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Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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surface of the ground, forming sinter or tufa. If the latter



U2



PHYSICAL GEOGKAPHY



happens to cover loose rock waste or soil, a cavern or cave
will result if the material under it be removed.

In other instances the hot waters, charged with mineral
or metallic salts, flow into deep fissures in the rocks. As
the water cools the soluble matter is dejDosited on the walls
of the fissure until, finally, the latter is filled, thereby
forming a mineral vein or lode. All through the various




BLUE GROTTO, ISLAND OF CAPRL ITALY

mountain regions of the earth, gold, silver, copper, lead,
and other valuable metals have been deposited in such
fissures and veins." Thus underground waters are a
vehicle by which many useful metals are carried from the
interior to the surface of the earth.

Caverns. — Caverns and caves, although sometimes
formed at the surface in the manner already noted, for the
greater part are formed underground .by the action of
■water.'- The water merely dissolves the rock and carries



UNDERGROUND WATERS



1^3



it off, leaving a cavern. Clay,
slate, granite, and sandstones are
not readily dissolved ; and in re-
gions underlaid by such rocks,
caverns are rare. Limestones, on
the contrary, are quite soluble,
and in localities where they are
the prevailing rock, caves and
caverns are common. In the cav-
ern district of Kentucky, Tennes-
see, and Yirgiuia^^ small pieces
of sharp flint are plentifully dis-
tributed throughout the lime-
stone. These are tossed about
and carried along with the water
and thus become powerful cutting
tools.

Between the solvent power of
the water and the incessant cut-
ting done by the flint particles,
the vmderground channel is worn
deeper and wider till a cavern,
perhaps a score of miles long and
many feet deep, is formed. Very
likely it has hundreds of galleries
and branches ; time alone is nec-
essary to give it vast dimensions.

But time alone will see the fac-
tors that nuide the cavern destroy
it. In the first place the surface
waters are constantly at work
wearing away the rock that forms
the roof or dome of the cavern.
By and by breaks are made



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p-



U4



PHYSICAL GEOGRAPHY



tlirougli tliG roof ami sinkholes are thus formed. These
increase in size and in number until the dome is destroyed.
The river is then no longer an underground stream ; it is a
surface river flowing in a limestone canon. Natural Bridge,
in Virginia, is a remnant of one of these domes ; the rest of
the roof has been carried away."

In the second place, parts of these caverns are filled up by




A PASSAGE IN LURAY CAVERN— STALACTITES AND STALAGMITES

the limestone itself. In places the water charged with lime-
stone leaks or filters through at the top of the dome, fall-
ing drop by drop. A part of the water leaves a minute
portion of limestone at the roof ; the rest falls to the floor
of the cavern, where a little more of the water evaporates.
So, little by little, the deposited limestone gathers into
icicle-shaped columns, both at the roof and the floor of the



UNDERGROUND WATERS U5

cavern. The former are called sfalactifes, tlie latter stalag.
mites. In time the two join, forming a single column, and
as the water trickles clown their sides they increase in size,
and thus the cavern is filled.

Perhaps, in the course of time, this same mass of lime-
stone may be dissolved away and redeposited elsewhere.
At all events, the process illustrates the general law that
governs cavern-formation in these regions. Water in mo-
tion dissolves limestone and makes caverns ; still water de-
jjosits limestone and fills tliem up}^

QUESTIONS AND EXERCISES.— If possible find the depth of each
of half a dozen or more wells in the neighborhood in which you live ;
compare the distance of the surface of the ground to the surface of the
water in the wells.

To what depth must a well be sunk before it will fill with water ?

Will one be apt to find percolating waters in regions having but a
very little rain ?

Explain why water in very shallow wells is apt to be impure

How do springs become " mineral " in character ?

Why does rain water contain no mineral matter in solution ?

Why are geysers and hot springs confined usually to volcanic regions?

Under what circumstances or conditions can water be heated above
the ordinary boiling point ? (See almost any text-book in physics.)

From the diagram, p. 129, decide the conditions which will cause
underground streams or percolating waters to form a swamp.

Describe a way in which caverns may be formed at the foot of sea
cliffs that face heavy waves.

How are the sinkholes in the limestone regions formed ?

By using lime-water such as is obtainable at the druggist's, suggest
a way in which stalactites may be artificially formed.

