Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

. (page 4 of 25)
Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 4 of 25)
Font size
QR-code for this ebook

from any large body of land. There is no doubt about
the origin of some of them ; they consist of the lava that
has been ejected from volcanoes. In some instances these
islands are solitary, as Jan Mayen and St. Helena ; in
others they form a chain, as the Hawaiian group.

In the Pacific Ocean there is a large area in which isl-
ands are so numerous that they form the well-defined grand
division Polynesia ; find the meaning of this word from
the dictionary. These islands occur in quite regular chains
that are roughl}' parallel in direction ; they are therefore
thought to be the higher summits of submerged mountain-
ranges. In some instances a volcanic peak is in sight, but
for the greater part the position of each peak is marked
by the reef of coral growth that encircles it. The islands
themselves are popularly known as coral islands.^

It has been inferred that the coral polyps began their
growths ou the slopes of the volcanic peaks, and that the
latter gradually subsided imtil they were covered by the
sea. But while the peak Avas slowly sinking the coral
poh'ps steadily built their reefs upward, keeping the top
always even with the wash of the waves. This opinion,
first made prominent by Darwin, is borne out by the fact
that, while the coral polj'p cannot live more than twenty
fathoms below the surface of the sea, the reefs sometimes
extend almost vertically to a dej)th of several hundred

A peculiar feature about many of these islands is their
form. As a rule each consists of an irregular ring of reef
matter, broken and tcjssed up by the waves, surrounding
shallow water. The reef is called an atoll ; the enclosed


water a lagoon. Usually the atoll is broken in one or more
places, and in many instances the lagoons form good har-

^ bors. The reef is rarely more than a

/':>■■::, \ few feet high, and its vegetation is con-

/ f"^^ \ fined to a few species, mainly of palms.

/ a %.. \ The Sea. — The sea covers more

/ 1- s) \ than half the northern and about

I iK"'-^ \ seven eighths of the southern hemi-

1 tA. #1. \ sphere. Althon<i;li the area it covers

I i^i^ ^* ^ is continuous, it is separated by the

■^ ■* J. V\ continents into great divisions called

C\ ^: oceans. Name them. AVhicli one is

fV'' '-^f I nearly enclosed ? Compare the At-

|1l 1^ nl lautic in shape with the others. For

%# ^Cl-s. '^1 convenience, the polar circles are

I taken as the boundaries of the polar

oV / oceans, and the equator convention-

\ ^^;;-%:, / ally divides the two largest oceans

\ %#'*' .,. .-' ii^to northern and southern divisions.

\. /' Which of the oceans is nearly land-

'■■••.., / locked '? At what place do the Pacific

A GROUP OF CORAL and Arctic Oceans meet? the Atlan-

'^ITLn'iZ'''^?^^'-'' tic and Pacific? the Atlantic and


Indian? the Atlantic and Arctic?

The Pacific Ocean comprises about one-lialf the entire
Sea ; the Atlantic about one-quarter. The shore line of
the latter, however, is considerably longer ; explain why.
Why are not the polar oceans important routes of traffic ?
On a globe trace a northwest passage from London to
India ; why is not such a passage feasible as a trade route ?

In general, the average depth of the oceans varies with
their size — the larger the ocean the greater its depth. The
Pacific is about 2,500 fathoms, the Atlantic and Indian not
far from 2,000 fathoms. The polar oceans are shallower,


but not enough is known about their depth upon which an
average can be computed. The greatest ocean depths are
much in excess of the average depths. There is a large
3,000-fathom area in the north Pacific — compare it Avith
Australia in size — and several smaller areas in the Atlan-
tic and Indian Oceans. There are also several 4,000-fathoni
and at least two small 5,000-fathom ai'eas ; describe their
positions.*' The greatest deptli of the sea, it is seen,
scarcely surpasses the height of the loftiest mountain
peak ; yet while four-fifths of the sea basin is six thou-
sand feet lower than sea-level, less than a tenth of the land
reaches six thousand feet above it.

The floor or bed of the sea is bv no means so irregular
as the surface of the land ; and, the vicinity of the coral
islands and the continental shores excepted, no steep slopes
or abrupt changes of level are known to exist. The sound-
ings made for the telegraph cables disclosed no slopes nor
inclines too steep for a railway grade. After deep water
was reached, the soundings for the Atlantic cable of 1866
did not vary more than seven or eight hundred feet in two
thousand miles.

