COAST RECORDS OF THE RISE OR FALL OF THE LAND
=The characters in which the record has been preserved.=—The peculiar
forms into which the sea has etched and molded its shores have been
considered in the last chapter. Of these the more significant are
the notched rock cliff, the cut rock terrace, the sea cave, the sea
arch, the stack, and the sloping cliff and terrace, among the carved
features; and the barrier beach and built terrace, among the molded
forms. It is important to remember that the molded forms, by the very
manner of their formation, stand in a definite relationship to the
carved features; so that when either one has been in part effaced and
made difficult of determination, the discovery of the other in its
correct natural position may remove all doubt as to the origin of the
relic.
[Illustration:
FIG. 270.—The east coast of Florida, with shore line characteristic of
a raised coast.]
In studies of the change of level of the land, it is customary to refer
all variations to the sea level as a zero plane of reference. It is not
on this account necessary to assume that the changes measured from this
arbitrary datum plane are the absolute upward or downward oscillations
which would be measured from the earth’s center; for the sea, like the
land, has been subject to its changes of level. There need, however, be
no apology for the use of the sea surface as a plane of reference; for
it is all that we have available for the purpose, and the changes in
level, even if relative only, are of the greatest significance. It is
probable that in most cases where the coast line is rising from uplift,
some portion of the sea basin not far distant is becoming deepened,
so that the visible change of level is the algebraic sum of the two
effects.
=Even coast line the mark of uplift.=—It was early pointed out in
this volume (p. 158) that the floor of the sea in the neighborhood of
the land presents a relatively even surface. The carving by waves,
combined with the process of deposition of sediments, tends to fill up
the minor irregularities of surface and preserve only the features of
larger scale, and these in much softened outlines. Upon the continents,
on the contrary, the running water, taking advantage of every slight
difference in elevation and searching out the hidden structure planes
within the rock, soon etches out a surface of the most intricate
detail. The effect of elevation of the sea floor into the light of day
will therefore be to produce an even shore line devoid of harbors (Fig.
270). If the coast has risen along visible planes of faulting near the
sea margin, the coast line, in addition to being even, will usually be
made up of notably straight elements joined to one another.
[Illustration:
FIG. 271.—Ragged coast line of Alaska, the effect of subsidence.]
=A ragged coast line the mark of subsidence.=—When in place of uplift
a subsidence occurs upon the coast, the intricately etched surface,
resulting from erosion beneath the sky, comes to be invaded by the sea
along each trench and furrow, so that a most ragged outline is the
result (Fig. 271). Such a coast has many harbors, while the uplifted
coast is as remarkable for its lack of them.
=Slow uplift of the coast—the coastal plain and cuesta.=—A gradual
uplift of the coast is made apparent in a progressive retirement of the
sea across a portion of its floor, thus exposing this even surface of
recent sediments. The former shore land will be easily recognized by
it’s etched surface, which will now come into sharp contrast with the
new plain. It is therefore referred to as the _oldland_ and the newly
exposed _coastal plain_ as the _newland_ (Fig. 272).
[Illustration:
FIG. 272.—Portion of Atlantic coastal plain and neighboring oldland of
the Appalachian Mountains.]
But the near-shore deposits upon the sea floor had an initial dip
or slope to seaward, and this inclination has been increased in the
process of uplift. The streams from the oldland have trenched their way
across these deposits while the shore was rising. But the process being
a slow one, deposits have formed upon the seaward side of the plain
after the landward portion was above tide, and the coastal plain may
come to have a “belted” or zoned character. The streams tributary to
those larger ones which have trenched the plain may encounter in turn
harder and softer layers of the plain deposits, and at each hard layer
will be deflected along its margin so as to enter the main streams
more nearly at right angles. They will also, as time goes on, migrate
laterally seaward through undermining of the harder layers, and thus
will be shaped alternating belts of lowland separated by escarpments
in the harder rock from the residual higher slopes. Belts of upland of
this character upon a coastal plain are called _cuestas_ (Fig. 273).
[Illustration:
FIG. 273.—Ideal form of cuestas and intermediate lowlands carved from
a coastal plain (after Davis).]
=The sudden uplifts of the coasts.=—Elevations of the coast which
yield the coastal plain must be accounted among the slower earth
movements that result in changes of level. Such movements, instead of
being accompanied by disastrous earthquakes, were probably marked by
frequent slight shocks only, by subterranean rumblings, or, it may be,
the land rose gradually without manifestations of a sensible character.
