Books: The Elements of Geology

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Even the glacial epoch, during which vast ice sheets deposited
drift over northeastern North America, must have been brief as
well as recent, for many lofty mountains, such as the Rockies and
the Alps, still bear the marks of great glaciers which then filled
their valleys. Had the glacial epoch been long, as the earth
counts time, these mountains would have been worn low by ice; had
the epoch been remote, the marks of glaciation would already have
been largely destroyed by other agencies.

On the other hand, rivers are well-nigh universally at work over
the land surfaces of the globe, and ever since the dry land
appeared they have been constantly engaged in leveling the
continents and in delivering to the seas the waste which there is
built into the stratified rocks.

ICEBERGS. Tide glaciers, such as those of Greenland and Alaska,
are able to excavate their beds to a considerable distance below
sea level. From their fronts the buoyancy of sea water raises and
breaks away great masses of ice which float out to sea as
icebergs. Only about one seventh of a mass of glacier ice floats
above the surface, and a berg three hundred feet high may be
estimated to have been detached from a glacier not less than two
thousand feet thick where it met the sea.

Icebergs transport on their long journeys whatever drift they may
have carried when part of the glacier, and scatter it, as they
melt, over the ocean floor. In this way pebbles torn by the inland
ice from the rocks of the interior of Greenland and glaciated
during their carriage in the ground moraine are dropped at last
among the oozes of the bottom of the North Atlantic.

CHAPTER VI

THE WORK OF THE WIND

We are now to study the geological work of the currents of the
atmosphere, and to learn how they erode, and transport and deposit
waste as they sweep over the land. Illustrations of the wind's
work are at hand in dry weather on any windy day.

Clouds of dust are raised from the street and driven along by the
gale. Here the roadway is swept bare; and there, in sheltered
places, the dust settles in little windrows. The erosive power of
waste-laden currents of air is suggested as the sharp grains of
flying sand sting one's face or clatter against the window. In the
country one sometimes sees the dust whirled in clouds from dry,
plowed fields in spring and left in the lee of fences in small
drifts resembling in form those of snow in winter.

THE ESSENTIAL CONDITIONS for the wind's conspicuous work are
illustrated in these simple examples; they are aridity and the
absence of vegetation. In humid climates these conditions are only
rarely and locally met; for the most part a thick growth of
vegetation protects the moist soil from the wind with a cover of
leaves and stems and a mattress of interlacing roots. But in arid
regions either vegetation is wholly lacking, or scant growths are
found huddled in detached clumps, leaving interspaces of
unprotected ground (Fig. 119). Here, too, the mantle of waste,
which is formed chiefly under the action of temperature changes,
remains dry and loose for long periods. Little or no moisture is
present to cause its particles to cohere, and they are therefore
readily lifted and drifted by the wind.

TRANSPORTATION BY THE WIND

In the desert the finer waste is continually swept to and fro by
the ever-shifting wind. Even in quiet weather the air heated by
contact with the hot sands rises in whirls, and the dust is lifted
in stately columns, sometimes as much as one thousand feet in
height, which march slowly across the plain. In storms the sand is
driven along the ground in a continuous sheet, while the air is
tilled with dust. Explorers tell of sand storms in the deserts of
central Asia and Africa, in which the air grows murky and
suffocating. Even at midday it may become dark as night, and
nothing can be heard except the roar of the blast and the whir of
myriads of grains of sand as they fly past the ear.

Sand storms are by no means uncommon in the arid regions of the
western United States. In a recent year, six were reported from
Yuma, Arizona. Trains on transcontinental railways are
occasionally blockaded by drifting sand, and the dust sifts into
closed passenger coaches, covering the seats and floors. After
such a storm thirteen car loads of sand were removed from the
platform of a station on a western railway.

DUST FALLS. Dust launched by upward-whirling winds on the swift
currents of the upper air is often blown for hundreds of miles
beyond the arid region from which it was taken. Dust falls from
western storms are not unknown even as far east as the Great
Lakes. In 1896 a "black snow" fell in Chicago, and in another dust
storm in the same decade the amount of dust carried in the air
over Rock Island, Ill., was estimated at more than one thousand
tons to the cubic mile.

In March, 1901, a cyclonic storm carried vast quantities of dust
from the Sahara northward across the Mediterranean to fall over
southern and central Europe. On March 8th dust storms raged in
southern Algeria; two days later the dust fell in Italy; and on
the 11th it had reached central Germany and Denmark. It is
estimated that in these few days one million eight hundred
thousand tons of waste were carried from northern Africa and
deposited on European soil.

