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Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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tion, that a four or five knot current is constantly flowing into
it from the main body. From this inflow about 350,000 tons of
salt are deposited dailj-. Now, if this amount of salt were left
dissolved in the lake the latter would sooner or later become a
saturated brine. But because of this separation and deposit of
salt, the waters have not become perceptibly salter, in the time
since measurements have been made.

'- It seems a contradiction of facts to assert that a salt lake
may become fresh by a process of drying up ; nevertheless this
has been the history of many lakes. During a long-continued
period of deficient rainfall, a lake may dry up, leaving its mineral
salts as a deposit upon the bottom. In time the winds cover
this saline crust with a thick layer of fine soil ; and when the
lake again begins to fill, its waters are fresh. Pyramid and AVin-
nemueca Lakes in Nevada are illustrations ; their waters are
comparatively fresh.

'' Lake Agassiz, a body of water considerably larger than the
five great lakes, formerly covered a large part of the valley of the
Red River of the North. The destruction of this body of water
was caused probably by glacial action. It had several outlets,
one of which Avas the present channel of the Minnesota River.

'" This lake preceded any of the lakes now in the Basin Region,
and was older even than the Uinta Mountains. The bed of the
lake seems to have been lowered, and this, in part, was probably
one factor in its destruction.

'^ It is not unlikely that Lake Moeris, in Egypt, was destroyed
by winds. It was situated a few miles southwest of the Nile
delta and disappeared within historic times ; but until within a
few years its exact position was not known. In this region the
iiioveirient of wind-blown rock waste is incessant, and the amount
iiiovcfl in even a few days is enormous. Former canals across
the Isthmus of Suez, one after another have been filled by rock



188 PHYSICAL GEOGRAPHY

wasto, and there is every appearance to suggest that the isthmus
itself was foniied hirgely through teolian agency.

"The old shore lines of Great Salt Lake are still a marked
feature, and, excepting the few places where they have been
obliterated, they have been surveyed througliout the entire cir-
cuit of the lake. Old shore lines have been found above the
present level of Lakes Titicaca and Maracaibo, and also above
the level of the lakes of the western part of the Great Basin.
Two old shorelines of Lake Ontario have been found in New
York, one of which, the "Ridge Road," may be traced along
nearly the whole extent of the southern shore. In time its
level has been somewhat warped and it has now a grade of one or
two feet per mile.

" In many instances the carbonates of alkaline metals are pre-
sent in such quantities that the waters of the lake are strongly
alkaline. Many of the lakes of the Great Basin are alkaline.

'"Lake Maracaibo is a lagoon or "clover-leaf" bay, rather
than a lake of ordinary character.

'" It is difficult to draw the line between marsh lakes and
swamps on the one hand, and quite as difficult to distinguish
between the latter and meadow lands on the other. The differ-
ence is practically one of degree. A lake or a shallow lagoon
passes through all the intervening stages.

^° In many instances the emergence of underground waters to
the surface, by percolation (see illustration, p. 133), causes
swamps. The various holsas on the coast plain between Los
Angeles, California, and tlie ocean are formed in this manner.

'^ It is well to bear in mind that peat is not a plant, but a
condition of imperfect decomposition that, under certain condi-
tions, almost all vegetable tissue may assume. The softer and
moi'e soluble parts of the tissue, which have been changed to a
black slime, are really a mixture of nearly pure carbon and
hydrocarbons ; the wood fibre remains. It is likely that the in-
correct popular notion has arisen from the fact that nearly all
the peat used for fuel is derived from species named.

'" Although all lacustrine swamps are old lakes that have been
destroyed by vegetation, not all of them become peat- bogs. In
many instances the lake is situated north or south of the lati-
tude in which sphagnum thrives. The peat-bogs of Ireland
are historic, but they are not more extensive than those of the



IMPERFECT AND OBSTEUCTED DRAINAGE 189

Danube. They occur in nearly every country in which sphag-
num grows.

" Quaking bogs are very common in the swamps of the South
Atlantic States. Usually the mat of sphagnum spreads from the
margin toward the centre, but in many instances patches of
the plant accumulate in the open water, forming islands.
Generally the insular patches are attached to the bottom, but not
infrequently they float hither and thither. In time they spread
marginally until the surface is finally covered. The mat of
accumulated sphagnum receives more or less earthy matter and
becomes a tolerably firm surface. In California one of the lines
controlled by the Southern Pacific Company was built across a
quaking bog a distance of several miles. It finally caved in,
however, engulfing several cars of a freight train.

