ALLUVIUM.
Alluvium described--Due to complicated causes--Of various ages, as
shown in Auvergne--How distinguished from rocks in
situ--River-terraces--Parallel roads of Glen Roy--Various theories
respecting their origin.
Between the superficial covering of vegetable mould and the subjacent rock
there usually intervenes in every district a deposit of loose gravel, sand,
and mud, to which the name of alluvium has been applied. The term is
derived from _alluvio_, an inundation, or _alluo_, to wash, because the
pebbles and sand commonly resemble those of a river's bed or the mud and
gravel spread over low lands by a flood.
A partial covering of such alluvium is found alike in all climates, from
the equatorial to the polar regions; but in the higher latitudes of Europe
and North America it assumes a distinct character, being very frequently
devoid of stratification, and containing huge fragments of rock, some
angular and others rounded, which have been transported to great distances
from their parent mountains. When it presents itself in this form, it has
been called "diluvium," "drift," or the "boulder formation;" and its
probable connexion with the agency of floating ice and glaciers will be
treated of more particularly in the eleventh and twelfth chapters.
[Illustration: Fig. 99. Lavas of Auvergne resting on alluviums of
different ages.]
The student will be prepared, by what I have said in the last chapter on
denudation, to hear that loose gravel and sand are often met with, not
only on the low grounds bordering rivers, but also at various points on
the sides or even summits of mountains. For, in the course of those
changes in physical geography which may take place during the gradual
emergence of the bottom of the sea and its conversion into dry land, any
spot may either have been a sunken reef, or a bay, or estuary, or
sea-shore, or the bed of a river. For this reason it would be
unreasonable to hope that we should ever be able to account for all the
alluvial phenomena of each particular country, seeing that the causes of
their origin are so complicated. Moreover, the last operations of water
have a tendency to disturb and confound together all pre-existing
alluviums. Hence we are always in danger of regarding as the work of a
single era, and the effect of one cause, what has in reality been the
result of a variety of distinct agents, during a long succession of
geological epochs. Much useful instruction may therefore be gained from
the exploration of a country like Auvergne, where the superficial gravel
of very different eras happens to have been preserved by sheets of lava,
which were poured out one after the other at periods when the
denudation, and probably the upheaval, of rocks were in progress. That
region had already acquired in some degree its present configuration
before any volcanos were in activity, and before any igneous matter was
superimposed upon the granitic and fossiliferous formations. The pebbles
therefore in the older gravels are exclusively constituted of granite
and other aboriginal rocks; and afterwards, when volcanic vents burst
forth into eruption, those earlier alluviums were covered by streams of
lava, which protected them from intermixture with gravel of subsequent
date. In the course of ages, a new system of valleys was excavated, so
that the rivers ran at lower levels than those at which the first
alluviums and sheets of lava were formed. When, therefore, fresh
eruptions gave rise to new lava, the melted matter was poured out over
lower grounds; and the gravel of these plains differed from the first
or upland alluvium, by containing in it rounded fragments of various
volcanic rocks, and often bones belonging to distinct groups of land
animals which flourished in the country in succession.
The annexed drawing will explain the different heights at which beds of
lava and gravel, each distinct from the other in composition and age, are
observed, some on the flat tops of hills, 700 or 800 feet high, others on
the slope of the same hills, and the newest of all in the channel of the
existing river where there is usually gravel alone, but in some cases a
narrow stripe of solid lava sharing the bottom of the valley with the
river. In all these accumulations of transported matter of different ages
the bones of extinct quadrupeds have been found belonging to assemblages of
land mammalia which flourished in the country in succession, and which vary
specifically, the one from the other, in a greater or less degree, in
proportion as the time which separated their entombment has been more or
less protracted. The streams in the same district are still undermining
their banks and grinding down into pebbles or sand, columns of basalt and
fragments of granite and gneiss; but the older alluviums, with the fossil
remains belonging to them, are prevented from being mingled with the gravel
of recent date by the cappings of lava before mentioned. But for the
accidental interference, therefore, of this peculiar cause, all the
alluviums might have passed so insensibly the one into the other, that
those formed at the remotest era might have appeared of the same date as
the newest, and the whole formation might have been regarded by some
geologists as the result of one sudden and violent catastrophe.