We may see from these examples the importance of the wind as an
agent of transportation, and how vast in the aggregate are the
loads which it carries. There are striking differences between air
and water as carriers of waste. Rivers flow in fixed and narrow
channels to definite goals. The channelless streams of the air
sweep across broad areas, and, shifting about continually, carry
their loads back and forth, now in one direction and now in
another.

WIND DEPOSITS

The mantle of waste of deserts is rapidly sorted by the wind. The
coarser rubbish, too heavy to be lifted into the air, is left to
strew wide tracts with residual gravels (Fig. 120). The sand
derived from the disintegration of desert rocks gathers in vast
fields. About one eighth of the surface of the Sahara is said to
be thus covered with drifting sand. In desert mountains, as those
of Sinai, it lies like fields of snow in the high valleys below
the sharp peaks. On more level tracts it accumulates in seas of
sand, sometimes, as in the deserts of Arabia, two hundred and more
feet deep.

DUNES. The sand thus accumulated by the wind is heaped in wavelike
hills called dunes. In the desert of northwestern India, where the
prevalent wind is of great strength, the sand is laid in
longitudinal dunes, i.e. in stripes running parallel with the
direction of the wind; but commonly dunes lie, like ripple marks,
transverse to the wind current. On the windward side they show a
long, gentle slope, up which grains of sand can readily be moved;
while to the lee their slope is frequently as great as the angle
of repose (Fig. 122). Dunes whose sands are not fixed by
vegetation travel slowly with the wind; for their material is ever
shifted forward as the grains are driven up the windward slope
and, falling over the crest, are deposited in slanting layers in
the quiet of the lee.

Like river deposits, wind-blown sands are stratified, since they
are laid by currents of air varying in intensity, and therefore
in transporting power, which carry now finer and now coarser
materials and lay them down where their velocity is checked (Fig.
123). Since the wind varies in direction, the strata dip in
various directions. They also dip at various angles, according to
the inclination of the surface on which they were laid.

Dunes occur not only in arid regions, but also wherever loose sand
lies unprotected by vegetation from the wind. From the beaches of
sea and lake shores the wind drives inland the surface sand left
dry between tides and after storms, piling it in dunes which may
invade forests and fields and bury villages beneath their slowly
advancing waves. On flood plains during summer droughts river
deposits are often worked over by the wind; the sand is heaped in
hummocks and much of the fine silt is caught and held by the
forests and grassy fields of the bordering hills.

The sand of shore dunes differs little in composition and the
shape of its grains from that of the beach from which it was
derived. But in deserts, by the long wear of grain on grain as
they are blown hither and thither by the wind, all soft minerals
are ground to powder and the sand comes to consist almost wholly
of smooth round grams of hard quartz.

Some marine sandstones, such as the St. Peter sandstone of the
upper Mississippi valley, are composed so entirely of polished
spherules of quartz that it has been believed by some that their
grains were long blown about in ancient deserts before they were
deposited in the sea.

DUST DEPOSITS. As desert sands are composed almost wholly of
quartz, we may ask what has become of the softer minerals of which
the rocks whose disintegration has supplied the sand were in part,
and often in large part, composed. The softer minerals have been
ground to powder, and little by little the quartz sand also is
worn by attrition to fine dust. Yet dust deposits are scant and
few in great deserts such as the Sahara. The finer waste is blown
beyond its limits and laid in adjacent oceans, where it adds to
the muds and oozes of their floors, and on bordering steppes and
forest lands, where it is bound fast by vegetation and slowly
accumulates in deposits of unstratified loose yellow earth. The
fine waste of the Sahara has been identified in dredgings from the
bottom of the Atlantic Ocean, taken hundreds of miles from the
coast of Africa.

LOESS. In northern China an area as large as France is deeply
covered with a yellow pulverulent earth called loess (German,
loose), which many consider a dust deposit blown from the great
Mongolian desert lying to the west. Loess mantles the recently
uplifted mountains to the height of eight thousand feet and
descends on the plains nearly to sea level. Its texture and lack
of stratification give it a vertical cleavage; hence it stands in
steep cliffs on the sides of the deep and narrow trenches which
have been cut in it by streams.

