THE SEA AND ITS WORK
It is well known that the waters of the sea cover nearly three-fourths
of the surface of the earth. We think of the United States as being a
large piece of land of over three million square miles--but the sea
is about forty-five times as large, that is, it covers approximately
140,000,000 square miles. It is a remarkable fact that the average
depth of the great oceans of the earth is nearly two and one-half
miles. If the sea were universally present everywhere with the same
depth, it would be almost two miles deep. Yet this vast body of water
is an extremely thin layer when compared with the earth's diameter
of 8,000 miles. The Pacific is the deepest of the oceans with an
average depth of about two and three-fourths miles. The deepest ocean
water ever sounded is 32,114 feet (over six miles), not far from
the Philippine Islands. This is known as the Planet Deep, and was
discovered in 1912. Second deepest is 30,930 feet near the island of
Guam. In the Pacific Ocean there are five places where the water is
over five miles deep and eleven places were it is over four miles deep.
The deepest sounding ever made in the Atlantic Ocean was 27,972 feet,
not far from Porto Rico.
Many substances are known to be in solution in sea water, but in spite
of this the composition is remarkably uniform. The most abundant
substance by far in solution is common salt. In every 100 pounds of
sea water, there are 3.5 pounds of mineral matter of various kinds
dissolved. Nearly 78 per cent of the dissolved matter is common salt.
The principal other constituents in solution are chloride and sulphate
of magnesia, and the sulphates of lime and potash. All other dissolved
mineral substances together make up less than one per cent of the
total. It has been estimated that if all the dissolved mineral matter
should be brought together, it would form a layer 175 feet thick over
the whole sea bottom. The salts of the sea have been mostly supplied by
the rivers, which in turn have derived them from the disintegration and
chemical decay of the rocks.
If we make a general comparison with the surface of the land, the
floor of the ocean is a vast monotonous plain. None of the sea bottom
compares with the ruggedness of mountains, and even the more level
portions of land surface show many sharp minor irregularities such as
stream trenches. But the sea bottom is characterized by its smoothness
of surface. There are under the sea, however, mountain-like ridges,
plateaus, submarine volcanoes and valleys known as "deeps." But these
rarely show ruggedness of relief like similar features on land.
One of the most remarkable relief features of the ocean bottom is the
so-called "continental shelf." This is a relatively narrow platform
covered by shallow water bordering nearly all the lands of the earth.
Seaward, the depth of water is greatest, and it is seldom over 600 or
800 feet. The continental shelves of the world cover about 10,000,000
square miles or about one-fourteenth of the area of the sea.
Viewed in a broad way, there are two great classes of marine deposits;
first, those laid down comparatively near the borders of the land,
that is, on the continental shelf and continental slope, and second,
the abysmal deposits laid down on the bottom of the deep ocean. Those
found along and near the continental borders are largely land-derived
materials, that is to say, they are mostly sediments carried from the
land into the sea by rivers, and to a lesser extent rock material
broken up by waves along many shores. Practically all such land-derived
material is deposited within 100 to 300 miles of the shores. The
continental border deposits are extremely variable. Near shore they
are chiefly gravel and sands, while farther out they become gradually
finer, and on the continental slope only very fine muds are deposited.
These deposits usually contain more or less organic materials and
shells or skeletons of organisms. In some cases the shells or skeletons
of organisms predominate or even exist to the exclusion of nearly
all other material, as is true of the coral deposits or reefs which
form only in shallow water. Deposits like those just described as
accumulating on the bottom of the shallow sea, comparatively near the
lands, are of great significance to the geologist because just such
marine deposits now consolidated into sandstone, conglomerate, shale,
and limestone, are so widely exposed over the various continents.
A knowledge of the conditions under which shallow sea deposits are
now forming, is, therefore, of great value in interpreting events of
earth history as they are recorded in similar rocks which have been
accumulating through millions of years of time. One specific instance
will make this matter clearer. Using the method outlined in Chapter I
for the determination of earth chronology, and our knowledge of present
conditions under which shallow sea deposits are formed, it has been
well established that a shallow sea spread over fully four-fifths of
the area of North America during the middle Ordovician period of the
early Paleozoic era. Beyond this main conclusion, a careful study of
these rocks has revealed many important facts regarding the physical
geography, life, and climate of that time. The importance of this whole
matter is still further emphasized by the statement that five-sixths of
the exposed rocks of the earth are strata--mostly of shallow sea origin.
The deposits on the deep sea bottom are very largely either organic or
the shells and skeletons of organisms which have fallen to the bottom
from near the surface as already explained. Most common of these are
the deep sea "oozes" which are made up of the remains and shells of
tiny organisms called "foraminifers." These "oozes" cover about 50
million square miles of the sea bottom down to depths of from two to
three miles.
At depths greater than from two to three miles, a peculiar red clay is
the prevailing deposit. This is most extensive of all, covering an area
of 55 million square miles, or nearly the total area of lands of the
earth. Some remains of organisms are mixed with this clay, but since
most of the shells are of carbonate of lime and very thin, they are
dissolved without reaching the bottom in the deep sea water which is
under great pressure and rich in carbonic acid gas.