In almost every country, the alluvium consists in its upper part of
transported materials, but it often passes downwards into a mass of
broken and angular fragments derived from the subjacent rock. To this
mass the provincial name of "rubble," or "brash," is given in many
parts of England. It may be referred to the weathering or disintegration
of stone on the spot, the effects of air and water, sun and frost,
and chemical decomposition.
[Illustration: Fig. 100. Cross section.
_a._ Vegetable soil.
_b._ Alluvium.
_c._ Mass of same, apparently detached.]
The inferior surface of alluvial deposits is often very irregular,
conforming to all the inequalities of the fundamental rocks (fig. 100.).
Occasionally, a small mass, as at _c_, appears detached, and as if included
in the subjacent formation. Such isolated portions are usually sections of
winding subterranean hollows filled up with alluvium. They may have been
the courses of springs or subterranean streamlets, which have flowed
through and enlarged natural rents; or, when on a small scale and in soft
strata, they may be spaces which the roots of large trees have once
occupied, gravel and sand having been introduced after their decay.
[Illustration: Fig. 101. Sand-pipes in the chalk at Eaton, near Norwich.]
But there are other deep hollows of a cylindrical form found in England,
France, and elsewhere, penetrating the white chalk, and filled with sand
and gravel, which are not so readily explained. They are sometimes called
"sand-pipes," or "sand-galls," and "puits naturels," in France. Those
represented in the annexed cut were observed by me in 1839, laid open in a
large chalk-pit near Norwich. They were of very symmetrical form, the
largest more than 12 feet in diameter, and some of them had been traced, by
boring, to the depth of more than 60 feet. The smaller ones varied from a
few inches to a foot in diameter, and seldom descended more than 12 feet
below the surface. Even where three of them occurred, as at _a_, fig. 101.,
very close together, the parting walls of soft white chalk were not broken
through. They all taper downwards and end in a point. As a general rule,
sand and pebbles occupy the central parts of each pipe, while the sides and
bottom are lined with clay.
Mr. Trimmer, in speaking of appearances of the same kind in the Kentish
chalk, attributes the origin of such "sand-galls" to the action of the sea
on a beach or shoal, where the waves, charged with shingle and sand, not
only wear out longitudinal furrows, such as may be observed on the surface
of the chalk near Norwich when the incumbent gravel is removed, but also
drill deep circular hollows by the rotatory motion imparted to sand and
pebbles. Such furrows, as well as vertical cavities, are now formed, he
observes, on the coast where the shores are composed of chalk.[82-A]
That the commencement of many of the tubular cavities now under
consideration has been due to the cause here assigned, I have little doubt.
But such mechanical action could not have hollowed out the whole of the
sand-pipes _c_ and _d_, fig. 101., because several large chalk-flints seen
protruding from the walls of the pipes have not been eroded, while sand and
gravel have penetrated many feet below them. In other cases, as at _b b_,
similar unrounded nodules of flint, still preserving their irregular form
and white coating, are found at various depths in the midst of the loose
materials filling the pipe. These have evidently been detached from regular
layers of flints occurring above. It is also to be remarked that the course
of the same sand-pipe, _b b_, is traceable above the level of the chalk for
some distance upwards, through the incumbent gravel and sand, by the
obliteration of all signs of stratification. Occasionally, also, as in the
pipe _d_, the overlying beds of gravel bend downwards into the mouth of the
pipe, so as to become in part vertical, as would happen if horizontal
layers had sunk gradually in consequence of a failure of support. All these
phenomena may be accounted for by attributing the enlargement and deepening
of the sand-pipes to the chemical action of water charged with carbonic
acid, derived from the vegetable soil and the decaying roots of trees. Such
acid might corrode the chalk, and deepen indefinitely any previously
existing hollow, but could not dissolve the flints. The water, after it had
become saturated with carbonate of lime, might freely percolate the
surrounding porous walls of chalk, and escape through them and from the
bottom of the tube, so as to carry away in the course of time large masses
of dissolved calcareous rock[83-A], and leave behind it on the edges of
each tubular hollow a coating of fine clay, which the white chalk contains.