On loess hillsides in China are thousands of villages whose
eavelike dwellings have been excavated in this soft, yet firm, dry
loam. While dust falls are common at the present time in this
region, the loess is now being rapidly denuded by streams, and its
yellow silt gives name to the muddy Hwang-ho (Yellow River), and
to the Yellow Sea, whose waters it discolors for scores of miles
from shore.

Wind deposits both of dust and of sand may be expected to contain
the remains of land shells, bits of wood, and bones of land
animals, testifying to the fact that they were accumulated in open
air and not in the sea or in bodies of fresh water.

WIND EROSION

Sand-laden currents of air abrade and smooth and polish exposed
rock surfaces, acting in much the same way as does the jet of
steam fed with sharp sand, which is used in the manufacture of
ground glass. Indeed, in a single storm at Cape Cod a plate glass
of a lighthouse was so ground by flying sand that its transparency
was destroyed and its removal made necessary.

Telegraph poles and wires whetted by wind-blown sands are
destroyed within a few years. In rocks of unequal resistance the
harder parts are left in relief, while the softer are etched away.
Thus in the pass of San Bernardino, Cal., through which strong
winds stream from the west, crystals of garnet are left projecting
on delicate rock fingers from the softer rock in which they were
imbedded.

Wind-carved pebbles are characteristically planed, the facets
meeting along a summit ridge or at a point like that of a pyramid.
We may suppose that these facets were ground by prevalent winds
from certain directions, or that from time to time the stone was
undermined and rolled over as the sand beneath it was blown away
on the windward side, thus exposing fresh surfaces to the driving
sand. Such wind-carved pebbles are sometimes found in ancient
rocks and may be accepted as evidence that the sands of which the
rocks are composed were blown about by the wind.

DEFLATION. In the denudation of an arid region, wind erosion is
comparatively ineffective as compared with deflation (Latin, de,
from; flare, to blow),--a term by which is meant the constant
removal of waste by the wind, leaving the rocks bare to the
continuous attack of the weather. In moist climates denudation is
continually impeded by the mantle of waste and its cover of
vegetation, and the land surface can be lowered no faster than the
waste is removed by running water. Deep residual soils come to
protect all regions of moderate slope, concealing from view the
rock structure, and the various forms of the land are due more to
the agencies of erosion and transportation than to differences in
the resistance of the underlying rocks.

But in arid regions the mantle is rapidly removed, even from well-
nigh level plains and plateaus, by the sweep of the wind and the
wash of occasional rains. The geological structure of these
regions of naked rock can be read as far as the eye can see, and
it is to this structure that the forms of the land are there
largely due. In a land mass of horizontal strata, for example, any
softer surface rocks wear down to some underlying, resistant
stratum, and this for a while forms the surface of a level plateau
(Fig. 129). The edges of the capping layer, together with those of
any softer layers beneath it, wear back in steep cliffs, dissected
by the valleys of wet-weather streams and often swept bare to the
base by the wind. As they are little protected by talus, which
commonly is removed about as fast as formed, these escarpments and
the walls of the valleys retreat indefinitely, exposing some hard
stratum beneath which forms the floor of a widening terrace.

The high plateaus of northern Arizona and southern Utah, north of
the Grand Canyon of the Colorado River, are composed of stratified
rocks more than ten thousand feet thick and of very gentle
inclination northward. From the broad plat form in which the
canyon has been cut rises a series of gigantic stairs, which are
often more than one thousand feet high and a score or more of
miles in breadth. The retreating escarpments, the cliffs of the
mesas and buttes which they have left behind as outliers, and the
walls of the ravines are carved into noble architectural forms--
into cathedrals, pyramids, amphitheaters, towers, arches, and
colonnades--by the processes of weathering aided by deflation. It
is thus by the help of the action of the wind that great plateaus
in arid regions are dissected and at last are smoothed away to
waterless plains, either composed of naked rock, or strewed with
residual gravels, or covered with drifting residual sand.

The specific gravity of air is 1/823 that of water. How does this
fact affect the weight of the material which each can carry at the
same velocity?

If the rainfall should lessen in your own state to from five to
ten inches a year, what changes would take place in the vegetation
of the country? in the soil? in the streams? in the erosion of
valleys? in the agencies chiefly at work in denuding the land?

In what way can a wind-carved pebble be distinguished from a
river-worn pebble? from a glaciated pebble?