COLLATKRAL READING.
Shaler. — First Book in (ieoloi?y, pp. 66-87.
Shaler.— Aspects of the Earth, pp. 06-143.
Powell.— Irrigation and Artesian Wells, pp. 203-290. United

States Oeo/of/ic(tl Siii-neiy, 11th Anir/iaJ licport. Part 2.
Le CoxTE.^Elenients of (jreolot;y, pp. l();5-li:{.
United States Geological Survey. — Map of Yellowstone
National Park.



146 PHYSICAL GEOGRAPHY



NOTES

^ If the rocks are near the surface and the amount of water is
considerable, swamps may result. That is, swamps may be an
incident of imperfect underground drainage, as they are of im-
perfect surface drainage.

' The fissures between the ends of faulted strata are very fre-
quently the channels of springs, and sooner or later the fissure is
likely to be closed up by deposits from the spring water.

3 This may be seen in the cases of streams that flow through a
region of pervious soil. Such streams steadily increase in volume,
althougli for many miles tlaey receive no tributaries. As an ex-
ample, Spanish Fork, on the west slope of the Wasatch Mountains,
receives only two or three small tributaries from the summit to
the base of the mountains. It begins as a rivulet, scarcely larger
than one's arm ; it reaches the base of the range, a mountain tor-
rent twenty feet across. Almost the whole increment is due to
percolation.

* The sand valleys are apparently Tdlls, but, in most cases
they are valleys filled with rock waste carried thither by winds.
In tlie saturation of these accumulations of rock waste capillary
attraction is an important factor ; for this little-understood
force is not only an agent of accumulation, but one of reten-
tion also.

5 The amount of desert land made productive solely by artesian
wells has been greatly exaggerated by senseless guesses. Such es-
timates commonly make the aggregate as ' ' millions of square
miles." As a matter of fact all the artesian wells in the world
do not supply an area equal to that of the State of Delaware with
the water necessary to produce tlie whole of its crops.

" The difference between springs and percolating waters is
mainly one of degree ; issuing from a channel it is a spring, but
if the water merely oozes through the soil it is considered only
as an example of percolation. In Florida there are a number of
springs, so-called, that discharge each an amount of water suffi-
cient to fill a river bed. Orange and Silver Springs are so large
that small river craft easily enter the mouths. Tliese springs, as
a matter of fact, are the exits of underground rivers.

' From time immemorial geograpliers have explained the peri-



UNDERGROUND WATERS 147

odieal spring on the supposed existence ot a siplion-shaped chan-
nel. Doubtless such channels exist, but not a single one has ever
been discovered. In a tew instances the pressure ot accumulating
gases is known to be a cause ot uiteruiittent How, but in the great
majority ot eases the cause of periodicity is unknown. One of the
most remarkable periodical springs occurs in Palestine near the old
convent of Mar Jirius. This spring is quiescent for about two and
a half days, and its period of activity lasts for several hours. It
is probable that the stream flowing from this spring is the Sab-
batic River described by Josephus, which rested for six days and
flowed on the seventh. The fact that such springs gradually de-
crease their periods ot quiescence, and finally become regular,
bears out this supposition . A spring near Rogersville, Tennessee,
IS celebrated for the enormous quantity of water ejected. Its
period of flow occurs about every half hour, lasting only a few
minutes. The Bullerborn, once a famous intermittent spring of
Westphalia, has now' a constant flow. In regions of very high
tides, periodical springs are sometimes formed by tidal action.
The fresh water is pushed back by the tide, until it emerges to
the surface through self-made channels.

8 In desert regions, where the heat is intense, there are many
instances ot rivers that are dry "washes" in the daytime, and
fair-sized streams at night. Water nearly always can be found
at a slight depth, by digging for it. In the daytime, the enor-
mous evaporation causes the water to disappear. In the night,
or during cloudy days, the evaporation is lessened and the perco-
lating waters rise to the surface. This plienomenon is occasion-
ally noticed in the lower courses of Humboldt, Carson, and Reese
Rivers, in Nevada. TIu; underground part of the river is nearly
always to be found.

'•• Considerable trouble from this cause occurred near tlu^ junc-
tion of Oxford Street and Edgeware Road, London, and the rea-
son was the fact that the famous Tyburn flowed in tliis locality,
crossing Oxford Street a little to the eastward of the entrance
to Hyde Park. .About four hundred S(iuare feet of Broad-
way, New York, recently caved in from a, similar cause. The
foundations of a costly church in Philadelphia sank in the
(juicksand before they were completcil, and the large sewer under
one of the principal streets has caved in several times— all be-
cause they were iindci-rnined hy hiii-jed streams.