I have seen tubes precisely similar and from 1 to 5 feet in diameter
traversing vertically the upper half of the soft calcareous building
stone, or chalk without flints, constituting St. Peter's Mount,
Maestricht. These hollows are filled with pebbles and clay, derived from
overlying beds of gravel, and all terminate downwards like those of
Norfolk. I was informed that, 6 miles from Maestricht, one of these
pipes, 2 feet in diameter, was traced downwards to a bed of flattened
flints, forming an almost continuous layer in the chalk. Here it
terminated abruptly, but a few small root-like prolongations of it were
detected immediately below, probably where the dissolving substance had
penetrated at some points through openings in the siliceous mass.
It is not so easy as may at first appear to draw a clear line of
distinction between the _fixed_ rocks, or regular strata (rocks _in
situ_ or _in place_), and _alluvium_. If the bed of a torrent or river
be dried up, we call the gravel, sand, and mud left in their channels,
or whatever, during floods, they may have scattered over the
neighbouring plains, alluvium. The very same materials carried into a
lake, where they become sorted by water and arranged in more distinct
layers, especially if they inclose the remains of plants, shells, or
other fossils, are termed regular strata.
In like manner we may sometimes compare the gravel, sand, and broken
shells, strewed along the path of a rapid marine current, with a deposit
formed contemporaneously by the discharge of similar materials, year after
year, into a deeper and more tranquil part of the sea. In such cases, when
we detect marine shells or other organic remains entombed in the strata,
which enable us to determine their age and mode of origin, we regard them
as part of the regular series of fossiliferous formations, whereas, if
there are no fossils, we have frequently no power of separating them from
the general mass of superficial alluvium.
The usual rarity of organic remains in beds of loose gravel and sand is
partly owing to the rapid and turbid water in which they were formed having
been in a condition unfavourable to the habitation of aquatic beings, and
partly to their porous nature, which, by allowing the free percolation of
rain-water, has promoted the decomposition and removal of organic matter.
It has long been a matter of common observation that most rivers are now
cutting their channels through alluvial deposits of greater depth and
extent than could ever have been formed by the present streams. From this
fact a rash inference has sometimes been drawn, that rivers in general have
grown smaller, or become less liable to be flooded than formerly. But such
phenomena would be a natural result of any considerable oscillations in the
level of the land experienced since the existing valleys originated.
Suppose part of a continent, comprising within it a large hydrographical
basin like that of the Mississippi, to subside several inches or feet in
a century, as the west coast of Greenland, extending 600 miles north and
south, has been sinking for three or four centuries, between the
latitudes 60 deg. and 69 deg. N.[84-A] There might be no encroachment of the sea
at the river's mouth in consequence of this change of level, but the
fall of the waters flowing from the interior being lessened, the main
river and its tributaries would have less power to carry down to its
delta, and to discharge into the ocean, the sedimentary matter with
which they are annually loaded. They would all begin to raise their
channels and alluvial plains by depositing in them the heavier sand and
pebbles washed down from the upland country, and this operation would
take place most effectively if the amount of subsidence in the interior
was unequal, and especially if, on the whole, it exceeded that of the
region near the sea. If then the same area of land be again upheaved to
its former height, the fall, and consequently the velocity, of every
river would begin to augment. Each of them would be less given to
overflow its alluvial plain; and their power of carrying earthy matter
seaward, and of scouring out and deepening their channels, would
continue till, after a lapse of many thousand years, each of them would
have eroded a new channel or valley through a fluviatile formation of
modern date. The surface of what was once the river-plain at the period
of greatest depression, would remain fringing the valley sides in the
form of a terrace apparently flat, but in reality sloping down with the
general inclination of the river. Everywhere this terrace would present
cliffs of gravel and sand, facing the river. That such a series of
movements has actually taken place in the main valley of the Mississippi
and in its tributary valleys during oscillations of level, I have
endeavoured to show in my description of that country[85-A]; and the
freshwater shells of existing species and bones of land quadrupeds,
partly of extinct races preserved in the terraces of fluviatile origin,
attest the exclusion of the sea during the whole process of filling up
and partial re-excavation.
