Chapter XIII: Part IV: Dynamical Geology (2)

Besides its chemical activity water exerts among subterranean rocks a
mechanical influence which leads to important changes in the
topography of the surface. In removing the mineral matter, either in
solution or as fine sediment, it sometimes loosens the support of
overlying masses of rock which may ultimately give way on sloping
ground, and rush down the declivities in the form of landslips. These
destructive effects are specially frequent on the sides of valleys in
mountainous countries and on lines of sea-cliff.

3. _Brooks and Rivers._--As geological agents the running waters on
the face of the land play an important part in epigene changes. Like
rain and springs they have both a chemical and a mechanical action.
The latter receives most attention, as it undoubtedly is the more
important; but the former ought not to be omitted in any survey of the
general waste of the earth's surface. The water of rivers must possess
the powers of a chemical solvent like rain and springs, though its
actual work in this respect can be less easily measured, seeing that
river water is directly derived from rain and springs, and necessarily
contains in solution mineral substances supplied to it by them and not
by its own operation. Nevertheless, it is sometimes easy to prove that
streams dissolve chemically the rocks of their channels. Thus, in
limestone districts the base of the cliffs of river ravines may be
found eaten away into tunnels, arches, and overhanging projections,
presenting in their smooth surfaces a great contrast to the angular
jointed faces of the same rock, where now exposed to the influence
only of the weather on the higher parts of the cliff.

The mechanical action of rivers consists (a) in transporting mud,
sand, gravel and blocks of stone from higher to lower levels; (b) in
using these loose materials to widen and deepen their channels by
erosion; (c) in depositing their load of detritus wherever possible
and thus to make new geological formations.

(a) _Transporting Power._--River-water is distinguished from that of
springs by being less transparent, because it contains more or less
mineral matter in suspension, derived mainly from what is washed down
by rain, or carried in by brooks, but partly also from the abrasion of
the water-channels by the erosive action of the rivers themselves. The
progress of this burden of detritus may be instructively followed from
the mountain-tributaries of a river down to the mouth of the main
stream. In the high grounds the water-courses may be observed to be
choked with large fragments of rock disengaged from the cliffs and
crags on either side. Traced downwards the blocks are seen to become
gradually smaller and more rounded. They are ground against each
other, and upon the rocky sides and bottom of the channel, getting
more and more reduced as they descend, and at the same time abrading
the rocks over or against which they are driven. Hence a great deal of
debris is produced, and is swept along by the onward and downward
movement of the water. The finer portions, such as mud and fine sand,
are carried in suspension, and impart the characteristic turbidity to
river-water; the coarser sand and gravel are driven along the
river-bottom. The proportion of suspended mineral matter has been
ascertained with more or less precision for a number of rivers. As an
illustrative example of a river draining a vast area with different
climates, forms of surface and geological structure the Mississippi
may be cited. The average proportion of sediment in its water was
ascertained by Humphreys and Abbot to be 1/1500 by weight or 1/2900 by
volume. These engineers found that, in addition to this suspended
material, coarse detritus is constantly being pushed forward along the
bed of the river into the Gulf of Mexico, to an amount which they
estimated at about 750,000,000 cubic ft. of sand, earth and gravel;
they concluded that the Mississippi carries into the gulf every year
an amount of mechanically transported sediment sufficient to make a
prism one square mile in area and 268 ft. in height.

(b) _Excavating Power._--It is by means of the sand, gravel and stones
which they drive against the sides and bottoms of their channels that
streams have hollowed out the beds in which they flow. Not only is the
coarse detritus reduced in size by the friction of the stones against
each other, but, at the same time, these materials abrade the rocks
against which they are driven by the current. Where, owing to the
shape of the bottom of the channel, the stones are caught in eddies,
and are kept whirling round there, they become more and more worn down
themselves, and at the same time scour out basin-shaped cavities, or
"pot-holes," in the solid rock below. The uneven bed of a swiftly
flowing stream may in this way be honeycombed with such eroded basins
which coalesce and thus appreciably lower the surface of the bed. The
steeper the channel, other conditions being equal, the more rapid will
be the erosion. Geological structure also affects the character and
rate of the excavation. Where the rocks are so arranged as to favour
the formation and persistence of a waterfall, a long chasm may be
hollowed out like that of the Niagara below the falls, where a hard
thick bed of nearly flat limestone lies on softer and more easily
eroded shales. The latter are scooped out from underneath the
limestone, which from time to time breaks off in large masses and the
waterfall gradually retreats up stream, while the ravine is
proportionately lengthened. To the excavating power of rivers the
origin of the valley systems of the dry land must be mainly assigned
(see Part VIII.).

