steamboats, like moving palaces, stem the force of
the current, or shoot rapidly down the descending stream, through the
solitudes of the forests and prairies. Already does the flourishing
population of the great valley far exceed that of the thirteen United
States when first they declared their independence. Such is the state of
a continent where trees and stones are hurried annually by a thousand
torrents, from the mountains to the plains, and where sand and finer
matter are swept down by a vast current to the sea, together with the
wreck of countless forests and the bones of animals which perish in the
inundations. When these materials reach the gulf, they do not render the
waters unfit for aquatic animals; but on the contrary, the ocean here
swarms with life, as it generally does where the influx of a great river
furnishes a copious supply of organic and mineral matter. Yet many
geologists, when they behold the spoils of the land heaped in successive
strata, and blended confusedly with the remains of fishes, or
interspersed with broken shells and corals; when they see portions of
erect trunks of trees with their roots still retaining their natural
position, and one tier of these preserved in a fossil state above
another, imagine that they are viewing the signs of a turbulent instead
of a tranquil and settled state of the planet. They read in such
phenomena the proof of chaotic disorder and reiterated catastrophes,
instead of indications of a surface as habitable as the most delicious
and fertile districts now tenanted by man.
DELTA OF THE GANGES AND BRAHMAPOOTRA.
[Illustration: Fig. 25.
MAP OF THE DELTA OF THE GANGES AND BRAHMAPOOTRA.]
As an example of a still larger delta advancing upon the sea in
opposition to more powerful tides, I shall next describe that of the
Ganges and Brahmapootra (or Burrampooter). These, the two principal
rivers of India, descend from the highest mountains in the world, and
partially mingle their waters in the low plains of Hindostan, before
reaching the head of the Bay of Bengal. The Brahmapootra, somewhat the
larger of the two, formerly passed to the east of Dacca, even so lately
as the beginning of the present century, pouring most of its waters into
one of the numerous channels in the delta called "the Megna." By that
name the main stream was always spoken of by Rennell and others in their
memoirs on this region. But the main trunk now unites with an arm of the
Ganges considerably higher up, at a point about 100 miles distant from
the sea; and it is constantly, according to Dr. Hooker, working its way
westward, having formerly, as may be seen by ancient maps, moved
eastward for a long period.
The area of the delta of the combined rivers, for it is impossible now
to distinguish what belongs to each, is considerably more than double
that of the Nile, even if we exclude from the delta a large extent of
low, flat, alluvial plain, doubtless of fluviatile origin, which
stretches more than 100 miles to the hills west of Calcutta (see map,
fig. 25), and much farther in a northerly direction beyond the head of
the great delta. The head of a delta is that point where the first arm
is given off. Above that point a river receives the waters of
tributaries flowing from higher levels; below it, on the contrary, it
gives out portions of its waters to lower levels, through channels which
flow into adjoining swamps, or which run directly to the sea. The
Mississippi, as before described, has a single head, which originated at
an unknown period when the Red River joined it. In the great delta of
Bengal there may be said to be two heads nearly equidistant from the
sea, that of the Ganges (G, map, fig. 25), about 30 miles below
Rajmahal, or 216 statute miles in a direct line from the sea, and that
of the Brahmapootra (B), below Chirapoonjee, where the river issues from
the Khasia mountains, a distance of 224 miles from the Bay of Bengal.
It will appear, by reference to the map, that the great body of fresh
water derived from the two rivers enters the bay on its eastern side;
and that a large part of the delta bordering on the sea is composed of a
labyrinth of rivers and creeks, all filled with salt water, except those
immediately communicating with the Hoogly, or principal arm of the
Ganges. This tract alone, known by the name of the Woods, or Sunderbunds
(more properly Soonderbuns), a wilderness infested by tigers and
crocodiles, is, according to Rennell, equal in extent to the whole
principality of Wales.[369]
On the sea-coast there are eight great openings, each of which has
evidently, at some ancient period, served in its turn as the principal
channel of discharge. Although the flux and reflux of the tide extend
even to the heads of the delta when the rivers are low, yet, when they
are periodically swollen by tropical rains, their volume and velocity
counteract the tidal current, so that, except very near the sea, the ebb
and flow become insensible. During the flood season, therefore, the
Ganges and Brahmapootra almost assume in their delta, the character of
rivers entering an inland sea; the movements of the ocean being then
subordinate to the force of the rivers, and only slightly disturbing
their operations. The great gain of the delta in height and area takes
place during the inundations; and, during other seasons of the year, the
ocean makes reprisals, scouring out the channels, and sometimes
devouring rich alluvial plains.