^^ The various species of rush, flag, reed, and sweet briar are
associated with swamps and contribute not a little to their for-
mation. The wild grape and several species of wild smilax are
also abundant in swamps. These species, however, are found
mainly south of the latitude in which sphagnum thrives.

^^ The mud-flat stage is always present; it is merely the area or
belt that is uncovered at low tide. If the slope is gentle this
belt may have considerable Avidth — and this is the case along the
coast of the South Atlantic States and the shores of the Gulf.
Along shores swept by fairly high tides the mud-flat belt is
usually wide.



CHAPTER XI

OCEAN WATERS AND THEIR MOVEMENTS: WAVES,
TIDES, AND CURRENTS

Almost all the plieuomena connected witli the wasting
of the land, with climate, and even with the existence of
life, in one way or another depend on the sea. In at least
two ways the sea differs from other bodies of water. It
is many thousand times the size of the largest body of
fresh water and, two or three inland lakes excepted, its
surface level is lower. Practically the sea supplies the
land with fresh water, and because of its lower level, al-
most all the waters of the land sooner or later flow back
into it.

Sea- water is briny and bitter ; doubtless it has always
been thus, but inasmuch as the stream waters flowing into
it are constantly dissolving mineral matter from the rock
waste and carrying it to the ocean, the amount in the latter
is constantly increasing. Every one hundred pounds of
sea-water, on an average, contains about three and one-
half pounds of saline matter ; most of this is common salt,
the remainder being chiefly lime and magnesia. The per-
centage of miner;d matter varies. In localities where
evaporation is rapid, the propoition of salt is larger.
Thus, in the Red Sea' it is more than four per cent.,
while in the Baltic Sea it is less than one-half as great.
It is somewhat greater in tropical than in polar regions.

Bulk for bulk, sea-water is heavier than fresh w^ater. A
cubic foot of fresh water weighs about 1,000 ounces ; on

190



192 PHYSICAL GEOGEAPHY

account of its minenil matter the same volume of sea-
water weighs at least thirty-five ounces more. Tempera-
ture also aflfects the density of water ; if 1,000 cubic inches
of water at the freezing-point be heated to the temperature
of a hot summer day, its volume will be increased seven or
eight cubic inches. The differences in temperature and
density have far-reaching results ; for upon these varia-
tions the general circulation of the waters of the sea in
part are due.

The temperature of the sea varies with both latitude and
depth. In general, the surface waters of equatorial ]"e-
gions are warmest, and in the broader extents of the sea
their temperature is not far from 26° (79° F.). Toward the
poles it gradually falls, and in polar regions it is rarely
much above the freezing-point. The variation of temper-
ature with latitude is by no means uniform, however, for
in various places warm water dragged by the " skin fric-
tion" of winds is frequently found in high latitudes.

With relation to depth the variation is remarkably uni-
form. In low latitudes the bottom temperature of deep
water is a degree or two above the freezing-point of fresh
water ; in polar latitudes, a degree or two below it. In
shallow waters and land-locked basins, however, the varia-
tions in temperature are usually very irregular. Thus, the
entrance to the Gulf of Mexico is blocked by a submarine
ridge whose ci'est is 1,200 feet below the surface, and be-
cause of this, water whose temperature is lower than that of
the 1,200-foot level cannot enter the Gulf. But even at a
depth of 12,000 feet, the temperature varies but little from
that of the 1,200-foot level.

The freezing temperature of salt water is lower by two or
three degrees than that of fresh water, the difference de-
pending mainly on the amount of mineral salts in solu-
tion. The ice of the sea is therefore formed in high lati-



WAVES, TIDES, AND CURRENTS



193



tudes, where the temperature is much belcnv the freezing-
point.'^

Sea-ice takes various forms.^ The nearly level and nar-
row shelf that in polar regions forms- along the shore, and
skirts almost its entire extent, is called the ice-foot. Any
considerable extent of undisturbed or unbroken ice forms
an ice-sheet or ice-Jield. When on-shore winds become so
strong that the ice-field is crushed and piled np against the




ICK OF THE SEA : FLOE, PACK, AND BERG.



shore, it forms pac^ ice.* Detached masses floating about
constitute y?oes ; finely broken ice floating on the surface
constitutes sludge.