Arms of the Sea. — In various iilaces the sea extends to
a considerable distance within the general outlines of the
continents, forming the bodies or arms called seas, gulfs,
bays, sounds, straits, etc. Many of the smaller coves and
estuaries are shore formations, having been made or
shaped by the action of waves or by currents of water.
The larger arms, however, are structural, and have resulted
from uplieaval or depression of the continent, or of some
part of it.

The liorders of a continent may be flanked by 'lofty
highlands, and the trend of the coast usually conforms to
the trend of the ranges. Thus, the bend that gives the
west coast of Africa its shape also gives a similar form to



the Gull" of Guinea. Where parallel rauges extenci sea-
ward, or form an angle with the coast, the sea usually en-
ters the valley to some distance between them. On a map
of North America, note the position of the Gulf of Cali-
fornia, and Puget Sound ; on a map of Europe, the Adri-
atic Sea. Note similar examples along the west coast of
Asia. Compare the coast lines of the grand divisions
with reference to indentations. Which has the longer
coast line — Europe or Africa ?

Unfit for commerce and a menace to navigation.

Almost any partly enclosed portion of an ocean is called
a sea, and the Caribbean and North Seas are examples of
a type of enclosed waters. There is another type, how-
ever, that is even more remarkable because practically
land-locked. Of this type the Mediterranean is an exam-
ple, and such arms of the ocean are now often called 7nedi-
terraneans. The Gulf of Mexico is properly included in
this class. Nearly all the larger arms of the sea are de-



pressed j)arts of the continents, or of the plateau on wbicli
tliey are situated.

Coast Forms. — The study of ahnost any good map of
a continent, or of any considerable part of its shore out-
lines, shows that various parts of the coast differ materially.
Compare, for instance, the coasts of Maine and Florida ;
of the Chesapeake Bay and southern California. The illus-
trations on P23. 46 and 52 are examples of shore forms.
One of them is a rock-bound coast deeply indented with
fjords and hemmed in by rocky islets. This coast has
been worn and frayed by the action of sheets of ice, but it
has also subsided until the valleys are submerged by the
sea. Name the various coasts that resemble it.

A CI.IFl i,li:l I, OAST, SAN JUAN. I'l IUIm K|(.^.

In the illustiation on p. 52, the plain bordering the sea
dips so gently below sea level that the water is shallow half
a mile or more from the shore. The drag of the waves roll-
ing in and cond:)iijg on the coast picks up sand and rock
waste brought down by muddy streams and piles it in the
form of long spits and beaches at a little distance from the
shore.'' Find other coasts that resemble it.



Along many parts of the coast the sea seems to be en-
croaching- on the land, and the waves beat against the
shore, breaking it aAvaj until there is a high clili" with a
narrow beach at its foot. A considerable extent of the
California coast is bordered by sea-cliffs, and they occur

here and there along the
North Atlantic coast, as
at the coast of Newport,
Rhode Island.

Coral formations are
very important factors in
shore lines. On shore
they are caWed fringi^ig
reefs; farther out, bar-
rier reefs. Almost the
entire east coast of Aus-
tralia is shut off from
open communication by
a barrier reef more than
twelve hundred miles
long. There are a few
channels across the reef,
but the latter is a great
obstacle to commerce.
Fringing reefs occui* on
the south coast of Flor-
ida, and they are per-
haps the most common
examples of coral formation. They are common along
the shores of the Bahama Islands, and occur here and
there along the Hawaiian coast.

Coral growths are confined to warm, littoral waters, and
the reef-building ])olyp is limited to waters Avhose temper-
ature does not fall beloAv 25° (67° F.). Absolutely clear


The barrier beaches nearly enclose the coast ; the
inlets are kept deep enough for navigation by the
tidal currents.


water is requisite, aud for this reason coral reefs are rarely
fouuJ along the shores of coutiuents, aud uever withiu the
reach of river sediments.