_Islands formed and destroyed._--Major R. H. Colebrooke, in his account
of the course of the Ganges, relates examples of the rapid filling up of
some of its branches, and the excavation of new channels, where the
number of square miles of soil removed in a short time (the column of
earth being 114 feet high) was truly astonishing. Forty square miles, or
25,600 acres, are mentioned as having been carried away, in one place,
in the course of a few years.[370] The immense transportation of earthy
matter by the Ganges and Brahmapootra is proved by the great magnitude
of the islands formed in their channels during a period far short of
that of a man's life. Some of these, many miles in extent, have
originated in large sand-banks thrown up round the points at the angular
turning of the rivers, and afterwards insulated by breaches of the
streams. Others, formed in the main channel, are caused by some
obstruction at the bottom. A large tree, or a sunken boat, is sometimes
sufficient to check the current, and cause a deposit of sand, which
accumulates till it usurps a considerable portion of the channel. The
river then undermines its banks on each side, to supply the deficiency
in its bed, and the island is afterwards raised by fresh deposits during
every flood. In the great gulf below Luckipour, formed by the united
waters of the Ganges and Megna, some of the islands, says Rennell, rival
in size and fertility the Isle of Wight. While the river is forming new
islands in one part, it is sweeping away old ones in others. Those newly
formed are soon overrun with reeds, long grass, the Tamarix Indica, and
other shrubs, forming impenetrable thickets, where the tiger, the
rhinoceros, the buffalo, deer, and other wild animals, take shelter. It
is easy, therefore, to perceive, that both animal and vegetable remains
may occasionally be precipitated into the flood, and become imbedded in
the sediment which subsides in the delta.
Three or four species of crocodile, of two distinct sub-genera, abound
in the Ganges, and its tributary and contiguous waters; and Mr. H. T.
Colebrooke informed me, that he had seen both forms in places far
inland, many hundred miles from the sea. The Gangetic crocodile, or
Gavial (in correct orthography, Garial), is confined to the fresh water,
living exclusively on fish, but the commoner kinds, called Koomiah and
Muggar, frequent both fresh and salt, being much larger and fiercer in
salt and brackish water.[371] These animals swarm in the brackish water
along the line of sand-banks, where the advance of the delta is most
rapid. Hundreds of them are seen together in the creeks of the delta,
or basking in the sun on the shoals without. They will attack men and
cattle, destroying the natives when bathing, and tame and wild animals
which come to drink. "I have not unfrequently," says Mr. Colebrooke,
"been witness to the horrid spectacle of a floating corpse seized by a
crocodile with such avidity, that he half emerged above the water with
his prey in his mouth." The geologist will not fail to observe how
peculiarly the habits and distribution of these saurians expose them to
become imbedded in the horizontal strata of fine mud, which are annually
deposited over many hundred square miles in the Bay of Bengal. The
inhabitants of the land, which happen to be drowned or thrown into the
water, are usually devoured by these voracious reptiles; but we may
suppose the remains of the saurians themselves to be continually
entombed in the new formations. The number, also, of bodies of the
poorer class of Hindoos thrown annually into the Ganges is so great,
that some of their bones or skeletons can hardly fail to be occasionally
enveloped in fluviatile mud.
It sometimes happens, at the season when the periodical flood is at its
height, that a strong gale of wind, conspiring with a high spring-tide,
checks the descending current of the river, and gives rise to most
destructive inundations. From this cause, in 1763, the waters at
Luckipour rose six feet above their ordinary level, and the inhabitants
of a considerable district, with their houses and cattle, were totally
swept away.
The population of all oceanic deltas are particularly exposed to suffer
by such catastrophes, recurring at considerable intervals of time; and
we may safely assume that such tragical events have happened again and
again since the Gangetic delta was inhabited by man. If human experience
and forethought cannot always guard against these calamities, still less
can the inferior animals avoid them; and the monuments of such
disastrous inundations must be looked for in great abundance in strata
of all ages, if the surface of our planet has always been governed by
the same laws. When we reflect on the general order and tranquillity
that reigns in the rich and populous delta of Bengal, notwithstanding
the havoc occasionally committed by the depredations of the ocean, we
perceive how unnecessary it is to attribute the imbedding of successive
races of animals in older strata to extraordinary energy in the causes
of decay and reproduction in the infancy of our planet, or to those
general catastrophes and sudden revolutions so often resorted to.
_Deposits in the delta._--The quantity of mud held in suspension by the
waters of the Ganges and Brahmapootra is found, as might be expected, to
exceed that of any of the rivers alluded to in this or the preceding
chapters; for, in the first place, their feeders flow from mountains of
unrivalled altitude, and do not clear themselves in any lakes, as does
the Rhine in the Lake of Constance, or the Rhone in that of Geneva. And,
secondly, their whole course is nearer the equator than that of the
Mississippi, or any great river, respecting which careful experiments
have been made, to determine the quantity of its water and earthy
contents. The fall of rain, moreover, as we have before seen, is
excessive on the southern flanks of the first range of mountains which
rise from the plains of Hindostan, and still more remarkable is the
quantity sometimes poured down in one day. (See above, p. 200.) The sea,
where the Ganges and Brahmapootra discharge their main stream at the
flood season, only recovers its transparency at the distance of from 60
to 100 miles from the delta; and we may take for granted that the
current continues to transport the finer particles much farther south
than where the surface water first becomes clear. The general slope,
therefore, of the new strata must be extremely gentle. According to the
best charts, there is a gradual deepening from four to about sixty
fathoms, as we proceed from the base of the delta to the distance of
about one hundred miles into the Bay of Bengal. At some few points
seventy, or even one hundred, fathoms are obtained at that distance.