A small part of the ice is caught by currents and winds,
and earned into warmer latitudes, where it finally melts.
By far the greater part, however, never leaves polar re-
gions ; possibly in a few instances it accumulates, but
most of it melts during the l)rief polar summer. A certain
amount of ice certainly floats into temperate latitudes, in



194



PHYSICAL GEOGEAPHY



the form of icebergs, but this ice is not boru of the sea ; it
is fresh water ice that is formed ou land, and, in the form
of glaciers, moves down the slopes until it breaks off."

Waves. — The alternate rising and falling of successive
ridges of water form waves. Thej vary in size from the




STORM WAVES : SURF BREAKERS.

tiny ripples made by a summer breeze, to the huge billows
that toss the largest ships.'' Every body of water upon the
earth is swept by waves, and these are caused by the fric-
tion of the air against the surface of the water.'

The motion of the water of the wave is simply up and
down, with a possible rotatory movement ; and if the wind
ceases for a moment, the i^heory holds true. Under a



WAVES, TIDES, AND CUREENTS 195

strong wind, however, the top of the wave is pushed for-
ward, and if the gale be very strong it breaks into foam,
forming " white caps" and "scud." Before the strongest
storm-winds not a little water is blown into spray, and the
whole surface of the ocean is covered with foam.

When waves roll in upon a shallow coast their motion
is also modified. The moment the bottom of the wave
touches ground it begins to drag. The top of the wave,
on the contrary, not being impeded, advances more rapidly,
and finally combn or falls forward, making breakers. The
water and foam that flow upon the shore constitute the
surf.

The distance from the shore at which waves begin to
comb depends partly on the depth of the wave, and partly
on the depth of water along the shore. Ordinary waves
rarely exceed three or four fathoms in depth, and therefore
do not comb until they are within a few rods of the shore.
Along certain shores of the Indian Ocean, on the other
hand, where the coast waters are shallow and the waves
are deep, the latter begin to comb at a distance of three
or four miles from shore.

For the formation of the highest and largest waves, a
deep, open sea is required, and in general the largest waves
are found in the broadest expanse of water. In calm
weather, the waves of the open sea are from six to ten
feet in height ; their breadth is about ten times the height.

With a wind of twenty or thirty miles an hour, the
height of the wave is somewhat increased; its breadth is
materially greater, and the largest steamships pitch con-
siderably as tliey rid(; over them. With the wind at sixty
or eighty miles the breadth of the wave is increased to
about two thousand feet ; its heiglit may reach tweut}' or
thirty feet, and its progressive velocity may reach forty
miles an hour.



19G . PHYSICAL GEOGRAPHY

It is a conimou belief that the waves run highest when
the wind is at its maximum velocity. This is not the case,
however ; they do uot reach their greatest height until the
lull of the wiud;*^ then they sometimes roll to a height of
forty-five or fifty feet.

The force with which waves strike an opposing surface
is greater than is generally imagined. Measurements on
the coast of Scotland, show that ordinary calm-weather
waves have a striking force of six hundred pounds per
square foot ; that of the heaviest storm-waves is about ten
times as great.

In navigation it is found that the chief damage from
storm-waves is due to the battering that the lighter wood-
work above deck receives.^ In recent years the old cus-
tom of spreading oil on the surface to the windward has
been revived."^' The oil covering the water presents a
surface that offers comparatively little friction to the wind.
As a result the waves, although rolling high, no longer




THE TIDE WAVE: MOON IN CONJUNCTION

break upon the vessel. The latter, therefore, is often
enabled to withstand storm-waves that otherwise would
demolish everything above her decks.

Notwithstanding their tremendous energy, waves are
superficial. The effects of ordinary, calm-weather waves
do not extend more than a few feet below the surface ;
the fiercest storm-waves do not reach more than two hun-
dred feet below the surface.



WAVES, TIDES, AND CURRENTS 197

Tides. — The alternate rise autl fall of the sea-level y
twice a day is a pheuomenon familiar to everyone who
has \isited the seashore. For six honrs the level of the
water, little by little rises, overflowing the shore and fill-
ing the river estuaries. For a few moments, the water is
stationary, and then for about six hours it falls — ever
repeating, never ceasing its oscillations.

Excepting certain estuaries and bays, neither the high
nor the low water level varies much throughout the year.
As the level rises and the water flows in upon the shore.