In many cases, the alluvium in which rivers are now cutting their channels,
originated when the land first rose out of the sea. If, for example, the
emergence was caused by a gradual and uniform motion, every bay and
estuary, or the straits between islands, would dry up slowly, and during
their conversion into valleys, every part of the upheaved area would in its
turn be a sea-shore, and might be strewed over with littoral sand and
pebbles, or each spot might be the point where a delta accumulated during
the retreat and exclusion of the sea. Materials so accumulated would
conform to the general slope of a valley from its head to the sea-coast.
_River terraces._--We often observe at a short distance from the present
bed of a river a steep cliff a few feet or yards high, and on a level with
the top of it a flat terrace corresponding in appearance to the alluvial
plain which immediately borders the river. This terrace is again bounded by
another cliff, above which a second terrace sometimes occurs: and in this
manner two or three ranges of cliffs and terraces are occasionally seen on
one or both sides of the stream, the number varying, but those on the
opposite sides often corresponding in height.
[Illustration: Fig. 102. River Terraces and Parallel Roads.]
These terraces are seldom continuous for great distances, and their
surface slopes downwards, with an inclination similar to that of the
river. They are readily explained if we adopt the hypothesis before
suggested, of a gradual rise of the land; especially if, while rivers
are shaping out their beds, the upheaving movement be intermittent, so
that long pauses shall occur, during which the stream will have time to
encroach upon one of its banks, so as to clear away and flatten a large
space. This operation being afterwards repeated at lower levels, there
will be several successive cliffs and terraces.
_Parallel roads._--The parallel shelves, or roads, as they have been
called, of Lochaber or Glen Roy and other contiguous valleys in Scotland,
are distinct both in character and origin from the terraces above
described; for they have no slope towards the sea like the channel of a
river, nor are they the effect of denudation. Glen Roy is situated in the
western Highlands, about ten miles north of Fort William, near the western
end of the great glen of Scotland, or Caledonian Canal, and near the foot
of the highest of the Grampians, Ben Nevis. Throughout its whole length, a
distance of more than ten miles, two, and in its lower part three, parallel
roads or shelves are traced along the steep sides of the mountains, as
represented in the annexed figure, fig. 102., each maintaining a perfect
horizontality, and continuing at exactly the same level on the opposite
sides of the glen. Seen at a distance, they appear like ledges or roads,
cut artificially out of the sides of the hills; but when we are upon them
we can scarcely recognize their existence, so uneven is their surface, and
so covered with boulders. They are from 10 to 60 feet broad, and merely
differ from the side of the mountain by being somewhat less steep.
On closer inspection, we find that these terraces are stratified in the
ordinary manner of alluvial or littoral deposits, as may be seen at those
points where ravines have been excavated by torrents. The parallel shelves,
therefore, have not been caused by denudation, but by the deposition of
detritus, precisely similar to that which is dispersed in smaller
quantities over the declivities of the hills above. These hills consist of
clay-slate, mica-schist, and granite, which rocks have been worn away and
laid bare at a few points only, in a line just above the parallel roads.