One remarkable exception, however, occurs to the regularity of the shape
of the bottom. Opposite the middle of the delta, at the distance of
thirty or forty miles from the coast, a deep submarine valley occurs,
called the "swatch of no ground," about fifteen miles in diameter, where
soundings of 180, and even 300, fathoms fail to reach the bottom. (See
map, p. 275.) This phenomenon is the more extraordinary, since the
depression runs north to within five miles of the line of shoals; and
not only do the waters charged with sediment pass over it continually,
but, during the monsoons, the sea, loaded with mud and sand, is beaten
back in that direction towards the delta. As the mud is known to extend
for eighty miles farther into the gulf, an enormous thickness of matter
must have been deposited in "the swatch." We may conclude, therefore,
either that the original depth of this part of the Bay of Bengal was
excessive, or that subsidences have occurred in modern times. The latter
conjecture is the less improbable, as the whole area of the delta has
been convulsed in the historical era by earthquakes, and actual
subsidences have taken place in the neighboring coast of Chittagong,
while "the swatch" lies not far from the volcanic band which connects
Sumatra, Barren Island, and Ramree.[372]
Opposite the mouth of the Hoogly river, and immediately south of Saugor
Island, four miles from the nearest land of the delta, a new islet was
formed about twenty years ago, called Edmonstone Island, on the centre
of which a beacon was erected as a landmark in 1817. In 1818 the island
had become two miles long and half a mile broad, and was covered with
vegetation and shrubs. Some houses were then built upon it, and in 1820
it was used as a pilot station. The severe gale of 1823 divided it into
two parts, and so reduced its size as to leave the beacon standing out
in the sea, where, after remaining seven years, it was washed away. The
islet in 1836 had been converted by successive storms into a sand-bank,
half a mile long, on which a sea-mark was placed.
Although there is evidence of gain at some points, the general progress
of the coast is very slow; for the tides, when the river water is low,
are actively employed in removing alluvial matter. In the Sunderbunds
the usual rise and fall of the tides is no more than eight feet, but, on
the east side of the delta, Dr. Hooker observed, in the winter of 1851,
a rise of from sixty to eighty feet, producing among the islands at the
mouths of the Megna and Fenny rivers, a lofty wave or "bore" as they
ascend, and causing the river water to be ponded back, and then to sweep
down with great violence when the tide ebbs. The bay for forty miles
south of Chittagong is so fresh that neither algae nor mangroves will
grow in it. We may, therefore, conceive how effective may be the current
formed by so great a volume of water in dispersing fine mud over a wide
area. Its power is sometimes augmented by the agitation of the bay
during hurricanes in the month of May. The new superficial strata
consists entirely of fine sand and mud; such, at least, are the only
materials which are exposed to view in regular beds on the banks of the
numerous creeks. Neither here or higher up the Ganges, could Dr. Hooker
discover any land or freshwater shells in sections of the banks, which
in the plains higher up sometimes form cliffs eighty feet in height at
low water. In like manner I have stated[373] that I was unable to find
any buried shells in the delta or modern river cliffs of the
Mississippi.