Upon those coasts which are often in the throes of seismic disturbance,
a quite different effect is to be observed. Here within the rocks
we will probably find the marks of recent faulting with large
displacements, and the movements have been upon such a scale that shore
features, little modified by subsequent weathering, stand well above
the present level of the seas. Above such coasts, then, we recognize
the characteristic marks of wave action, and the evidence that they
have been suddenly uplifted is beyond question.
[Illustration: FIG. 274.—Uplifted sea cave, ten feet above the water
upon the coast of California; the monument to a former earthquake
(after a photograph by Fairbanks).]
[Illustration: FIG. 275.—Double-notched cliff near Cape Tiro, Celebes
(after a photograph by Sarasin).]
=The upraised cliff.=—Upon the coast of southern California may be
found all the features of wave-cut shores now in perfect preservation,
and in some cases as much as fifteen hundred feet above the level of
the sea. These features are monuments to the grandest of earthquake
disturbances which in recent time have visited the region (Fig.
274). Quite as striking an example of similar movements is afforded
by notched cliffs in hard limestone upon the shore of the Island of
Celebes (Fig. 275). But the coast of California furnishes the other
characteristic coast features in the high sea arch and the stack
as additional monuments to the recent uplift. Let one but imagine
the stacks which now form the Seal Rocks off the Cliff House at San
Francisco to be suddenly raised high above the sea, and the forms which
they would then present would differ but little from those which are
shown in Fig. 276.
[Illustration: FIG. 276.—Jasper rock stacks uplifted on the coast of
California (after a photograph by Fairbanks).]
=The uplifted barrier beach.=—Within the reëntrants of the shore, the
wave-cut cliff is, as we know, replaced by the barrier beach, which
takes its course across the entrance to a bay. After an uplift, such
a barrier composed of sand or shingle should be connected with the
headlands, often with a partially filled lagoon behind it. Its cross
section should be steep in the direction of the lagoon, but quite
gradual in front (Fig. 277).
[Illustration: FIG. 277.—Uplifted shingle beach across the entrance to
a former bay upon the coast of southern California (after a photograph
by Fairbanks).]
[Illustration: FIG. 278.—Raised beach terraces near Elie, Fife,
Scotland.]
=Coast terraces.=—Upon those shores where to-day high mountains
front the sea, the coast may generally be seen to rise in a series of
terraces (Fig. 278). This is notably true of those coasts which are
to-day racked by earthquakes, such as is the eastern margin of the
Pacific from Alaska to Patagonia. The traveler by steamer along the
coast from San Francisco to Chili has for weeks almost constantly in
sight these giant steps on which the mountains have been uplifted from
the sea. In Alaska we are fortunate in having the history of the later
stages in this uplift (Fig. 279). As described in a former chapter,
portions of this shore rose in the month of September of the year 1899
in some places as high as forty-seven feet, to the accompaniment of
a terrific earthquake and sea wave. Above the terrace which marks
the beach line of 1899 there is a higher terrace of similar form now
overgrown with trees, but none the less clearly to be recognized as a
shore line of the past century which preceded in the long sequence the
uplift of 1899.
[Illustration: FIG. 279.—Uplifted sea cliffs and terraces on the coast
of Russell Fjord, Alaska (after Tarr and Martin).]
[Illustration: FIG. 280.—Diagrams to show how excessive sinking
upon the sea floor will cause the shore to migrate landward as it is
uplifted.]
[Illustration:
FIG. 281.—A drowned river mouth, or estuary upon a coastal plain.]
As was noted in our study of earthquakes, the recent instrumental
records of distant earthquakes tell us that the movements upon the sea
floor are many times larger than those upon the continents, and that
while the mountainous coasts are generally rising, the deeps of the sea
are sinking. The effect of this over-balance of sinking, or resultant
shrinking of the earth’s shell, may be to compress the mountain
district and so cause the shore line to move landward at the same time
that it moves upward (Fig. 280).
=The sunk or embayed coast.=—When now, upon the other hand, a section
of the coast line sinks with reference to the sea, the water invades
all the near-shore valleys, thus “drowning” them and yielding the
drowned river mouth or _estuary_. If the relief of the shore was
slight, as it generally is upon a coastal plain, slight depression only
will produce broad estuaries, such as Chesapeake Bay at the drowned
mouth of the Susquehanna (Fig. 281).