The highest of these roads is about 1250 feet above the level of the sea,
the next about 200 feet lower than the uppermost, and the third still lower
by about 50 feet. It is only this last, or the lowest of the three, which
is continued throughout Glen Spean, a large valley with which Glen Roy
unites. As the shelves are always at the same height above the sea, they
become continually more elevated above the river in proportion as we
descend each valley; and they at length terminate very abruptly, without
any obvious cause, either in the shape of the ground, or any change in the
composition or hardness of the rocks. I should exceed the limits of this
work, were I to attempt to give a full description of all the geographical
circumstances attending these singular terraces, or to discuss the
ingenious theories which have been severally proposed to account for them
by Dr. MacCulloch, Sir T. D. Lauder, and Messrs. Darwin, Agassiz, Milne,
and Chambers. There is one point, however, on which all are agreed, namely,
that these shelves are ancient beaches, or littoral formations accumulated
round the edges of one or more sheets of water which once stood at the
level, first of the highest shelf, and successively at the height of the
two others. It is well known, that wherever a lake or marine fiord exists
surrounded by steep mountains subject to disintegration by frost or the
action of torrents, some loose matter is washed down annually, especially
during the melting of snow, and a check is given to the descent of this
detritus at the point where it reaches the waters of the lake. The waves
then spread out the materials along the shore, and throw some of them upon
the beach; their dispersing power being aided by the ice, which often
adheres to pebbles during the winter months, and gives buoyancy to them.
The annexed diagram illustrates the manner in which Dr. MacCulloch and Mr.
Darwin suppose "the roads" to constitute mere indentations in a superficial
alluvial coating which rests upon the hillside, and consists chiefly of
clay and sharp unrounded stones.
[Illustration: Fig. 103. Cross section.
A B. Supposed original surface of rock.
C D. Roads or shelves in the outer alluvial covering of the hill.]
Among other proofs that the parallel roads have really been formed along
the margin of a sheet of water, it may be mentioned, that wherever an
isolated hill rises in the middle of the glen above the level of any
particular shelf, a corresponding shelf is seen at the same level passing
round the hill, as would have happened if it had once formed an island in a
lake or fiord. Another very remarkable peculiarity in these terraces is
this; each of them comes in some portion of its course to a _col_, or
passage between the heads of glens, the explanation of which will be
considered in the sequel.
Those writers who first advocated the doctrine that the roads were the
ancient beaches of freshwater lakes, were unable to offer any probable
hypothesis respecting the formation and subsequent removal of barriers of
sufficient height and solidity to dam up the water. To introduce any
violent convulsion for their removal was inconsistent with the
uninterrupted horizontality of the roads, and with the undisturbed aspect
of those parts of the glens where the shelves come suddenly to an end. Mr.
Agassiz and Dr. Buckland, desirous, like the defenders of the lake theory,
to account for the limitation of the shelves to certain glens, and their
absence in contiguous glens, where the rocks are of the same composition,
and the slope and inclination of the ground very similar, started the
conjecture that these valleys were once blocked up by enormous glaciers
descending from Ben Nevis, giving rise to what are called in Switzerland
and in the Tyrol, glacier-lakes. After a time the icy barrier was broken
down, or melted, first, to the level of the second, and afterwards to that
of the third road or shelf.
In corroboration of this view, they contended that the alluvium of Glen
Roy, as well as of other parts of Scotland, agrees in character with the
moraines of glaciers seen in the Alpine valleys of Switzerland. Allusion
will be made in the eleventh chapter to the former existence of glaciers in
the Grampians: in the mean time it will readily be conceded that this
hypothesis is preferable to any previous lacustrine theory, by accounting
more easily for the temporary existence and entire disappearance of lofty
transverse barriers, although the height required for the imaginary dams
of ice may be startling.
Before the idea last alluded to had been entertained, Mr. Darwin examined
Glen Roy, and came to the opinion that the shelves were formed when the
glens were still arms of the sea, and, consequently, that there never were
any barriers. According to him, the land emerged during a slow and uniform
upward movement, like that now experienced throughout a large part of
Sweden and Finland; but there were certain pauses in the upheaving process,
at which times the waters of the sea remained stationary for so many
centuries as to allow of the accumulation of an extraordinary quantity of
detrital matter, and the excavation, at points immediately above, of many
deep notches and bare cliffs in the hard and solid rock.