No substance so coarse as gravel occurs in any part of the delta of the
Ganges and Brahmapootra, nor nearer the sea than 400 miles. Yet it is
remarkable that the boring of an Artesian well at Fort William, near
Calcutta, in the years 1835-1840, displayed, at the depth of 120 feet,
clay and sand with pebbles. This boring was carried to a depth of 481
feet below the level of Calcutta, and the geological section obtained in
the operation has been recorded with great care. Under the surface soil,
at a depth of about ten feet, they came to a stiff blue clay about forty
feet in thickness; below which was sandy clay, containing in its lower
portion abundance of decayed vegetable matter, which at the bottom
assumed the character of a stratum of black peat two feet thick. This
peaty mass was considered as a clear indication (like the "dirt-bed" of
Portland) of an ancient terrestrial surface, with a forest or Sunderbund
vegetation. Logs and branches of a red-colored wood occur both above and
immediately below the peat, so little altered that Dr. Wallich was able
to identify them with the Soondri tree, _Heritiera littoralis_, one of
the most prevalent forms, at the base of the delta. Dr. Falconer tells
me that similar peat has been met with at other points round Calcutta at
the depth of nine feet and twenty-five feet. It appears, therefore, that
there has been a sinking down of what was originally land in this
region, to the amount of seventy feet or more perpendicular; for
Calcutta is only a few feet above the level of the sea, and the
successive peat-beds seem to imply that the subsidence of the ground was
gradual or interrupted by several pauses. Below the vegetable mass they
entered upon a stratum of yellowish clay about ten feet thick,
containing horizontal layers of kunkar (or kankar), a nodular,
concretionary, argillaceous limestone, met with abundantly at greater or
less depths in all parts of the valley of the Ganges, over many
thousand square miles, and always presenting the same characters, even
at a distance of one thousand miles north of Calcutta. Some of this
kunkar is said to be of very recent origin in deposits formed by river
inundations near Saharanpoor. After penetrating 120 feet, they found
loam containing water-worn fragments of mica-slate and other kinds of
rock, which the current of the Ganges can no longer transport to this
region. In the various beds pierced through below, consisting of clay,
marl, and friable sandstone, with kunkar here and there intermixed, no
organic remains of decidedly marine origin were met with. Too positive a
conclusion ought not, it is true, to be drawn from such a fact, when we
consider the narrow bore of the auger and its effect in crushing shells
and bones. Nevertheless, it is worthy of remark, that the only fossils
obtained in a recognizable state were of a fluviatile or terrestrial
character. Thus, at the depth of 350 feet, the bony shell of a tortoise,
or trionyx, a freshwater genus, was found in sand, resembling the living
species of Bengal. From the same stratum, also, they drew up the lower
half of the humerus of a ruminant, at first referred to a hyaena. It was
the size and shape, says Dr. Falconer, of the shoulder-bone of the
_Cervus porcinus_, or common hog-deer, of India. At the depth of 380
feet, clay with fragments of lacustrine shells was incumbent on what
appears clearly to have been another "dirt-bed," or stratum of decayed
wood, implying a period of repose of some duration, and a forest-covered
land, which must have subsided 300 feet, to admit of the subsequent
superposition of the overlying deposits. It has been conjectured that,
at the time when this area supported trees, the land extended much
farther out into the Bay of Bengal than now, and that in later times the
Ganges, while enlarging its delta, has been only recovering lost ground
from the sea.
At the depth of about 400 feet below the surface, an abrupt change was
observed in the character of the strata, which were composed in great
part of sand, shingle, and boulders, the only fossils observed being the
vertebrae of a crocodile, shell of a trionyx, and fragments of wood very
little altered, and similar to that buried in beds far above. These
gravelly beds constituted the bottom of the section at the depth of 481
feet, when the operations were discontinued, in consequence of an
accident which happened to the auger.
The occurrence of pebbles at the depths of 120 and 400 feet implies an
important change in the geographical condition of the region around or
near Calcutta. The fall of the river, or the general slope of the
alluvial plain may have been formerly greater; or, before a general and
perhaps unequal subsidence, hills once nearer the present base of the
delta may have risen several hundred feet, forming islands in the bay,
which may have sunk gradually, and become buried under fluviatile
sediment.
_Antiquity of the delta._--It would be a matter of no small scientific
interest, if experiments were made to enable us to determine, with some
degree of accuracy, the mean quantity of earthy matter discharged
annually into the sea by the united waters of the Ganges and
Brahmapootra. The Rev. Mr. Everest instituted, in 1831-2, a series of
observations on the earthy matter brought down by the Ganges, at
Ghazepoor, 500 miles from the sea. He found that, in 1831, the number of
cubic feet of water discharged by the river per second at that place
was, during the
Rains (4 months) 494,208
Winter (5 months) 71,200
Hot weather (3 months) 36,330
so that we may state in round numbers that 500,000 cubic feet per second
flow down during the four months of the flood season, from June to
September, and less than 60,000 per second during the remaining eight
months.
The average quantity of solid matter suspended in the water during the
rains was, by weight, 1/428th part; but as the water is about one-half
the specific gravity of the dried mud, the solid matter discharged is
1/856th part in bulk, or 577 cubic feet per second. This gives a total
of 6,082,041,600 cubic feet for the discharge in the 122 days of the
rain. The proportion of sediment in the waters at other seasons was
comparatively insignificant, the total amount during the five winter
months being only 247,881,600 cubic feet, and during the three months of
hot weather 38,154,240 cubic feet. The total annual discharge, then,
would be 6,368,077,440 cubic feet.