If, on the other hand, the relief of the shore is strong and the
subsidence is large, the entire coast line will be transformed into
an archipelago of steep-walled rocky islets which rise abruptly from
the sea (Figs. 282 and 284). A plateau which is intersected by deep
and steep-walled valleys of U-section (p. 341) under large submergence
yields the _fjords_ so characteristic of Scandinavia or Alaska. A
ragged coast line, fringed with islands as a result of submergence, is
described as an _embayed coast_.
[Illustration:
FIG. 282.—Archipelago of steep rocky islets due to large submergence
of a coast having strong relief. Entrance to Esquimalt Harbor,
Vancouver Island (after a photograph by Fairbanks).]
[Illustration:
FIG. 283.—The submerged Hudsonian channel which continues the Hudson
River across the continental shelf.]
=Submerged river channels.=—The sinking of a coast of small relief be
sufficient to completely submerge river valleys, whose channels then
begin to fill with sediment and whose courses can only be followed in
soundings. One of the most interesting of such channels is that which
continues the Hudson River across the continental shelf into the deeper
sea (Fig. 283).
=Records of an oscillation of movement.=—Because a coast is deeply
embayed is no ground for assuming that a subsidence is now in progress,
or is, in fact, the latest movement recorded upon the coast. In many
cases it is easy to see that such is not the case. The coast of Maine
is perhaps as typical of an embayed shore line as any that might be
cited, but a study of the river valleys in the neighborhood shows
clearly that the present submergence of their mouths is a fraction only
of an earlier one which has left a record of its existence in beds of
marine clay which outline the earlier and far deeper indentations (Fig.
284).
[Illustration:
FIG. 284.—Marine clay deposits near the mouths of the rivers of Maine
which preserve a record of earlier subsidence (after Stone).]
If now we give a closer examination to the coast, it is found that
there are marks of recent uplift in an abandoned shore line now far
above the reach of the waves. There is here, then, the record, first of
subsidence and consequent embayment, and, later, of an uplift which has
reduced the raggedness of the coast outline exposed the clay deposits,
and raised the strands of the period of deep subsidence to their
present position.
In countries which possess a more ancient civilization than our
own, the record of such oscillations in the level of the ground has
sometimes been entered upon human monuments, so that it is possible to
date more or less definitely the periods of subsidence or elevation. At
the little town of Pozzuoli, upon the shore of the Bay of Naples, is
found one of the mos instructive of these records.
[Illustration:
FIG. 285.—View of the three standing columns of the temple of Jupiter
Serapis at Pozzuoli, showing the dark and rough band nine feet in width
affected by the rock-boring mollusks which now live in the Bay of
Naples.]
In the ruins of the ancient temple of Jupiter Serapis are three marble
monoliths 40 feet in height, curiously marked by a roughened surface
between the heights of 12 and 21 feet above their pedestals (Fig. 285).
Closer inspection shows that this roughened surface has been produced
by a marine, rock-boring mollusk, the _lithodomus_, which lives in
the waters of the Bay of Naples, and the shells of this animal are
still to be found within the cavities upon the surface of the columns.
Without recounting details which have been many times recited since
these interesting monuments were first geologically explored by Babbage
and Lyell, it may be stated that a record is here preserved, first of
subsidence amounting to some 40 feet, and of subsequent elevation, of
the low coast land on which stood the temple in the old Roman city of
Puteoli (Fig. 286).
At the time of deepest submergence the top of the lithodomus zone
upon the column stood at the level of the water in the Bay of Naples,
the smoother lower zone being buried at the time in the sand at the
bottom, and thus made inaccessible for the lithodomi. It is to be added
that studies made in the environs of Pozzuoli have fully confirmed
the changes of level revealed by the columns, through the discovery
of now elevated shore lines which are referable to the period of deep
submergence.
=Simultaneous contrary movements upon a coast.=—In our study of
the changes in the level of the ground that take place during
earthquakes, it was learned that neighboring sections of the earth’s
crust may be moved at different rates or even in opposite directions,
notwithstanding the fact that the general movement of the province is
one of uplift. Thus during the Alaskan earthquake of 1899, although
portions of the coast line were elevated by as much as forty-seven
feet, neighboring sections were raised by smaller amounts, and some
small sections were sunk and so far submerged that the salt water and
the beach sand were washed about the roots of forest trees.
[Illustration: FIG. 286.—Pozzuoli in the 3rd, 9th, and 20th
Centuries.]