148 PHYSICAL GEOGRAPHY

•"At Orangovillo, liuliana., an underground stream comes to
the surface and Hows vvitli sufficient force to turn a mill-wheel.
Only a few miles away, Lost River, a considerable stream, sinks
out of sight. San Pedro Springs, near San Antonio, Texas,
is the outlet of an underground stream. Giant Spring, near
(ireat Falls, Montana, is the outlet of Little Belt River, which
disappears and flows underground for thirty miles of its course.
In Alabama, the engineers of the Anniston and Atlantic Railway
discovered an underground stream sixty feet below the bed of
Coosa River. According to Greek legends, the Alpheus, the river
of Peloponnesus, which Hercules turned through the Augean
stables, sank underground and emerged to the surface somewhere
in Sicily. As a matter of fact a considerable part of the course
of the Alpheus is underground, and there is a spring in Sicily
discharging a large volume of water. It is hardly necessary to
add that the two have no connection.

" As a rule such veins have a very symmetrical banded appear-
ance, the stripes on the right hand corresponding with those on
the left, in California, these veins are called "ribbon " rock.

'2 There are several instances in which caves have been formed
in volcanic rocks. Fingal's Cave, on the Island of StafTa, west
of Scotland, is an example. It is more than two hundred feet in
length, and is surmounted by a dome sixty feet high. In many
instances waves have hollowed out caverns in rock cliffs.

^^ Mammotli Cave, Kentucky, is a labyrinth of passages aggre-
gating more than two hundred miles ; the length of the cave on
a straight line is about ten miles. Some of the vaults and domes
are two hundred and fifty feet high. There are several othei
caves in the vicinity nearly if not quite as large. Weir's Cave
and Luray Cavern, both in Virginia, are smaller than Mammoth
Cave. Being limestone caverns they do not differ from the latter.
Howe's Cave, Schoharie County, New York, is one of the few
large caverns of interest in the northern Appalachian region. In
the grotto of Lueg, Illyria, there are three galleries, one over
anotlier. The cavern of Adelsberg, Austria, is the abandoned
channel of the Poik. Its length is not far from two miles ; its
labyrinthine passages aggregate many miles. A considerable
part of the course of the Poik is underground. Probably the un-
derground passage and caverns of the Timavo have been more
thoroughly investigated than those of any other stream. The



UNDERGROUND WATERS 149

river flows to the Adriatic, a few miles north of Trieste, and its
c'iiaracter lius been Ivnown for more tlian two thousand years.
Concerning it Virgil wrote :

. . et fontem superare Timavi
undo i)cr ora noveni vasto cum iiiurniure nioutis
it marc proruptum, et pelago premit arva sonanti.

—^Eneid I, 247.

Virgil's description is no longer true of the delta, for the nine
mouths have become only three in number.

^* Many similar natural bridges are known to exist in various
parts of the world. Near Bogota, Colombia, a natural arch spans
a chasm nearly four hundred feet deep. The arch is a double
one, the lower one being composed of three large fragments that,
detached from the upper arch, fell in such a manner as to wedge
themselves between the perpendicular walls. In one of the deep
canons of Arizona a huge mass of rock has fallen and become
wedged between the walls of the chasm, thereby forming a sort
of natural bridge. A natural bridge spans Pine Creek, in Gila
County, Arizona. Like that In Virginia, it is the fragment of the
dome of a stream that once flowed underground. The arch is
about four hundred feet wide and the span is about a thousand
feet in length. The underside of the arch is water- worn, but
since it was formed the creek has cut its channel more than two
hundred feet downward. In several instances the arch more
properly constitutes a tunnel. One, near Clinch River, Virginia,
is more than half a mile long, and is a part ot the route selected
for a railway, in almost every in.stance a stream ot water flows
under the arch, and its current carries aAvay the fragments that
fall from the roof.

'^ The distribution and also the concentration of certain eco-
nomic minerals, in many instances, lias resulted from the flow of
underground waters. Gold has been dissolved from certain rocks
and gradually concentrated in veins through their action. Iron
salts have been leached from rocks and deposited in other rocks
by the same agency. Beds of sand through which water con-
taining lime percolates, in time, become sandstone, the grains of
sand being cemented together by the lime carried in the water.