This quantity of mud would in one year raise a surface of 228-1/2 square
miles, or a square space, each side of which should measure 15 miles, a
height of one foot. To give some idea of the magnitude of this result,
we will assume that the specific gravity of the dried mud is only
one-half that of granite (it would, however, be more); in that case, the
earthy matter discharged in a year would equal 3,184,038,720 cubic feet
of granite. Now about 12-1/2 cubic feet of granite weigh one ton; and it
is computed that the great Pyramid of Egypt, if it were a solid mass of
granite, would weigh about 600,000,000 tons. The mass of matter,
therefore, carried down annually would, according to this estimate, more
than equal in weight and bulk forty-two of the great pyramids of Egypt,
and that borne down in the four months of the rains would equal forty
pyramids. But if, without any conjecture as to what may have been the
specific gravity of the mud, we attend merely to the weight of solid
matter actually proved by Mr. Everest to have been contained in the
water, we find that the number of tons weight which passed down in the
122 days of the rainy season was 339,413,760, which would give the
weight of fifty-six pyramids and a half; and in the whole year
355,361,464 tons, or nearly the weight of sixty pyramids.
The base of the great Pyramid of Egypt covers eleven acres, and its
perpendicular height is about five hundred feet. It is scarcely possible
to present any picture to the mind which will convey an adequate
conception of the mighty scale of this operation, so tranquilly and
almost insensibly carried on by the Ganges, as it glides through its
alluvial plain, even at a distance of 500 miles from the sea. It may,
however, be stated, that if a fleet of more than eighty Indiamen, each
freighted with about 1400 tons' weight of mud, were to sail down the
river every hour of every day and night for four months continuously,
they would only transport from the higher country to the sea a mass of
solid matter equal to that borne down by the Ganges, even in this part
of its course, in the four months of the flood season. Or the exertions
of a fleet of about 2000 such ships going down daily with the same
burden, and discharging it into the gulf, would be no more than
equivalent to the operations of the great river.
The most voluminous current of lava which has flowed from Etna within
historical times was that of 1669. Ferrara, after correcting Borelli's
estimate, calculated the quantity of cubic yards of lava in this current
at 140,000,000. Now, this would not equal in bulk one-fifth of the
sedimentary matter which is carried down in a single year by the Ganges,
past Ghazepoor, according to the estimate above explained; so that it
would require five grand eruptions of Etna to transfer a mass of lava
from the subterranean regions to the surface, equal in volume to the mud
carried down in one year to that place.
Captain R. Strachey, of the Bengal Engineers, has remarked to me, not
only that Ghazepoor, where Mr. Everest's observations were made, is 500
miles from the sea, but that the Ganges has not been joined there by its
most important feeders. These drain upon the whole 750 miles of the
Himalaya, and no more than 150 miles of that mountain-chain have sent
their contributions to the main trunk at Ghazepoor. Below that place,
the Ganges is joined by the Gogra, Gunduk, Khosee, and Teesta from the
north, to say nothing of the Sone flowing from the south, one of the
largest of the rivers which rise in the table-land of central India.
(See map, fig. 25, p. 275.) Moreover the remaining 600 miles of the
Himalaya comprise that eastern portion of the basin where the rains are
heaviest. (See above, p. 200.) The quantity of water therefore carried
down to the sea may probably be four or five times as much as that which
passes Ghazepoor.
The Brahmapootra, according to Major Wilcox,[374] in the month of
January, when it is near its minimum, discharges 150,000 cubic feet of
water per second at Gwalpara, not many miles above the head of its
delta. Taking the proportions observed at Ghazepoor at the different
seasons as a guide, the probable average discharge of the Brahmapootra
for the whole year may be estimated at about the same as that of the
Ganges. Assuming this; and secondly, in order to avoid the risk of
exaggeration, that the proportion of sediment in their waters is about a
third less than Mr. Everest's estimate, the mud borne down to the Bay of
Bengal in one year would equal 40,000 millions of cubic feet, or between
six and seven times as much as that brought down to Ghazepoor, according
to Mr. Everest's calculations in 1831, and ten times as much as that
conveyed annually by the Mississippi to the Gulf of Mexico.
Captain Strachey estimates the annually inundated portion of the delta
at 250 miles in length by 80 in breadth, making an area of 20,000 square
miles. The space south of this in the bay, where sediment is thrown
down, may be 300 miles from E. to W. by 150 N. and S., or 45,000 square
miles, which, added to the former, gives a surface of 65,000 square
miles, over which the sediment is spread out by the two rivers. Suppose
then the solid matter to amount to 40,000 millions of cubic feet per
annum, the deposit, he observes, must be continued for forty-five years
and three-tenths to raise the whole area a height of one foot, or 13,600
years to raise it 300 feet; and this, as we have seen, is much less than
the thickness of the fluviatile strata actually penetrated, (and the
bottom not reached) by the auger at Calcutta.