A region racked by heavy earthquakes, where the present configuration
of the ground speaks strongly for a movement of somewhat similar
nature, but with average movement of elevation much greater to the
northward than in the opposite direction, is the extended coast line
of Chili. This country is characterized by a great central north and
south valley which separates the coast range from the high chain of the
Cordilleras to the eastward. To the southward the floor of this valley
descends, and has its continuance in the Gulf of Corcovado behind the
island of Chiloe and the Chonos archipelago. The known recent uplift of
the coast of Chili, particularly in the northern sections and during
the earthquakes of the eighteenth, nineteenth, and twentieth centuries,
lends great interest to this topographic peculiarity. Indications are
not lacking that, during the earthquake of Concepcion in 1835, and of
Valparaiso in 1907, the measure of uplift was greater to the north than
it was to the south.
[Illustration: FIG. 287.—Map of San Clemente Island, California,
showing the characteristic topography of recent uplift (after U. S.
Coast and Geodetic Survey).]
=The contrasted islands of San Clemente and Santa Catalina.=—Perhaps
the most striking example of simultaneous opposite movements observable
in neighboring portions of the earth’s crust is furnished by the coast
of southern California. The coast itself at San Pedro and the island
of San Clemente, some fifty miles off this point, in common with most
portions of the neighboring coast land, have been rising in interrupted
movements from the sea, and offer in rare perfection the characteristic
coast terraces (Fig. 287 and Fig. 278, p. 250). Midway between these
two rising sections of the crust, and less than twenty-five miles
distant from either, is the island of Santa Catalina, which has been
sinking beneath the waves, and apparently at a similarly rapid rate
(Fig. 288). The topography of the island shows the intricate detail of
a maturely eroded surface, while that of the neighboring San Clemente
shows only the widely spaced, deep cañons of the infantile stage of
erosion (Fig. 165 and pl. 12 A). While Santa Catalina has been sinking,
San Pedro Hill has risen 1240 feet, and San Clemente, 1500 feet. It
is characteristic of a sinking coast line that the cliff recession is
abnormally rapid, and evidence for this is furnished by the shores of
Santa Catalina, upon which the waves are cutting the cliffs back into
the beds of cañons, and so causing small falls to develop at the cañon
mouths.
┌─────────────────────────────────────────────────────────────────────┐
│ PLATE 12. │
│ │
│ [Illustration: _A._ V-shaped cañon cut in an upland recently │
│ elevated from the sea, San Clemente Island, California (after W. S. │
│ Tangier-Smith).] │
│ │
│ [Illustration: _B._ A “hogback” at the base of the Bighorn │
│ Mountains, Wyoming (after Darton).] │
└─────────────────────────────────────────────────────────────────────┘
[Illustration:
FIG. 288.—Map of Santa Catalina Island, California, showing the
characteristic surface of an area which has long been above the waves,
and the entire absence of coast terraces (after U. S. C. and G. S.).]
=The Blue Grotto of Capri.=—We may now return to the Bay of Naples for
additional evidence that oscillations of level in neighboring portions
of the same coast are not necessarily synchronous, and that near-lying
sections may even move in opposite directions at the same time, as has
already been shown for the islands off the California coast. For the
Pozzuoli shore of the bay it was learned that within the Christian
Era a complete cycle of downward, followed by later upward, movement
has been largely accomplished. Across the bay, and less than 20 miles
distant, is the Blue Grotto of Capri, a sea cave cut in limestone above
an earlier cave of the same nature which is now deep below the water
surface. It is the refracted sunlight which enters the cave through
this lower submerged opening and has been robbed on the way of all but
its blue rays which gives to the famous grotto its special charm (Fig.
289).
[Illustration:
FIG. 289.—Cross section of the Blue Grotto on the Island of Capri,
showing the submerged sea cave through which most of the light enters
the grotto, and the higher artificial window now widened by wave action
(after von Knebel).]
It is known that the former, and now submerged, sea cave was in use
by Roman patricians as a cool retreat from the oppressive hot wind
known as the sirocco, and that an artificial entrance or window was
cut where is now the only accessible entrance to the grotto. In the
ancient writings, no mention is made, however, of the remarkable blue
illumination for which it is now famous, and the conditions at the
time, as we may see, were not such as to make this possible. Later
subsidence of the coast has brought the ancient window to the sea
level, where it has been considerably enlarged by the waves. The
earlier grotto, abandoned as its entrance was closed, was rediscovered
in 1826 by the painter and poet, August Kopisch.
A grotto with green illumination (the Grotto Verde) is situated upon
the opposite side of the island, and a blue grotto, having its origin
in similar conditions to those of the famous Blue Grotto, is found upon
the island of Busi off the Dalmatian coast.