Coast Outlines and Civilization. — The coast forms of
a country have not a little bearing on its prosperity and
its enlightenment as well. A coast with good harbors in-
vites commerce and intercommunication. Along the North
Atlantic coast of the United States, where a rugged surface
slopes abruptly below sea-level, good harbors are nume-
rous. The same conditions prevail on the coast of Europe.
Of two regions, one having good, the other poor harbors,
commerce and intercommunication will seek the former.
Africa and South America have but very few good harbors,
and to this fact the half- savage condition of the native
peoples is largely due. The great stride in the progress
of the Japanese people was begun when they opened their
ports to foreign trade.

QUESTIONS AND EXERCISES-— How have good harbors affected
the progress of the Englich people .-* What has been the effect of closed
ports on the Chinese ?

Compare the commerce of the North Atlantic coast of the United
States with that ot the South Atlantic coast. To which type does each
of these coast forms belong ? Where are most of the large seaports of
the Atlantic coast of the United States ? Explain the reason for their

Why should Australia be considered a continent rather than an
island ?

Does the cutting of the Suez Canal give Africa any insular properties
that it did not possess before ?

Make a list of the principal mediterranean seas of the world.

Mention several instances in which peninsulas enclose waters so as
to form gulfs or bays.

From a good map of the British Isles find the names used as syno-
nymes of " cape " and " strait."

Find the centre of each hemisphere on p. 40.

Study the position of the submerged part of the continents on the
map, pp. 45-46-



Dana.— Manual of Geology, pp. 145-152.

Redway. — New Basis of Geography. Chapter IV.

Shaler. — Sea and Land, pp. 187-322.

United States Geological, Survey. — Norwich and New
London Sheet (drowned valleys) ; Sandy Hook and Barnegat
Sheets (spits and barrier beachesj ; Port Washington Sheet


' It is commonly asserted that the same amount of water ex-
ists on the earth at the present time as during remote geological
periods. This is doubtless true, but it is also true that not all
the water is in the same form now as in prior times. When
the earth was younger thei-e was much water in a liquid form
that is now chemically combined with various mineral elements.
Nearly all the minerals, especially those in a crystalline form,
contain notable proportions of water in combination.

-' This separation of the land masses has been aptly called
the "zone of fracture." The isthmus of Panama is scarcely
thirty miles wide and the isthmus of Suez is only one hundred
miles across. Yet these two necks of land are all that connect
the divisions of each continent. That is, twenty-five thousand
miles of open navigation are obstructed by less than one hundred
and thirty miles of land. Even these barriers are disappearing
because of canals either completed or projected.

^ It is now the custom to restrict the latter term to the largest
land masses, but it is sometimes more convenient to apply it to a
grand division, Europe and Asia are also called continents, but
the only real boundary that separates them is the desert high-
land that separates western from oriental civilization. Physically
it is better to treat Eurasia as a whole — politically and histori-
cally the two divisions are best considered separately.

* This margin is also called the continental plateau, the conti-
nental border, and the submerged border.

^ The coral polyp is a zoophyte form of marine animal growth
not unlike a tree with its branches. The mouths of the polyp


completely cover its upper surface in much the same manner as
the flowers of the hollyhock or mullein cluster about the stem.
In a single community the growth of the polyp is chieHy upward,
but where the eonnnunities are thickly clustered, their branches
interlock and finally form a compact mass. The living portion
of a coral is found at the surface of the water or a few feet below
it ; the dead portion may extend a hundred fathoms or more be-
low the surface.

•^ The deepest soundings so far obtained are 4, 655 fathoms by the
U. S. S. Tuscarora, east of Japan, in an area now known as Tus-
carora Deep ; 5,147 fathoms, one hundred miles E. N. E. of Sun-
day Island ; and 5,155 fathoms a few leagues east of Macarthy Isl-
and, not far from the Kermadec group. The two last were made
by Commander Balfour, H. M. S. Penguin. North of Puerto Rico
a sounding of 4,651 fathoms has been obtained. The cable ship
Nero reported a sounding of 5,200 fathoms east of the Hawaiian
Islands. Formerly deep sea soundings were made with heavy
Manila rope, and in very deep water it was impossible to tell
when the sinker had reached bottom. With tlie method perfected
by Admiral Belknap and Captain Sigsbee, steel piano wire takes
the place of the rope. The Avire carries at its lower end a sinker
which detaches itself on touching bottom, at the same time clos-
ing a cup that secures a specimen of the bottom. Very few of
the deep-sea soundings made prior to ISTO are now considered

^Marine currents frequently attempt to carry away the rock
waste piled up Ijy the waves, and between the two it is dragged
into a curved form making a hook. Sandy Hook, New Jersey,
is an exam[)le, and similar examples are found along the shorep
of Marthas Vineyard and Nantucket.