Nevertheless we can by no means deduce from these data alone, what will
be the future rate of advance of the delta, nor even predict whether the
land will gain on the sea, or remain stationary. At the end of 13,000
years the bay may be less shallow than now, provided a moderate
depression, corresponding to that experienced in part of Greenland for
many centuries shall take place (see chap. 30). A subsidence quite
insensible to the inhabitants of Bengal, not exceeding two feet three
inches in a century, would be more than sufficient to counterbalance all
the efforts of the two mighty rivers to extend the limits of their
delta. We have seen that the Artesian borings at Calcutta attest, what
the vast depth of the "swatch" may also in all likelihood indicate, that
the antagonist force of subsidence has predominated for ages over the
influx of fluviatile mud, preventing it from raising the plains of
Bengal, or from filling up a larger portion of the bay.
CONCLUDING REMARKS ON DELTAS.
_Convergence of deltas._--If we possessed an accurate series of maps of
the Adriatic for many thousand years, our retrospect would, without
doubt, carry us gradually back to the time when the number of rivers
descending from the mountains into that gulf by independent deltas was
far greater in number. The deltas of the Po and the Adige, for instance,
would separate themselves within the _recent_ era, as, in all
probability, would those of the Isonzo and the Torre. If, on the other
hand, we speculate on future changes, we may anticipate the period when
the number of deltas will greatly diminish; for the Po cannot continue
to encroach at the rate of a mile in a hundred years, and other rivers
to gain as much in six or seven centuries upon the shallow gulf, without
new junctions occurring from time to time; so that Eridanus, "the king
of rivers," will continually boast a greater number of tributaries. The
Ganges and the Brahmapootra have perhaps become partially confluent in
the same delta within the historical, or at least within the human era;
and the date of the junction of the Red River and the Mississippi would,
in all likelihood, have been known, if America had not been so recently
discovered. The union of the Tigris and the Euphrates must undoubtedly
have been one of the modern geographical changes of our Earth, for Col.
Rawlinson informs me that the delta of those rivers has advanced two
miles in the last sixty years, and is supposed to have encroached about
forty miles upon the Gulf of Persia in the course of the last
twenty-five centuries.
When the deltas of rivers, having many mouths, converge, a partial union
at first takes place by the confluence of some one or more of their
arms; but it is not until the main trunks are connected above the head
of the common delta, that a complete intermixture of their joint waters
and sediment takes place. The union, therefore, of the Po and Adige, and
of the Ganges and Brahmapootra, is still incomplete. If we reflect on
the geographical extent of surface drained by rivers such as now enter
the Bay of Bengal, and then consider how complete the blending together
of the greater part of their transported matter has already become, and
throughout how vast a delta it is spread by numerous arms, we no longer
feel so much surprise at the area occupied by some ancient formations of
homogeneous mineral composition. But our surprise will be still farther
lessened, when we afterwards inquire (ch. 21) into the action of tides
and currents in disseminating sediment.
_Age of existing deltas._--If we could take for granted, that the
relative level of land and sea had remained stationary ever since all
the existing deltas began to be formed--could we assume that their
growth commenced at one and the same instant when the present continents
acquired their actual shape--we might understand the language of
geologists who speak of "the epoch of existing continents." They
endeavor to calculate the age of deltas from this imaginary fixed
period; and they calculate the gain of new land upon the sea, at the
mouths of rivers, as having begun everywhere simultaneously. But the
more we study the history of deltas, the more we become convinced that
upward and downward movements of the land and contiguous bed of the sea
have exerted, and continue to exert, an influence on the physical
geography of many hydrographical basins, on a scale comparable in
magnitude or importance to the amount of fluviatile deposition effected
in an equal lapse of time. In the basin of the Mississippi, for example,
proofs both of descending and ascending movements to a vertical amount
of several hundred feet can be shown to have taken place since the
existing species of land and freshwater shells lived in that
region.[375]
The deltas also of the Po and Ganges have each, as we have seen (p.
257), when probed by the Artesian auger, borne testimony to a gradual
subsidence of land to the extent of several hundred feet--old
terrestrial surfaces, turf, peat, forest-land, and "dirt-beds," having
been pierced at various depths. The changes of level at the mouth of the
Indus in Cutch (see below, chap. 27), and those of New Madrid in the
valley of the Mississippi (see p. 270, and chap. 27), are equally
instructive, as demonstrating unceasing fluctuations in the levels of
those areas into which running water is transporting sediment. If,
therefore, the exact age of all modern deltas could be known, it is
scarcely probable that we should find any two of them in the world to
have coincided in date, or in the time when their earliest deposits
originated.
_Grouping of strata in deltas._--The changes which have taken place in
deltas, even within the times of history, may suggest many important
considerations in regard to the manner in which subaqueous sediment is
distributed. With the exception of some cases hereafter to be noticed,
there are some general laws of arrangement which must evidently hold
good in almost all the lakes and seas now filling up. If a lake, for
example, be encircled on two sides by lofty mountains, receiving from
them many rivers and torrents of different sizes, and if it be bounded
on the other sides, where the surplus waters issue, by a comparatively
low country, it is not difficult to define some of the leading
geological features which must characterize the lacustrine formation,
when this basin shall have been gradually converted into dry land by the
influx of sediment. The strata would be divisible into two principal
groups: the _older_ comprising those deposits which originated on the
side adjoining the mountains, where numerous deltas first began to form;
and the _newer_ group consisting of beds deposited in the more central
parts of the basin, and towards the side farthest from the mountains.