(c) _Reproductive Power._--So long as a stream flows over a steep
declivity its velocity suffices to keep the sediment in suspension,
but when from any cause, such as a diminution of slope, the velocity
is checked, the transporting power is lessened and the sediment begins
to fall to the bottom and to remain there. Hence various river-formed
or "alluvial" deposits are laid down. These sometimes cover
considerable spaces at the foot of mountains. The floors of valleys
are strewn with detritus, and their level may thereby be sensibly
raised. In floods the ground inundated on either side of a stream
intercepts some part of the detritus, which is then spread over the
flood-plain and gradually heightens it. At the same time the stream
continues to erode the channel, and ultimately is unable to reach the
old flood-plain. It consequently forms a new plain at a lower level,
and thus, by degrees, it comes to be flanked on either side by a
series of successive terraces or platforms, each of which marks one of
its former levels. Where a river enters a large body of water its
current is checked. Some of its sediment is consequently dropped, and
by slow accumulation forms a delta (q.v.). On land, every lake in
mountain districts furnishes instances of this kind of alluvium. But
the most important deltas are those formed in the sea at the mouths of
the larger rivers of the globe. Off many coast-lines the detritus
washed from the land gathers into bars, which enclose long strips of
water more or less completely separated from the sea outside and known
as lagoons. A chain of such lagoon-barriers stretches for hundreds of
miles round the Gulf of Mexico and the eastern shores of the United
States.

4. _Lakes._--These sheets of water, considered as a whole, do not
belong to the normal system of drainage on the land whereby valleys
are excavated. On the contrary they are exceptional to it; for the
constant tendency of running water is to fill them up, or to drain
them by wearing down the barriers that contain them at their outflow.
Some of them are referable to movements of the terrestrial crust
whereby depressions arise on the surface of the land, as has been
noted after earthquakes. Others have arisen from solution such as that
of rock-salt or of limestone, the removal of which by underground
water causes a subsidence of the ground above. A third type of
lake-basin occurs in regions that are now or have once been subject to
the erosive action of glaciers (see under next subdivision,
_Terrestrial Ice_). Many small lakes or tarns have been caused by the
deposit of debris across a valley as by landslips or moraines.
Considered from a geological point of view, lakes perform an important
function in regulating the drainage of the ground below their outfall
and diminishing the destructive effects of floods, in filtering the
water received from their affluent streams, and in providing
undisturbed areas of deposit in which thick and extensive lacustrine
formations may be accumulated. In the inland basins of some dry
climates the lakes are salt, owing to excess of evaporation, and their
bottoms become the sites of chemical deposits, particularly of
chlorides of sodium and magnesium, and calcium sulphate and carbonate.

5. _Terrestrial Ice._--Each of the forms assumed by frozen water has
its own characteristic action in geological processes. Frost has a
powerful influence in breaking up damp soils and surfaces of stone in
the pores or cracks of which moisture has lodged. The water in
freezing expands, and in so doing pushes asunder the component
particles of soil or stone, or widens the space between the walls of
joints or crevices. When the ice melts the loosened grains remain
apart ready to be washed away by rain or blown off by wind, while by
the widening of joints large blocks of rock are detached from the
faces of cliffs. Where rivers or lakes are frozen over the ice exerts
a marked pressure on their banks; and when it breaks up large sheets
of it are driven ashore, pushing up quantities of gravel and stones
above the level of the water. The piling up of the disrupted ice
against obstructions in rivers ponds back the water, and often leads
to destructive floods when the ice barriers break. Where the ice has
formed round boulders in shallow water, or at the bottom
("anchor-ice"), it may lift these up when the frost gives way, and may
transport them for some distance. Ice formed in the atmosphere, and
descending to the ground in the form of hail, often causes great
destruction to vegetation and not infrequently to animal life. Where
the frozen moisture reaches the earth as snow, it serves to protect
rock, soil and vegetation from the effects of frost; but on sloping
ground it is apt to give rise to destructive avalanches or landslips,
while indirectly, by its rapid melting, it may cause serious floods in
rivers.