The deep sea deposits, both "oozes" and red clay, do, however, contain
some land-derived and other materials. Thus off the west coast of
Africa some dust carried by the prevailing winds from the Sahara
Desert, is known to fall in the deep sea several hundred miles from
shore. Volcanic dust is carried for many miles and deposited in the
deep sea--particularly in the South Pacific Ocean. Bits of porous
volcanic rock called "pumice" sometimes float long distances out over
the deep sea, before becoming water soaked. Icebergs often drift far
out from the polar regions over the deep sea, and on melting the rock
débris which they carry is dropped to the sea bottom. Also, particles
of iron and dust from meteorites ("shooting stars") have been dredged
from the deep sea.
One important geological significance of the deep sea deposits is
the proof which they furnish that, from at least as far back as the
beginning of the Paleozoic era, fully twenty-five million years ago, to
the present time, the two great deep ocean basins--the Atlantic and the
Pacific--have maintained essentially the same positions on the earth.
This is proved by the fact that nowhere, on any continent among the
rocks of all ages, as old at least as the early Paleozoic, do we find
any really typical deep-sea deposits. There is then no evidence that a
deep sea ever spread over any considerable part of any continent, and
this in spite of the fact that marine deposits of shallow water origin
furnish abundant evidence of former sea extensions. The shallow seas
have at various times spread over large portions of the continents.
On many rocky coasts the waves are incessantly pounding and wearing
away the rocks. In such places the sea, like a mighty horizontal saw,
is cutting into the borders of the lands. The finer materials produced
by the grinding up of the rocks are carried seaward by the undertow.
But, if the land remains stationary with reference to the sea, this
landward cutting by the waves reaches a limit. Since even big waves
have very little effect in water 100 or 200 feet deep, a shelf is cut
by the waves and this shelf, not many miles wide, is covered by shallow
water. The finer ground-up rock materials carried out by the undertow
are dumped just beyond the edge of the shelf which is thus built out
seaward as a terrace. In traveling over this shelf and terrace, the
waves, due to friction, lose their power. With gradually sinking land,
a much wider shelf may be cut, because the power of the waves is then
allowed to continue.
It might be of interest to cite a few cases of relatively rapid coast
destruction by the waves which have come under human observation. A
remarkable example is the island of Heligoland on which is (or was)
located the powerful German fort which guards the entrance to the Kiel
Canal. In the year 800 A. D. this island had 120 miles of shore line;
in 1300 it had 45 miles of shore; in 1649 only 8 miles; and in 1900
but 3 miles of shore line remained. In southeastern England "whole
farms and villages have been washed away in the last few centuries,
the sea cliffs retreating from 7 to 15 feet a year." A church located
a mile from the sea shore near the mouth of the Thames river, in the
sixteenth century, now stands on a cliff overlooking the sea. An island
in Chesapeake Bay covered over 400 acres in 1848, and the waves have
since reduced it to about fifty acres. Study showed that the relatively
soft unconsolidated strata of the Nashaquitsa Cliffs on the island of
Martha's Vineyard, were cut back at the rate of 5-1/2 feet per year,
between 1846 and 1886.
If part of the relatively smooth sea bottom should be raised into land,
the resulting shore line would of course, be regular and free from
indentations or sharp embayments. Examples of such coast which are
very young are at Cape Nome, Alaska; the northern coast of Spain; and
the west coast of northern South America. Soon, however, such a shore
line is attacked, and, either where the waves are greatest or the rocks
are weakest, indentations will result and the whole coast is gradually
eaten back until the power of the waves is largely spent in traveling
across the shallow water shelf. Sand bars are then built across the
mouths of the bays or indentations which later the rivers gradually
fill up with sediment. The result is a relatively straight or regular
old shore line. The coast of Texas has about reached this stage.
If a portion of the relatively rugged land surface should become
submerged under the sea, a very irregular, deeply indented shore line
would result, due to the entrance of tidewater into the valleys. The
deeply indented coast of Maine is a fine example of a very irregular
youthful shore line produced by geologically recent sinking of a
rugged, hilly region so that tidewater backs for miles into the
lower reaches of the river valleys. The promontories and islands are
undergoing rapid wear, and the development of bars across the inlets
has scarcely begun. Other excellent examples are the coasts of Norway
and southern Alaska. Such a coast is then attacked by the ocean waves
and the promontories are cut back until the broad shallow water shelf
is formed, after which sand bars are built across the remaining
embayments and the shore line becomes relatively regular.
It is, then, a remarkable fact that, whether shore lines originate
by emergence of sea bottom, or by sinking of land, there is a very
strong tendency on the part of nature to develop regular shore lines.
It should be stated that the principles of wave work and shore-form
development just outlined apply almost equally well to lakes,
especially large ones.
Before leaving this subject of shore-line development, mention should
be made of the fact that bars and beaches are often built part way
or wholly across embayments of the coast with surprising rapidity.
To illustrate, Sandy Hook, New Jersey, is advancing northward, while
Rockaway Beach, New York, is extending westward, the tendency being
to close up the entrance to New York harbor and to make the line of
seashore more nearly regular. Records show that Rockaway Beach actually
advanced westward more than three miles between the years 1835 and
1908.