The following characters would form the principal marks of distinction
between the strata in each series:--The more ancient system would be
composed, for the most part, of coarser materials, containing many beds
of pebbles and sand, often of great thickness, and sometimes dipping at
a considerable angle. These, with associated beds of finer ingredients,
would, if traced round the borders of the basin, be seen to vary greatly
in color and mineral composition, and would also be very irregular in
thickness. The beds, on the contrary, in the newer group, would consist
of finer particles, and would be horizontal, or very slightly inclined.
Their color and mineral composition would be very homogeneous throughout
large areas, and would differ from almost all the separate beds in the
older series.
The following causes would produce the diversity here alluded to between
the two great members of such lacustrine formations:--When the rivers
and torrents first reach the edge of the lake, the detritus washed down
by them from the adjoining heights sinks at once into deep water, all
the heavier pebbles and sand subsiding near the shore. The finer mud is
carried somewhat farther out, but not to the distance of many miles, for
the greater part may be seen, as, for example, where the Rhone enters
the Lake of Geneva, to fall down in clouds to the bottom, not far from
the river's mouth. Thus alluvial tracts are soon formed at the mouths of
every torrent and river, and many of these in the course of ages become
of considerable extent. Pebbles and sand are then transported farther
from the mountains; but in their passage they decrease in size by
attrition, and are in part converted into mud and sand. At length some
of the numerous deltas, which are all directed towards a common centre,
approach near to each other; those of adjoining torrents become united,
and each is merged, in its turn, in the delta of the largest river,
which advances most rapidly into the lake, and renders all the minor
streams, one after the other, its tributaries. The various mineral
ingredients of all are thus blended together into one homogeneous
mixture, and the sediment is poured out from a common channel into the
lake.
As the average size of the transported particles decreases, while the
force and volume of the main river augments, the newer deposits are
diffused continually over a wider area, and are consequently more
horizontal than the older. When at first there were many independent
deltas near the borders of the basin, their separate deposits differed
entirely from each other; one may have been charged, like the Arve where
it joins the Rhone, with white sand and sediment derived from
granite--another may have been black, like many streams in the Tyrol,
flowing from the waste of decomposing rocks of dark slate--a third may
have been colored by ochreous sediment, like the Red River in
Louisiana--a fourth, like the Elsa in Tuscany, may have held much
carbonate of lime in solution. At first they would each form distinct
deposits of sand, gravel, limestone, marl, or other materials; but,
after their junction, new chemical combinations and a distinct color
would be the result, and the particles, having been conveyed ten,
twenty, or a greater number of miles over alluvial plains, would become
finer.
In those deltas where the tides and strong marine currents interfere,
the above description would only be applicable, with certain
modifications. If a series of earthquakes accompany the growth of a
delta, and change the levels of the land from time to time, as in the
region where the Indus now enters the sea, the phenomena will depart
still more widely from the ordinary type. If, after a protracted period
of rest, a delta sinks down, pebbles may be borne along in shallow water
near the foot of the boundary hills, so as to form conglomerates
overlying the fine mud previously thrown into deeper water in the same
area.
_Causes of stratification in deltas._--The stratified arrangement, which
is observed to prevail so generally in aqueous deposits, is most
frequently due to variations in the velocity of running water, which
cannot sweep along particles of more than a certain size and weight when
moving at a given rate. Hence, as the force of the stream augments or
decreases, the materials thrown down in successive layers at particular
places are rudely sorted, according to their dimensions, form, and
specific gravity. Where this cause has not operated, as where sand, mud,
and fragments of rock are conveyed by a glacier, a confused heap of
rubbish devoid of all stratification is produced.
Natural divisions are also occasioned in deltas, by the interval of time
which separates annually the deposition of matter during the periodical
rains, or melting of snow upon the mountains. The deposit of each year
may acquire some degree of consistency before that of the succeeding
year is superimposed. A variety of circumstances also give rise
annually, or sometimes from day to day, to slight variations in color,
fineness of the particles, and other characters, by which alternations
of strata distinct in texture and mineral ingredients must be produced.