But the most striking geological work performed by terrestrial ice is
that achieved by glaciers (q.v.) and ice-sheets. These vast masses of
moving ice, when they descend from mountains where the steeper rocks
are clear of snow, receive on their surface the debris detached by
frost from the declivities above, and bear these materials to lower
levels or to the sea. Enormous quantities of rock-rubbish are thus
transported in the Alps and other high mountain ranges. When the ice
retreats the boulders carried by it are dropped where it melts, and
left there as memorials of the former extension of the glaciers.
Evidence of this nature proves the much wider extent of the Alpine ice
at a comparatively recent geological date. It can also be shown that
detritus from Scandinavia has been ice-borne to the south-east of
England and far into the heart of Europe.

The ice, by means of grains of sand and pieces of stone which it drags
along, scores, scratches and polishes the surfaces of rock underneath
it, and, in this way, produces the abundant fine sediment that gives
the characteristic milky appearance to the rivers that issue from the
lower ends of glaciers. By such long-continued attrition the rocks are
worn down, portions of them of softer nature, or where the ice acts
with especial vigour, are hollowed out into cavities which, on the
disappearance of the ice, may be filled with water and become tarns or
lakes. Rocks over which land-ice has passed are marked by a peculiar
smooth, flowing outline, which forms a contrast to the more rugged
surface produced by ordinary weathering. They are covered with
groovings, which range from the finest striae left by sharp grains of
sand to deep ruts ground out by blocks of stone. The trend of these
markings shows the direction in which the ice flowed. By their
evidence the position and movement of former glaciers in countries
from which the ice has entirely vanished may be clearly determined
(see GLACIAL PERIOD).

6. _The Sea._--The physical features of the sea are discussed in
separate articles (see OCEAN AND OCEANOGRAPHY). The sea must be
regarded as the great regulator of temperature and climate over the
globe, and as thus exerting a profound influence on the distribution
of plant and animal life. Its distinctly geological work is partly
erosive and partly reproductive. As an eroding agent it must to some
extent effect chemical decompositions in the rocks and sediments over
which it spreads; but these changes have not yet been satisfactorily
studied. Undoubtedly, its chief destructive power is of a mechanical
kind, and arises from the action of its waves in beating upon
shore-cliffs. By the alternate compression and expansion of the air in
crevices of the rocks on which heavy breakers fall, and by the
hydraulic pressure which these masses of sea-water exert on the walls
of the fissures into which they rush, large masses of rock are
loosened and detached, and caves and tunnels are drilled along the
base of sea-cliffs. Probably still more efficacious are the blows of
the loose shingle, which, caught up and hurled forward by the waves,
falls with great force upon the shore rocks, battering them as with a
kind of artillery until they are worn away. The smooth surfaces of the
rocks within reach of the waves contrasted with their angular forms
above that limit bear witness to the amount of waste, while the
rounded forms of the boulders and shingle show that they too are being
continually reduced in size. Thus the sea, by its action on the
coasts, produces much sediment, which is swept away by its waves and
currents and strewn over its floor. Besides this material, it is
constantly receiving the fine silt and sand carried down by rivers. As
the floor of the ocean is thus the final receptacle for the waste of
the land, it becomes the chief era on the surface of the globe for the
accumulation of new stratified formations. And such has been one of
its great functions since the beginning of geological time, as is
proved by the rocks that form the visible part of the earth's crust,
and consist in great part of marine deposits. Chemical precipitates
take place more especially in enclosed parts of the sea, where
concentration of the water by evaporation can take place, and where
layers of sodium chloride, calcium sulphate and carbonate, and other
salts are laid down. But the chief marine accumulations are of
detrital origin. Near the land and for a variable distance extending
sometimes to 200 or 300 m. from shore the deposits consist chiefly of
sediments derived from the waste of the land, the finer silts being
transported farthest from their source. At greater depths and
distances the ocean floor receives a slow deposit of exceedingly fine
clay, which is believed to be derived from the decomposition of pumice
and volcanic dust from insular or submarine volcanoes. Wide tracts of
the bottom are covered with various forms of ooze derived from the
accumulation of the remains of minute organisms.

(C) _Life._

Among the agents by which geological changes are carried on upon the
surface of the globe living organisms must be enumerated. Both plants
and animals co-operate with the inorganic agents in promoting the
degradation of the land. In some cases, on the other hand, they
protect rocks from decay, while, by the accumulation of their remains,
they give rise to extensive formations both upon the land and in the
sea. Their operations may hence be described as alike destructive,
conservative and reproductive. Under this heading also the influence
of Man as a geological agent deserves notice.