CHAPTER VII

THE SEA AND ITS SHORES

We have already seen that the ocean is the goal at which the waste
of the land arrives. The mantle of rock waste, creeping down
slopes, is washed to the sea by streams, together with the
material which the streams have worn from their beds and that
dissolved by underground waters. In arid regions the winds sweep
waste either into bordering oceans or into more humid regions
where rivers take it up and carry it on to the sea. Glaciers
deliver the load of their moraines either directly to the sea or
leave it for streams to transport to the same goal. All deposits
made on the land, such as the flood plains of rivers, the silts of
lake beds, dune sands, and sheets of glacial drift, mark but
pauses in the process which is to bring all the materials of the
land now above sea level to rest upon the ocean bed.

But the sea is also at work along all its shores as an agent of
destruction, and we must first take up its work in erosion before
we consider how it transports and deposits the waste of the land.

SEA EROSION

THE SEA CLIFF AND THE ROCK BENCH. On many coasts the land fronts
the ocean in a line of cliffs. To the edge of the cliffs there
lead down valleys and ridges, carved by running water, which, if
extended, would meet the water surface some way out from shore.
Evidently they are now abruptly cut short at the present shore
line because the land has been cut back.

Along the foot of the cliff lies a gently shelving bench of rock,
more or less thickly veneered with sand and shingle. At low tide
its inner margin is laid bare, but at high tide it is covered
wholly, and the sea washes the base of the cliffs. A notch, of
which the SEA CLIFF and the ROCK BENCH are the two sides, has been
cut along the shore.

WAVES. The position of the rock bench, with its inner margin
slightly above low tide, shows that it has been cut by some agent
which acts like a horizontal saw set at about sea level. This
agent is clearly the surface agitation of the water; it is the
wind-raised wave.

As a wave comes up the shelving bench the crest topples forward
and the wave "breaks," striking a blow whose force is measured by
the momentum of all its tons of falling water. On the coast of
Scotland the force of the blows struck by the waves of the
heaviest storms has sometimes exceeded three tons to the square
foot. But even a calm sea constantly chafes the shore. It heaves
in gentle undulations known as the ground swell, the result of
storms perhaps a thousand miles distant, and breaks on the shore
in surf.

The blows of the waves are not struck with clear water only, else
they would have little effect on cliffs of solid rock. Storm waves
arm themselves with the sand and gravel, the cobbles, and even the
large bowlders which lie at the base of the cliff, and beat
against it with these hammers of stone.

Where a precipice descends sheer into deep water, waves swash up
and down the face of the rocks but cannot break and strike
effective blows. They therefore erode but little until the talus
fallen from the cliff is gradually built up beneath the sea to the
level at which the waves drag bottom upon it and break.

Compare the ways in which different agents abrade. The wind
lightly brushes sand and dust over exposed surfaces of rock.
Running water sweeps fragments of various sizes along its
channels, holding them with a loose hand. Glacial ice grinds the
stones of its ground moraine against the underlying rock with the
pressure of its enormous weight. The wave hurls fragments of rock
against the sea cliff, bruising and battering it by the blow. It
also rasps the bench as it drags sand and gravel to and fro upon
it.

WEATHERING OF SEA CLIFFS. The sea cliff furnishes the weapons for
its own destruction. They are broken from it not only by the wave
but also by the weather. Indeed the sea cliff weathers more
rapidly, as a rule, than do rock ledges inland. It is abundantly
wet with spray. Along its base the ground water of the neighboring
land finds its natural outlet in springs which under mine it.
Moreover, it is unprotected by any shield of talus. Fragments of
rock as they fall from its face are battered to pieces by the
waves and swept out to sea. The cliff is thus left exposed to the
attack of the weather, and its retreat would be comparatively
rapid for this reason alone.

Sea cliffs seldom overhang, but commonly, as in Figure 134, slope
seaward, showing that the upper portion has retreated at a more
rapid rate than has the base. Which do you infer is on the whole
the more destructive agent, weathering or the wave?

Draw a section of a sea cliff cut in well jointed rocks whose
joints dip toward the land. Draw a diagram of a sea cliff where
the joints dip toward the sea.

SEA CAVES. The wave does not merely batter the face of the cliff.
Like a skillful quarryman it inserts wedges in all natural
fissures, such as joints, and uses explosive forces. As a wave
flaps against a crevice it compresses the air within with the
sudden stroke; as it falls back the air as suddenly expands. On
lighthouses heavily barred doors have been burst outward by the
explosive force of the air within, as it was released from
pressure when a partial vacuum was formed by the refluence of the
wave. Where a crevice is filled with water the entire force of the
blow of the wave is transmitted by hydraulic pressure to the sides
of the fissure. Thus storm waves little by little pry and suck the
rock loose, and in this way, and by the blows which they strike
with the stones of the beach, they quarry out about a joint, or
wherever the rock may be weak, a recess known as a SEA CAVE,
provided that the rock above is coherent enough to form a roof.
Otherwise an open chasm results.