CHAPTER IX

THE WASTING OF THE LAND: THE WOKK OP
AVALANCHES AND GLACIERS

A GREAT deal of the moisture miugled with the air falls
upon the laud in the form of snow. Excepting very cold
regions, the snow that falls at altitudes below three or
four thousand feet melts with the coming of spring and
flows away in the various stream channels. In high
mountain regions more or less snow falls at altitudes in
which the temperature is rarely higher than the melting
point of the snow. In such localities, therefore, but little
of the snow can melt where it falls.'

In the Alps and in the higher ranges of the western
United States, the heaviest snows fall between the altitudes
of six thousand and nine thousand feet. Very little accu-
mulates below four chousand feet, and but little falls above
twelve thousand feet; in fact but httle moisture exists at
such high altitudes.

At high elevations, even though the fall might be slight,
it would seem as though the accumulation would increase
until the mass of snow exceeded that of the mountains.
In certain polar lands it is possible that tliis may be oc-
curring, but in high mountain regions various agencies
operate to prevent such enormous accumulation. Among
them are evaporation, wind, avalanches, and glaciers.
They not only remove the snow and ice, but they are also
powerful factors in wearing away the land and in trans-
porting rock waste.

150



WORK OF AVALANCHES AND GLACIERS 151



Evaporation is a veiy active agent in the removal of
snow. Ice and snow evaporate jiist as does water ; and
at great heights, where the air does not press so heavily as
at sea-level, evaporation is very rapid. This is seen when
frozen roads become dry and dusty without thawing.'-^

Winds are also a very potent factor. In high mountain
regions the wind has a force that is almost unknown in
lowlands, and the gales that rage among snow-covered
peaks quickly clear the dry snow-dust from every exposed
surface and drift it into ravines and canons.'^

There are two factors at work, however, that are inter-
esting, not only because they remove an enormous amount
of snow, but also because in transporting it they become
physiographic agents of
very great importance.
These are avalanches
and glaciers.

Avalanches. — When
a great body of snow,
resting on a steep slope,
suddenly slips and
plunges down the in-
cline, the moving mass
is called an avalanche,
or challanche. Excepting
the matter of the mate-
rial transported, which
is mainly snow, the ava-
lanche does not differ
materially from an ordi-
nary landslide. But
while it is very rare that a second landslide takes place
in the same track, it is evident that an avalanche may
occur every time the snow falls on the slope. The snow




AVALANCHE UASIN, MONTANA

Tlie slopes arc loo sleep lo permit the accuitnitation
of snow, and the latter. g,itlieri/ig- ■zvithin the
basin, has formed the lake at the bottom of the
cliff.



153 PHYSICAL GEOGRArilY

acemiiulates ou the steep slope uutil its great Aveight
causes it to slip, and the great mass gathering speed,
moves downward with a terrific roar.

lu the Alps, where as a rule, the slopes are steep,^ such
downfalls take place frequently and regularly. In many
places the avalanche tracks are as definitely marked as the
river channels. Indeed, one may consider the avalanche
track as the torrential part of a stream Avhose flow is occa-
sional and spasmodic. Like the mountain torrent, too, it
carries to lower levels an enormous amount of rock waste
stripped from the slopes. Not only are avalanche courses
distinctly marked, but expert mountaineers who have ac-
quired experience in discerning weather signs are able to
predict the occurrence of the snoAvslide with great cer-
tainty. The avalanche, therefore, is a feature of mountain
economy not less normal than the mountain torrent.

The most destructive avalanches occur in the first hours
of sunshine, just after a snow-storm.^ The flakes are then
so fine and smooth that they have but little coherence, and
almost any disturbance may start them. The footstep of
the chamois or a gnst of wind imparts motion to a handful
of snow, and it begins its descent. Gathering fresh mate-
rial as it advances, and increasing in velocity every moment,
it soon becomes a force that sweeps everything before it,
carrying havoc and destruction perhaps into the region
of cultivated fields and human habitations, far beyond the
foot of the slope. Rocks crash right and left and the whirl
of the wind carries eddies of snow a thousand feet or more
into the air.

When avalanches follow their customary tracks they are
neither especially dangerous nor destructive, unless the
snow and rock waste reach beyond their ordinary limits.
But in many instances they have taken place in localities
previously free from them, and these are the cases in which



WORK OF AVALANCHES AND GLACIERS 153

the destruction is greatest. Not oul}- is everytliiug de-
stroyed aloDg the path of the moving snow, but the effects
are even more apparent along the edges; for the bhists of
wind set in motion by the swiftly mo^dug snow, fell every
vestige of timber a thousand feet or more on both sides.
In recent years, places that the experienced mountaineers
have discovered to be possible avalanche tracks, have been
artificially guarded, so as to prevent the formation of
dangerous snowslides.