The larger vertical forms of the land are the results of
the slow movements of the rock envelope. Any consider-
able area of laud but little higher than sea-level is called
a plain ; if considerably higher, a plateau ; if wrinkled,
folded, and broken, a mountain system. There is no fixed
elevation at which an area ceases to be a lowland, or vice
versa, but in general, surfaces more than two thousand feet
above sea-level are called highlands, while those of less alti-
tude are lowlands.

As a rule, the various features that constitute topogra-
phy are distinct one from another ; but in many instances
lowlands gradually increase in altitude and become high-
lands ; an almost imperceptible swell in a level plain may
develop into a cliff or a ridge ; and a mountain-range, little
by little, may lose its characteristic form among other
features of the landscape. So it often happens that a sin-
gle topographic form may have the character of several
kinds of relief.

Plains. — Any level or nearly level stretch of land is
commonly called a plain. Most plains are lowlands, but in
a few instances the name is applied to surfaces that are
more than six thousand feet above sea-level— an elevation
considerably greater than that of some mountain-ranges.
The plain east of the Rocky Mountains is an example ;



it is higher than the crests of the AppaLachian Mountains,
and about as high as the highest peaks.

Plains are variously named. The grassy plains of the
New World were named savannas by the Spanish, and
prairies by the French — both of which names are very
commonly employed. In South America the vast plains
of Argentina are called pampas ; the grassy plains of the
Orinoco, llanos ; and the forest-covered plains of the Ama-


The forestry i> drficii-nt, and the soil only uiodi-ulcly fertile.

zon, silvas} In Eurasia, the vast plains that almost girdle
the Arctic Ocean are known as steppes, their frozen, swamp}^
coast fringe being known as tundras. In England and
Scotland the terms, meculoiv, heath, and moor, are used.

Origin of Plains. — Most plains have been formed by
the action of water, or have received their surface configu-
ration by it. If shai)od Ijy comparatively still water they


are known as marine or lacustrine plains ; the former being
old sea-bottoms ; the latter lake basins. If formed of sedi-
ments deposited by running streams they are alluvial
plains ; if levelled off by moving ice, diluvial plains ; if on
the margin of the sea or a lake, coast plains.

Marine and lacustrine plains constitute by far the greater
area of the lowland siirface of the earth. Originally old
sea or lake bottoms, their surfaces are level, because the
sediments forming them were deposited in still water. In
some instances the floor was filled and levelled off by the
remains of minute animals ; in others by dead and decay-
ing vegetation.

In time these old bottoms were raised above water-level
and, if their surfaces were not wrinkled and folded, they
constitute the plains of to-day. Thus, the larger part of
the Great Central Plain of North America is an old sea-
bottom, and so, too, is most of the great northern plain of
Eurasia. Of lacustrine plains, one of the finest examples
is the valley of Red River of the North. This plain re-
sulted from the draining of a lake, and was so recently
formed that its surface has scarcely been notched by the
river that now imperfectly drains it.

The plain surrounding the Caspian Sea is an excellent
example of a plain in the process of formation. On the
northern side, the gradual shrinkage of the lake has left a
plain more than two hundred miles wide, and when at
length the lake disappears, a broad, wind-swept plain will
take its place.'' The yalley or basin of Great Salt Lake
possibly is passing through a similar period of growth
and development.

Alluvial plains are usually best developed along the
lower courses of rivers, although they exist in narrow
reaches along almost the entire length of the stream. The
bottom-lands of the lower Mississippi and the Danube ;


the mazy deltas of tlie Nile and the Ganges-Brahmaputra,
and the broad, fertile plains of the Po are examples.'
Name other illustrations.