(a) _Plants._--Vegetation promotes the disintegration of rocks and
soil in the following ways: (1) By keeping the surfaces of stone
moist, and thus promoting both mechanical and chemical dissolution, as
is especially shown by liverworts, mosses and other moisture-loving
plants. (2) By producing through their decay carbonic and other
acids, which, together with decaying organic matter taken up by
passing moisture, become potent in effecting the chemical
decomposition of rocks and in promoting the disintegration of soils.
(3) By inserting their roots or branches between joints of rock, which
are thereby loosened, so that large slices may be eventually wedged
off. (4) By attracting rain, as thick woods, forests and peat-mosses
do, and thus accelerating the general waste of a country by running
water. (5) By promoting the decay of diseased and dead plants and
animals, as when fungi overspread a damp rotting tree or the carcase
of a dead animal.

That plants also exert a conservative influence on the surface of the
land is shown in various ways. (1) The formation of a stratum of turf
protects the soil and rocks underneath from being rapidly
disintegrated and washed away by atmospheric action. (2) Many plants,
even without forming a layer of turf, serve by their roots or branches
to protect the loose sand or soil on which they grow from being
removed by wind. The common sand-carex and other arenaceous plants
bind the loose sand-dunes of our coasts, and give them a permanence,
which would at once be destroyed were the sand laid bare again to
storms. The growth of shrubs and brushwood along the course of a
stream not only keeps the alluvial banks from being so easily
undermined and removed as would otherwise be the case, but serves to
arrest the sediment in floods, filtering the water and thereby adding
to the height of the flood plain. (3) Some marine plants, like the
calcareous nullipores, afford protection to shore rocks by covering
them with a hard incrustation. The tangles and smaller Fuci which grow
abundantly on the littoral zone break the force of the waves or
diminish the effects of ground swell. (4) Forests and brushwood
protect the soil, especially on slopes, from being washed away by rain
or ploughed up by avalanches.

Plants contribute by the aggregation of their remains to the formation
of stratified deposits. Some marine algae which secrete carbonate of
lime not only encrust rocks but give rise to sheets of submarine
limestone. An analogous part is played in fresh-water lakes by various
lime-secreting plants, such as _Chara_. Long-continued growth of
vegetation has, in some regions, produced thick accumulations of a
dark loam, as in the black cotton soil (_regur_) of India, and the
black earth (_tchernozom_) of Russia. Peat-mosses are formed in
temperate and arctic climates by the growth of marsh-loving plants,
sometimes to a thickness of 40 or 50 ft. In tropical regions the
mangrove swamps on low moist shores form a dense jungle, sometimes 20
m. broad, which protects these shores from the sea until, by the
arrest of sediment and the constant contribution of decayed
vegetation, the spongy ground is at last turned into firm soil. Some
plants (diatoms) can abstract silica and build it into their
framework, so that their remains form a siliceous deposit or ooze
which covers spaces of the deep sea-floor estimated at more than ten
millions of square miles in extent.

(b) _Animals._--These exert a destructive influence in the following
ways: (1) By seriously affecting the composition and arrangement of
the vegetable soil. Worms bring up the lower portions of the soil to
the surface, and while thus promoting its fertility increase its
liability to be washed away by rain. Burrowing animals, by throwing up
the soil and subsoil, expose these to be dried and blown away by the
wind. At the same time their subterranean passages serve to drain off
the superficial water and to injure the stability of the surface of
the ground above them. In Britain the mole and rabbit are familiar
examples. (2) By interfering with or even diverting the flow of
streams. Thus beaver-dams check the current of water-courses,
intercept floating materials, and sometimes turn streams into new
channels. The embankments of the Mississippi are sometimes weakened to
such an extent by the burrowings of the cray-fish as to give way and
allow the river to inundate the surrounding country. Similar results
have happened in Europe from subterranean operations of rats. (3) Some
mollusca bore into stone or wood and by the number of contiguous
perforations greatly weaken the material. (4) Many animals exercise a
ruinously destructive influence upon vegetation. Of the numerous
plagues of this kind the locust, phylloxera and Colorado beetle may be
cited.