=Character profiles.=—In the landscape of a coast which has been
slowly uplifted the characteristic line is the profile of the cuesta,
with short perpendicular element joined to a gently sloping and longer
section and continued in the horizontal portion corresponding to the
lowland (Fig. 290). Rapidly uplifted coasts offer in contrast the
lines characteristic of wave erosion and deposition, but at higher
levels and in repeated sections. Most prominent of all is the staircase
constructed of coast terraces, with either vertical or sloping risers
and with outwardly inclining and gently graded treads. Near the steep
riser in the staircase may sometimes be seen the sugar-loaf outline of
the stack cut in softer material, or the obelisk-like pillar undercut
at its base, which is carved in firmer rock masses. With excessively
rapid uplift, the double-notched cliff or the double sea arch may
appear in the landscape. Upon a submerged coast the most significant
lines in the view are those of the rock islet and the steep-walled
fjord.
[Illustration: FIG. 290.—Character profiles in coast landscapes where
there has been either elevation or depression.]
READING REFERENCES FOR CHAPTER XIX
General:—
SIR CH. LYELL. Principles of Geology, vol. 2, pp. 180-197.
ED. SUESS. The Face of the Earth, Clarendon Press, Oxford, 1906, vol.
2, Chapters i and xiv, pp. 1-29, 535-556.
ROBERT SIEGER. Seenschwankungen und Strandverschiebungen in
Scandinavien, Zeit. d. Gesell. f. Erdk., Berlin, vol. 28, 1893, pp.
1-106, 393-688, pl. 7.
Elevated shore lines:—
F. B. TAYLOR. The Highest Old Shore Line of Mackinac Island, Am. Jour.
Sci., vol. 43, 1892, pp. 210-218.
THOMAS L. WATSON. Evidences of Recent Elevation of the Southern Coast
of Baffins Land, Jour. Geol., vol. 5, 1897, pp. 17-33.
J. W. GOLDTHWAIT. The Abandoned Shore Lines of Eastern Wisconsin.
Bull. 17, Wis. Geol. and Nat. Hist. Surv., 1907, pp. 134, pls. 1-37.
Evidences of depression:—
W. B. SCOTT. Introduction to Geology, New York, 1907, pp. 33-36.
W. J. MCGEE. The Gulf of Mexico as a Measure of Isostacy, Am. Jour.
Sci. (3), vol. 44, 1892, pp. 177-192.
A. LINDENKOHL. Notes on the Submarine Channel of the Hudson River,
etc., Am. Jour. Sci. (3), vol. 41, 1891, pp. 489-499, pl. 18.
J. W. SPENCER. The Submarine Great Cañon of the Hudson River, _ibid._
(4), vol. 19, 1905, pp. 1-15; Submarine Valleys off the American Coast
and in the North Atlantic, Bull. Geol. Soc. Am., vol. 14, 1903, pp.
207-226, pls. 19-20.
F. NANSEN. The Bathymetrical Features of the North Polar Sea, with
a Discussion of the Continental Shelves and Previous Oscillations
of Shore Line, Norwegian North Polar Expedition, vol. 4, 1904, pp.
99-231, pl. 1.
W. V. KNEBEL. Höhlenkunde, etc., Braunschweig, 1906, pp. 175-177 (the
blue grotto of Capri).
Oscillation of movement:—
C. LYELL. Principles of Geology, vol. 2, pp. 164-176 (Temple of
Jupiter Serapis).
E. RAY LANKESTER. Extinct Animals, New York, 1905, pp. 31-42.
H. W. FAIRBANKS. Oscillations of the Coast of California during the
Pliocene and Pleistocene, Am. Geol., vol. 20, 1897, pp. 213-245.
G. H. STONE. Mon. 34, U. S. Geol. Surv., 1899, pp. 56-58, pl. 2.
BAILEY WILLIS. Ames Knob, North Haven, Maine. Bull. Geol. Soc. Am.,
vol. 14, 1903, pp. 201-206, pls. 17-18.
Simultaneous contrary movements on a coast:—
A. C. LAWSON. The Post-Pliocene Diastrophism of the Coast of Southern
California, Bull. Univ. Calif. Dept. Geol., vol. 1, 1893, pp. 115-160,
pls. 8-9.
W. S. TANGIER-SMITH. A Geological Sketch of San Clemente Island, 18th
Ann. Rept. U. S. Geol. Surv., Pt. ii, 1898, pp. 459-496, pls. 84-96.
R. S. TARR and L. MARTIN. Recent Changes of Level in the Yakutat Bay
Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64, pls.
12-23.