Thus, for example, at one period of the year, drift-wood may be carried
down, and, at another, mud, as was before stated to be the case in the
delta of the Mississippi; or at one time, when the volume and velocity
of the stream are greatest, pebbles and sand may be spread over a
certain area, over which, when the waters are low, fine matter or
chemical precipitates are formed. During inundations, the turbid current
of fresh water often repels the sea for many miles; but when the river
is low, salt water again occupies the same space. When two deltas are
converging, the intermediate space is often, for reasons before
explained, alternately the receptacle of different sediments derived
from the converging streams (see p. 272). The one is, perhaps, charged
with calcareous, the other with argillaceous matter; or one sweeps down
sand and pebbles, the other impalpable mud. These differences may be
repeated with considerable regularity, until a thickness of hundreds of
feet of alternating beds is accumulated. The multiplication, also, of
shells and corals in particular spots, and for limited periods, gives
rise occasionally to lines of separation, and divides a mass which might
otherwise be homogeneous into distinct strata.
An examination of the shell marl now forming in the Scotch lakes, or the
sediment termed "warp," which subsides from the muddy water of the
Humber and other rivers, shows that recent deposits are often composed
of a great number of extremely thin layers, either even or slightly
undulating, and preserving a general parallelism to the planes of
stratification. Sometimes, however, the laminae in modern strata are
disposed diagonally at a considerable angle, which appears to take place
where there are conflicting movements in the waters. In January, 1829, I
visited, in company with Professor L. A. Necker, of Geneva, the
confluence of the Rhone and Arve, when those rivers were very low, and
were cutting channels through the vast heaps of dabris thrown down from
the waters of the Arve in the preceding spring. One of the sandbanks
which had formed, in the spring of 1828, where the opposing currents of
the two rivers neutralized each other, and caused a retardation in the
motion, had been undermined; and the following is an exact
representation of the arrangement of laminae exposed in a vertical
section. The length of the portion here seen is about twelve feet, and
the height five. The strata A A consist of irregular alternations of
pebbles and sand in undulating beds: below these are seams of very fine
sand B B, some as thin as paper, others about a quarter of an inch
thick. The strata C C are composed of layers of fine greenish-gray sand
as thin as paper. Some of the inclined beds will be seen to be thicker
at their upper, others at their lower extremity, the inclination of some
being very considerable. These layers must have accumulated one on the
other by lateral apposition, probably when one of the rivers was very
gradually increasing or diminishing in velocity, so that the point of
greatest retardation caused by their conflicting currents shifted
slowly, allowing the sediment to be thrown down in successive layers on
a sloping bank. The same phenomenon is exhibited in older strata of all
ages.[376]
[Illustration: Fig. 26.
Section of a sand-bank in the bed of the Arve at its confluence with the
Rhone, showing the stratification of deposits where currents meet.]
If the bed of a lake or of the sea be sinking, whether at a uniform or
an unequal rate, or oscillating in level during the deposition of
sediment, these movements will give rise to a different class of
phenomena, as, for example, to repeated alternations of shallow-water
and deep-water deposits, each with peculiar organic remains, or to
frequent repetitions of similar beds, formed at a uniform depth, and
inclosing the same organic remains, and to other results too complicated
and varied to admit of enumeration here.
_Formation of conglomerates._--Along the base of the Maritime Alps,
between Toulon and Genoa, the rivers, with few exceptions, are now
forming strata of conglomerate and sand. Their channels are often
several miles in breadth, some of them being dry, and the rest easily
forded for nearly eight months in the year, whereas during the melting
of the snow they are swollen, and a great transportation of mud and
pebbles takes place. In order to keep open the main road from France to
Italy, now carried along the sea-coast, it is necessary to remove
annually great masses of shingle brought down during the flood season. A
portion of the pebbles are seen in some localities, as near Nice, to
form beds of shingle along the shore, but the greater part are swept
into a deep sea. The small progress made by the deltas of minor rivers
on this coast need not surprise us, when we recollect that there is
sometimes a depth of two thousand feet at a few hundred yards from the
beach, as near Nice. Similar observations might be made respecting a
large proportion of the rivers in Sicily, and among others, respecting
that which, immediately north of the port of Messina, hurries annually
vast masses of granitic pebbles into the sea.
_Constant interchange of land and sea._--I may here conclude my remarks
on deltas, observing that, imperfect as is our information of the
changes which they have undergone within the last three thousand years,
they are sufficient to show how constant an interchange of sea and land
is taking place on the face of our globe. In the Mediterranean alone,
many flourishing inland towns, and a still greater number of ports, now
stand where the sea rolled its waves since the era of the early
civilization of Europe. If we could compare with equal accuracy the
ancient and actual state of all the islands and continents, we should
probably discover that millions of our race are now supported by lands
situated where deep seas prevailed in earlier ages. In many districts
not yet occupied by man, land animals and forests now abound where ships
once sailed; and, on the other hand, we shall find, on inquiry, that
inroads of the ocean have been no less considerable. When to these
revolutions, produced by aqueous causes, we add analogous changes
wrought by igneous agency, we shall, perhaps, acknowledge the justice of
the conclusion of Aristotle, who declared that the whole land and sea on
our globe periodically changed places.[377]