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.

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.

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).

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).

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.

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.

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).

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.

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.

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).

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.

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.

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.

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.

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).

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.

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.

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