THE TIDK WAVE : MOON IN OPPOSITION

the tide isjfood ; as it recedes it is ehJ) ; its highest level is
high vxder, and its lowest loto water. During the few
minutes at the turn of the tide it is slack loater.

This rise and fall of water is ascribed to the attraction
of the sun and the moon ; in its nature, the movement of
the water is practically a wave several thousand miles
broad. Both the sun and the moon attract the earth.
The solid portion of the earth being rigid, however, does
not perceptibly bend or yield ; the water envelope, on
the contrary, is drawn into the elongated form," giving
the appearance of two wave-crests, one on each side of the
earth. No matter whether the sun and the moon are
on the same side, or on opposite sides, their combined
attraction Avill ])roduce the same results. If, however,
they have the position, so that they pull at right angles,
four tide-waves Avill be formed — two of the sun and two
of the moon.




THE TIDE: MOON IN QUADRATURE



198 PHYSICAL GEOGKAPHY

In most of the Northern Hemisphere, where the great
hmd masses interrupt the progress of the tide-waves, the
solar tides are merged into those of the moon. Only in
the broad expanse of the ocean, in the islands of the
South Pacific, are they distinguishable. When their

effects are added to or
subtracted from the lu-
nar waves, however, the
difference is consider-
able. Thus, at new and
full moon, when the pull
is exerted in a straight
line the tides are some-
what higher at flood
and lower at ebb ; these are the spring tides. When the
attraction is exerted at right angles they are neap tides.
In some instances the spring tides are twice as high as
the neap tides.

Thus it seems that the moon by its attractive force lifts
the waters of the sea into two great waves. Moreover, as
the moon revolves around the earth, these waves are each
drassed around at the same time, in much the same
manner as though they were fastened to it, each making a
passage in about twenty-eight days.

But while these waves are making each its revolution,
the earth at the same time is turning on its axis, every
twenty-four hours. The daihj motion of the tides, there-
fore, results from the earth's turning on its axis. Every
point on the earth, according^, overtakes and passes the
two waves daily, very much as though it were slipping
under them.

If the surface of the earth were covered with a uniform
depth of water, the direction of the tide-waves would be
from east to west. As a matter of fact, the position of the



WAVES, TIDES, AND CURKEXTS



199



continents prevents any sncli uniform direction. Every
mass of land is an obstacle in tlie path of the advancing
wave, and inasmuch as the latter cannot sweep over a con-
tinent, it must pass around it, or be checked.

Only in the broad, open waters of the Southern Hemi-
sphere do the tides move in their theoretical direction from
east to west. In the North Atlantic the wave is turned




CO-TIDAL I.INF.S
The IiHi-s show the position of the crest of the tide -wave for each two hours.



to the northward, and, entering the Arctic Ocean, it is
diverted to the eastward.^

The height of the tides is also aflfected by the land
masses. In mid-ocean the difference between high water
and low water is scarcely three feet. Along the coast of
the United States it varies from four to ten or twelve feet.
From New York to Savannah s]iring tides are about five
feet, and neap tides about four feet. In the Gulf of
Mexico the rise and fall is only about one-half as great ;



200 PHYSICAL GEOGRAPHY

along the Maine coast it is ten or twelve feet; and at
Sitka, Alaska, from twenty to thirty feet.

The great difference is due chiefly to the shape of the
shores. If the tide-wave faces a V-shaped estnary the
advancing body becomes constricted by the narrowing
shores. Not being able to spread out sideways, it is
therefore increased both in depth and velocity. In Minas
Basin, at the head of the Bay of Fundy, at times the water
advances as a solid wall twenty or thirty feet high. The
piling up of tide-waters iu the form of a wave is commonly
called a bore. It is a marked feature in the Amazon, the
Ganges, and the rivers of the China coast. It is also
noticeable iu many of the estuaries of the British Isles.
The spring tide in Bristol Channel is sometimes forty feet.