BLOWHOLES AND SEA ARCHES. As a sea cave is drilled back into the
rock, it may encounter a joint or crevice opened to the surface by
percolating water. The shock of the waves soon enlarges this to a
blowhole, which one may find on the breezy upland, perhaps a
hundred yards and more back from the cliff's edge. In quiet
weather the blowhole is a deep well; in storm it plays a fountain
as the waves drive through the long tunnel below and spout their
spray high in air in successive jets. As the roof of the cave thus
breaks down in the rear, there may remain in front for a while a
sea arch, similar to the natural bridges of land caverns.

STACKS AND WAVE-CUT ISLANDS. As the sea drives its tunnels and
open drifts into the cliff, it breaks through behind the
intervening portions and leaves them isolated as stacks, much as
monuments are detached from inland escarpments by the weather; and
as the sea cliff retreats, these remnant masses may be left behind
as rocky islets. Thus the rock bench is often set with stacks,
islets in all stages of destruction, and sunken reefs, all wrecks
of the land testifying to its retreat before the incessant attack
of the waves.

COVES. Where zones of soft or closely jointed rock outcrop along a
shore, or where minor water courses conic down to the sea and aid
in erosion, the shore is worn back in curved reentrants called
coves; while the more resistant rocks on either hand are left
projecting as headlands (Fig. 139). After coves are cut back a
short distance by the waves, the headlands come to protect them,
as with breakwaters, and prevent their indefinite retreat. The
shore takes a curve of equilibrium, along which the hard rock of
the exposed headland and the weak rock of the protected cove wear
back at an equal rate.

RATE OF RECESSION. The rate at which a shore recedes depends on
several factors. In soft or incoherent rocks exposed to violent
storms the retreat is so rapid as to be easily measured. The coast
of Yorkshire, England, whose cliffs are cut in glacial drift,
loses seven feet a year on the average, and since the Norman
conquest a strip a mile wide, with farmsteads and villages and
historic seaports, has been devoured by the sea. The sandy south
shore of Martha's Vineyard wears back three feet a year. But hard
rocks retreat so slowly that their recession has seldom been
measured by the records of history.

SHORE DRIFT

BOWLDER AND PEBBLE BEACHES. About as fast as formed the waste of
the sea cliff is swept both along the shore and out to sea. The
road of waste along shore is the BEACH. We may also define the
beach as the exposed edge of the sheet of sediment formed by the
carriage of land waste out to sea. At the foot of sea cliffs,
where the waves are pounding hardest, one commonly finds the rock
bench strewn on its inner margin with large stones, dislodged by
the waves and by the weather and some-what worn on their corners
and edges. From this BOWLDER BEACH the smaller fragments of waste
from the cliff and the fragments into which the bowlders are at
last broken drift on to more sheltered places and there accumulate
in a PEBBLE BEACH, made of pebbles well rounded by the wear which
they have suffered. Such beaches form a mill whose raw material is
constantly supplied by the cliff. The breakers of storms set it in
motion to a depth of several feet, grinding the pebbles together
with a clatter to be heard above the roar of the surf. In such a
rock crusher the life of a pebble is short. Where ships have
stranded on our Atlantic coast with cargoes of hard-burned brick
or of coal, a year of time and a drift of five miles along the
shore have proved enough to wear brick and coal to powder. At no
great distance from their source, therefore, pebble beaches give
place to beaches of sand, which occupy the more sheltered reaches
of the shore.

SAND BEACHES. The angular sand grains of various minerals into
which pebbles are broken by the waves are ground together under
the beating surf and rounded, and those of the softer minerals are
crushed to powder. The process, however, is a slow one, and if we
study these sand grains under a lens we may be surprised to see
that, though their corners and edges have been blunted, they are
yet far from the spherical form of the pebbles from which they
were derived. The grains are small, and in water they have lost
about half their weiglit in air; the blows which they strike one
another are therefore weak. Besides, each grain of sand of the wet
beach is protected by a cushion of water from the blows of its
neighbors.

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