The phenomena which are most difficult to reconcile with this theory
are, first, the abrupt cessation of the roads at certain points in the
different glens; secondly, their unequal number in different valleys
connecting with each other, there being three, for example, in Glen Roy
and only one in Glen Spean; thirdly, the precise horizontality of level
maintained by the same shelf over a space many leagues in length
requiring us to assume, that during a rise of 1250 feet no one portion
of the land was raised even a few yards above another; fourthly, the
coincidence of level already alluded to of each shelf with a _col_, or
the point forming the head of two glens, from which the rain-waters flow
in opposite directions. This last-mentioned feature in the physical
geography of Lochaber seems to have been explained in a satisfactory
manner by Mr. Darwin. He calls these _cols_ "landstraits," and regards
them as having been anciently sounds or channels between islands. He
points out that there is a tendency in such sounds to be silted up, and
always the more so in proportion to their narrowness. In a chart of the
Falkland Islands by Capt. Sullivan, R.N., it appears that there are
several examples there of straits where the soundings diminish regularly
towards the narrowest part. One is so nearly dry that it can be walked
over at low water, and another, no longer covered by the sea, is
supposed to have recently dried up in consequence of a small shift in
the relative level of sea and land. "Similar straits," observes Mr.
Chambers, "hovering, in character, between sea and land, and which may
be called fords, are met with in the Hebrides. Such, for example, is the
passage dividing the islands of Lewis and Harris, and that between North
Uist and Benbecula, both of which would undoubtedly appear as _cols_,
coinciding with a terrace or raised beach, all round the islands, if the
sea were to subside."[88-A]
The precise horizontality of level maintained by the roads or shelves of
Lochaber over an area many leagues in length and breadth, is a
difficulty common in some degree to all the rival hypotheses, whether of
lakes, or glaciers, or of the simple upheaval of the land above the sea.
For we cannot suppose the roads to be more ancient than the glacial
period, or the era of the boulder formation of Scotland, of which I
shall speak in the eleventh and twelfth chapters. Strata of that era of
marine origin containing northern shells of existing species have been
found at various heights in Scotland, some on the east, and others on
the west coast, from 20 to 400 feet high; and in one region in
Lanarkshire not less than 524 feet above high-water mark. It seems,
therefore, in the highest degree improbable that Glen Roy should have
escaped entirely the upward movement experienced in so many surrounding
regions,--a movement implied by the position of these marine deposits,
in which the shells are almost all of known recent species. But if the
motion has really extended to Glen Roy and the contiguous glens, it must
have uplifted them bodily, without in the slightest degree affecting
their horizontality; and this being admitted, the principal objection to
the theory of marine beaches, founded on the uniformity of upheaval, is
removed, or is at least common to every theory hitherto proposed.
To assume that the ocean has gone down from the level of the uppermost
shelf, or 1250 feet, simultaneously all over the globe, while the land
remained unmoved, is a view which will find favour with very few
geologists, for the reasons explained in the fifth chapter.
The student will perceive, from the above sketch of the controversy
respecting the formation of these curious shelves, that this problem, like
many others in geology, is as yet only solved in part; and that a larger
number of facts must be collected and reasoned upon before the question
can be finally settled.
FOOTNOTES:
[82-A] Trimmer, Proceedings of Geol. Soc. vol. iv. p. 7. 1842.
[83-A] See Lyell on Sand-pipes, &c., Phil. Mag., third series, vol.
xv. p. 257., Oct. 1839.
[84-A] Principles of Geology, 7th ed. p. 506., 8th ed. 509.
[85-A] Second Visit to the U. S. vol. ii. chap. 34.
[88-A] "Ancient Sea Margins," p. 114., by R. Chambers.