In many instances the shape of the shore is such that
the waters of the advancing tide are separated by an
island lying near the shore, again uniting in the narrow
strait between the mainland and the island. As a result
eddies and dangerous whirls are formed. Thus, at Long
Island the advancing wave is divided, one part entering
New York Bay, the other, Long Island Sound. The two
currents meet in the narrow Hell Gate, or "whirling strait."
The Maelstrom, an eddy formed by the Lofoten Islands,
off the coast of Norway, is a similar current.^^

Ocean Currents. — Throughout the greater part of its
extent the sea is traversed by cwrents that flow in defi-
nite directions with a fairly uniform velocity. The water
of an ocean current has an energy of its own, and its mo-
tion is practically the same as though it were flowing from
a higher to a lower level. There are other instances, how-
ever, in which the movement is almost entirely caused by
the wind, the direction being wholly a result of the wind.
The wind-blown waters are called drifts. Currents are
deep, sometimes extending to the bottom ; drifts, on the



202 PHYSICAL GEOGRAPHY

other hand, are superficial. The cnirreut may gradually
become a drift, aud a drift may become a current.

The winds, and the unequal heating of the waters in
equatorial and polar regions are thought to be the main
causes of the general movement of ocean waters ; the
winds and the rotation of the earth on its axis are the
chief factors that make them currents and determine
the direction of their flow.^^ The water in equatorial re-
gions receiving the vertical rays of the sun is heated to
a higher temperature than the water in higher latitudes.
Being expanded a flow toward polar regions occurs. At
the same time cooler water flows toward the equator in the
form of an undercurrent. Thus a constant circulation is
taking place — a surface movement from equatorial to po-
lar and an undercurrent from polar to equatorial latitudes.
This general movement is modified by the winds — and
undoubtedly by the rotation of the earth. ^^

In equatorial latitudes the prevailing direction of the
wuud is toward the west, aud this gives the waters a west-
erly movement. A flow of water, nearly 1,000 miles broad,
called the Equatorial Current, is the result, and, except at
the places where it is interrupted by the continents, it
girdles the earth. Its flow is scarcely more than a drift,
and its rate is about ten or fifteen miles per day. Most
of the warm currents of high and temperate latitudes are
branches of it.

The Atlantic part of this current is divided at the east-
ern angle of South America. The southern branch flows
along the eastern coast of this grand division for nearly
2,000 miles; what is its name? Gradually losing its
energy it becomes a drift, and finally it returns to the
equatorial current. Describe the course of the northern
branch ; what is its name after it emerges from the Carib-
bean Sea ? The Pacific part of the Equatorial Current is



WAVES, TIDES, AND CURRENTS 203

more than 9,000 miles long. At the edge of the Eastern
Continent it is again divided; what is the name of the
northern branch ? of the southern ? Describe the circuit
of each. In the midstream of the Equatorial Current is
found a narrow belt of water Howiug in the direction op-
posite that of the main stream. It is called the Equa-
torial Counter Current ; no satisfactory exj)lauation for it
is known.

The Gulf Stream is by far the most important of the
cm-rents of the Atlantic Ocean ; why ? Its sources are in
the Caribbean Sea. A part of its volume flows through
Santarem Channel ; a greater part is gathered into Yuca-
tan Channel ; a small but measurable part is drawn from
the Gulf of Mexico. These branches unite in Florida
Strait, and here the stream as a definite current begins.

At Florida Strait its velocity varies from three and one-
half to five and one-half miles an hour.^^ To the north-
ward it gradually decreases until, off the Labrador coast,
it ceases to have any motion of its own ; thereafter it is a
drift dragged by westerly Avinds.

The Gulf Stream is not only the swiftest of ocean cur-
rents, but it is also the warmest. Off the Florida coast
its summer temperature is 30° (86° F.), and even near the
Greenland coast it is tw^enty or thirty degrees (F.) warmer
than the surrounding waters. Contrary to common opinion,
it is not a shallow current. As a matter of fact, from Flor-
ida Strait to Cape Hatteras, it extends to the bottom of
the ocean. Its drift is pushed northward and eastward,
and much of it forms a circuit returning to the Equatorial
Cun-ent. A considerable volume, keeping northward, finds
an entrance to the gulfs and bays of western Europe, reach-
ing even to the north coast of Norway.

The Kuro Suoo is the Gulf Stream of the Pacific.
Some of its waters issue from the Bay of Bengal, but the



304 PHYSICAL GEOGRAPHY

greater part of its volume passes among the Malaysian
Islands. Thence it flows along the eastern coast of Asia.
Off the Japan Islands it becomes a drift, and its waters
are then pushed by the prevailing winds toward the North
American coast, performing an oval-shaped circuit like
that of the Gulf Stream.

The Kuro Siwo is not only a much feebler current than
the Gulf Stream, but it is a cooler stream as well. Its
summer temperature rarely exceeds 22° (72° F.), and its


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Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 13 of 25)