The surface of a coast plain is made level by the action
of the waves, and if an uplift of the surface is taking place,
the jalain gets gradually wider and wider as successive por-
tions of the sea-bottom are brought to the surface. The
coast plain along the South Atlantic and Gulf coast is an

A very fertiU- pnjine zvith coiishtctjblr forest growth.

excellent example.' Much of the material of which it is
composed is sediment brought down by the rivers, but the
waves have been the ciliief agent in building it. Through-
out its whole extent it is but little higher than tide-water.
The line along wliich the coast plain joins the older land is
marked by a rather abrupt slope called the "Fall Line,"
and in most places the lin(j where they meet is quite distinct.


Most of the rivers are navigable to the Fall Line, and along
the eastern side coast a line of cities marks the junction.

Almost every body of land is surrounded by a coast
plain ; indeed its formation and growth necessarily follows
the denudation or wasting of the land. Kock waste is con-
stantly being carried to sea-level by running waters, but
beyond this point it can go little or no farther ; so it is
distributed along the shore and levelled off by the waves.
In most instances slow, vertical movements of the rock en-
velope are concerned in the formation and development of
coast plains, but in many cases rivers, waves, and tidal car-
rents divide the work among themselves.

In various places surfaces formerly rugged have been
levelled off by the action of the sheet of ice that once cov-
ered portions of Europe and North America. Much of the
northern part of the United States received the configura-
tion of its surface by this process ; the moving sheet of
ice scoured off the rugged parts and filled the depressions
with the material removed.^

Distribution of Plains. — Alluvial and lacustrine plains,
of course, are incidents in the physiography of rivers and
lakes ; and coast plains are formed on nearly all shores.
The great marine plains of the world are mainly on the
slopes of the Arctic and the Atlantic Ocean. The most
extensive plain of the world is that which forms the
northern slope of Eurasia. From east to west it stretches
a distance of about nine thousand miles ; from north to
south, about three thousand miles. In Asia it is high and
rolling ; in Europe the greater part of its extent, how-
ever, is low and comparatively level.''

In the New World the great continental plain extends
from the Arctic Ocean to the Gulf of Mexico, and there is
an apparent extension from the Caribbean Sea southward
through South America. Its continuity is broken by


occasional ranges and arms of the sea. It presents cer-
tain marked contrasts to the plain of the Asian Continent.
The latter extends east and west ; the former, north and
south. The latter is a margin of the continent ; the for-
mer is an interior plain, bordered by mountain-ranges.

Physiographic Aspect of Plains. — Although water is
the chief agent in the formation of plains, it is likewise the
chief factor in their destruction. From the moment a plain
comes into existence, storm waters and running streams be-
gin to carve channels in its surface. These, extendiug in
area, carry the greater part — perhaps all the surface mate-
rial away.^ A plain thus channelled is said to be " dissect-
ed." The coast plain of much of the South Atlantic and
Gulf coast is young, especially near the sea. Its slope is
so gentle that the streams have not yet carved their chan-
nels to any great depth.

The plains bordering Lakes Erie and Ontario show^ signs
of greater age. The streams have accomplished a consid-
erable dissection and the channels are comparatively deep.
The " Bad Lands " of South Dakota and Nebraska are
remnants of an old lacustrine plain that has been so greatly
dissected that the region is well-nigh impassable through-
out much of its extent.

Economic Value of Plains. — Because of their com-
paratively level surface, plains are more accessible to
commerce than mountainous regions, Railways can be
built across them at the minimum of cost, and the rivers
that traverse them are usually navigable.

More than this, the soil of plains is usuall}' deep and
easily cultivated. Tlierefore the}' are capable of support-
ing a denser population than mountainous regions. In
remote times the alluvial plains of the Nile and of Meso-
potamia were the seats of dense population and vast in-
dustries. In later times the plains of Europe and of the


United States have become the great producers of wealth.
It ma}^ be said, therefore, that the greater part of the world's
wealth and power is centred in the plains of the temperate
zones. Only a small fraction of the world's population lives
above the altitude of 2,000 feet, and but few of the great
cities are more than six hundred feet above sea-level.

Plateaus. — Almost any broad extent of country having
an elevation of more than a few hundred feet, and an ir-
regular or dissected ' surface, is popularly called a plateau.
The name, originally meaning " Hat," or " level," has ac-
quired a signification almost the opposite. A plateau of
small area is usually called a mesa, a table-land, or a table-
mountain, according to its general form and structure.

Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 4 of 25)