The most important geological function performed by animals is the
formation of new deposits out of their remains. It is chiefly by the
lower grades of the animal kingdom that this work is accomplished,
especially by molluscs, corals and foraminifera. Shell-banks are
formed abundantly in such comparatively shallow and enclosed basins as
that of the North Sea, and on a much more extensive scale on the floor
of the West Indian seas. By the coral polyps thick masses of
limestones have been built up in the warmer seas of the globe (see
CORAL REEFS). The floor of the Atlantic and other oceans is covered
with a fine calcareous ooze derived mainly from the remains of
foraminifera, while in other regions the bottom shows a siliceous ooze
formed almost entirely of radiolaria. Vertebrate animals give rise to
phosphatic deposits formed sometimes of their excrement, as in guano
and coprolites, sometimes of an accumulation of their bones.

(c) _Man._--No survey of the geological workings of plant and animal
life upon the surface of the globe can be complete which does not take
account of the influence of man--an influence of enormous and
increasing consequence in physical geography, for man has introduced,
as it were, an element of antagonism to nature. His interference shows
itself in his relations to climate, where he has affected the
meteorological conditions of different countries: (1) By removing
forests, and laying bare to the sun and winds areas which were
previously kept cool and damp under trees, or which, lying on the lee
side, were protected from tempests. It is supposed that the wholesale
destruction of the woodlands formerly existing in countries bordering
the Mediterranean has been in part the cause of the present
desiccation of these districts. (2) By drainage, whereby the
discharged rainfall is rapidly removed, and the evaporation is
lessened, with a consequent diminution of rainfall and some increase
in the general temperature of a country. (3) By the other processes of
agriculture, such as the transformation of moor and bog into
cultivated land, and the clothing of bare hillsides with green crops
or plantations of coniferous and hardwood trees.

Still more obvious are the results of human interference with the flow
of water: (1) By increasing or diminishing the rainfall man directly
affects the volume of rivers. (2) By his drainage operations he makes
the rain to run off more rapidly than before, and thereby increases
the magnitude of floods and of the destruction caused by them. (3) By
wells, bores, mines, or other subterranean works he interferes with
the underground waters, and consequently with the discharge of
springs. (4) By embanking rivers he confines them to narrow channels,
sometimes increasing their scour, and enabling them to carry their
sediment further seaward, sometimes causing them to deposit it over
the plains and raise their level. (5) By his engineering operations
for water-supply he abstracts water from its natural basins and
depletes the streams.

In many ways man alters the aspect of a country: (1) By changing
forest into bare mountain, or clothing bare mountains with forest. (2)
By promoting the growth or causing the removal of peat-mosses. (3) By
heedlessly uncovering sand-dunes, and thereby setting in motion a
process of destruction which may convert hundreds of acres of fertile
land into waste sand, or by prudently planting the dunes with
sand-loving vegetation and thus arresting their landward progress. (4)
By so guiding the course of rivers as to make them aid him in
reclaiming waste land, and bringing it under cultivation. (5) By piers
and bulwarks, whereby the ravages of the sea are stayed, or by the
thoughtless removal from the beach of stones which the waves had
themselves thrown up, and which would have served for a time to
protect the land. (6) By forming new deposits either designedly or
incidentally. The roads, bridges, canals, railways, tunnels, villages
and towns with which man has covered the surface of the land will in
many cases form a permanent record of his presence. Under his hand the
whole surface of civilized countries is very slowly covered with a
stratum, either formed wholly by him or due in great measure to his
operations and containing many relics of his presence. The soil of
ancient towns has been increased to a depth of many feet by their
successive destructions and renovations.

Perhaps the most subtle of human influences are to be seen in the
distribution of plant and animal life upon the globe. Some of man's
doings in this domain are indeed plain enough, such as the extirpation
of wild animals, the diminution or destruction of some forms of
vegetation, the introduction of plants and animals useful to himself,
and especially the enormous predominance given by him to the cereals
and to the spread of sheep and cattle. But no such extensive
disturbance of the normal conditions of the distribution of life can
take place without carrying with it many secondary effects, and
setting in motion a wide cycle of change and of reaction in the animal
and vegetable kingdoms. For example, the incessant warfare waged by
man against birds and beasts of prey in districts given up to the
chase leads sometimes to unforeseen results. The weak game is allowed
to live, which would otherwise be killed off and give more room for
the healthy remainder. Other animals which feed perhaps on the same
materials as the game are by the same cause permitted to live
unchecked, and thereby to act as a further hindrance to the spread of
the protected species. But the indirect results of man's interference
with the regime of plants and animals still require much prolonged
observation.