Another form of avalanche occurs in the Alps late in
the season, at the beginning of Avarm weather. Instead of
light, powdery snow, its volume consists of ice and coarse
snow mixed with rock waste.'' The lower part of the snow
and ice are undermined by water as the ground on
which it rests thaws. Finally the whole mass slides down
the incline. These avalanches do not differ in any ma-
terial respect from landslides.

Glaciers. — A great part of the snow that falls on high
and steep slopes is either blown into ravines by the wind
or is tumbled into them by avalanclies. In the upper
part of the ravine the snow is light and flaky, but farther
down it has begun to melt, and instead of crystals it con-
sists of little granules of ice, called neve. Still farther
down the ravine, the neve has a striped or banded appear-
ance.^ Then the surface takes the form of irregular wave-
shaped ridges, and finally the surface is a field of ridges
and hummocks,^ half-drowned in streams of muddy water,
and ending in a mountain torrent.

All this mass of ice and snow constitutes a glacier. It
is in motion, and excepting the velocity, which is so slow
as to be almost imperceptible, its movements are much
like those of a stream of water. The flow is faster at the
surface than at the bottom, and it is also swifter in mid-
stream than at the edges.




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WORK OF AVALANCHES AXD GLACIERS 155



Because the glacier moves more rapidly in the centre
than at the sides of the stream, the surface is scored with
cracks and chasms called crevasses. These are roughl}'
parallel and cross the glacier in lines which, in many in-
stances, point upstream.^ In some cases the crevasses form
gently curving parallel lines that are not milike the rip-
ples in a river. Ordinaril}-, the crevasse is narrow and
only a few feet deep ; but in some places it becomes a
chasm fifty or sixty feet from top to bottom. Crevasses
are most numerous in that part of the glacier where the
slope is the steepest ; and, in general, they mark what in a
river would be the rapids. The velocity of the current
varies. On a



gentle
slope it may not be
more than three or four
inches a day ; on a
steep incline it may be
half as many feet. In
summer, when the tem-
perature is above the
freezing point, the mo-
tion is much swifter
than in winter — in some
instances twice as great.
As the ice-stream
makes its way down the
ravine, fragments of
rock fall from the con-
fining banks and lodge
at the edges. In time, these accumulate until they form
walls of considerable regularity. These walls constitute the
lateral moraines of the glacier. If two or more glaciers
flow into the same ravine, the moraines on the sides that
join unite to form a medial moraine. In some instances




CREVASSI-.S AND .Mwix\l\i, M^iji aLI.V
GLACIER, WASHINGTON



15G PHYSICAL GEOGRAPHY

several medial moraines may be seen stretching with great
regularity for a long distance.

Toward the lower end of the glacier, much of this sort
of rock waste gets to the bottom, in front of the ice-stream.
In summer, when the lower end of the glacier melts to a
considerable distance up-stream, the rock waste, consisting
mainly of large bowlders, is strewn along the bed. But in
Avinter, when the ice- front again advances, the scattered
bowhlers are pushed forward, forming across the path of
the glacier the long windrow of rock waste that constitutes
the terminal moraine.

The moraines of a glacier are one of its most interest-
ing and important features. Not infrequently the shape
of the ravine is such that the rocks com])osing the lateral
moraine are pushed against the sides, forming walls as
regular as though they had been laid by human hands.
Moreover, while the lateral moraines may decrease in size,
the terminal moraine is constantly growing in volume.

Glacial Ice Sheets. — Glacial movements are not con-
fined to the ice streams of ravines, however. The sheet of
snow that projects over the edge of a roof is a perfect
illustration of glacier motion ; and so, too, is the patch of
snow on a steep hillside that gradually creeps downward
or acquires a distorted shape.

But there are remarkable fields of ice many miles in
extent, that exhibit all the phenomena of glacier move-
ment. These vast fields are found mainly in polar regions.
They are not confined between the sides of ravines ; they
are ice sheets of vast extent. Probably the greater part
of the sheet is graduall}' settling downward ; certainly the
ice in many places is projecting beyond the edges and


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Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 10 of 25)