WORK OF THE WAVES.
Erosion.
The general effects of the waves and the other movements to which they give rise along shores are (1) the wear of the shores; (2) the transportation for greater or less distances of the products of wear; and (3) the deposition of the transported materials.
By waves and undertow.—In the dash of the waves against the shore, the chief wear is effected by the impact of the water and of the débris which the water carries. Lesser results are accomplished in other ways.
When the land at the margin of the water consists of unconsolidated material, or of fragmental material but slightly cemented, the impact of the water is sufficient to displace or erode it. If weak rock be associated with resistant rock within the zone of wave-work, the removal of the former may lead to the disruption and fall of the latter, especially when weak rock is washed out from beneath the strong. The impact of the water is competent also to break up and remove rock which was once resistant, but which has been superficially weakened by changes of temperature. Rock affected by numerous open joints is likewise attacked with success, for by the dash of the waves the blocks between the joints may be loosened and literally quarried out. It may, however, be doubted whether the dash of waves of clear water, even when their force is many tons to the square foot, has any appreciable power to wear rock which is thoroughly solid.
Fig. 301.—Angular blocks of rock which have fallen from the cliff above, as a result of undercutting by the waves. Grand Island, Lake Champlain. The rock is Black River limestone. Although from the shore of a lake instead of the sea, the principles illustrated are the same. (Perry.)
The impact of the waves is generally reinforced and made effective by the impact of the detritus they carry. The sand, the pebbles, and such stones as the waves can move are used as weapons of attack, being turned against one another and against the shore. Masses of rock too large for the waves to move ([Fig. 301]) are worn by the detritus
driven back and forth over them, and in time reduced to movable dimensions ([Fig. 302]). They then become the tools of the waves, and in use, are reduced to smaller and smaller size. Thus bowlders are reduced to cobbles, cobbles to pebbles, pebbles to sand, and sand to silt. The silt is readily held in suspension in agitated water, and thus is carried out beyond the range of breakers, and settles in water so deep as not to be effectively agitated to its bottom. Thus one generation of bowlders after another is worn out, and the comminuted products are carried out from the immediate shore and deposited in deeper water.
The effectiveness of waves, whether they work by impact of water alone, or by impact of water and detritus, is dependent on their strength and on the concentration of their blows.[156] The strength of waves is dependent on the strength of the winds (or other generating cause) and the depth and expanse of the water, and the concentration of their blows is conditioned by the slope against which they break. On exposed ocean-coasts the fetch of the waves is always great. The winds are variable. For a given coast they have an average strength, but the effectiveness of wave-erosion is determined less by the average strength of waves than by the strength of the storm-waves. This is often very great. On the Atlantic and North Sea coasts of Britain, winter breakers which exert a pressure of three tons per square foot are not infrequent.[157] So great is the force of exceptional storm-waves that blocks of rock exceeding 100 tons in weight are known to have been moved by them. Ground-swells, “even when no wind is blowing, often cover the cliffs of north Scotland with sheets of water and foam up to heights of 100 or even nearly 200 feet. During northeasterly gales the windows of the Dunnet Head lighthouse, at a height of upwards of 300 feet above high-water mark, are said to be sometimes broken by stones swept up the cliffs by sheets of sea-water.”[158] The average force of waves on the Atlantic coast of Britain has been found to be 611 lbs. per square foot in summer, and 2086 lbs. in winter.[159]
Where deep water extends up to the shore, the force of the wave is almost wholly expended near the water line; where shallow water borders the land, the force of the waves is expended over a greater area. Waves are, therefore, most efficient on bold coasts bordered by broad expanses of deep water.
The less familiar phases of wave-work are accomplished by hydraulic pressure, compressed air, the use of ice, etc. When the water of a wave is driven into an open joint or a cave, the hydraulic pressure is great, and if the structure be weak, the rock may be broken. When water is driven with force into a cave, the compression of the air may be great if the wave be high enough to close the entrance. When the water runs out of a cave, the air within may be greatly rarefied, while that above exerts its normal pressure. In either case the roof of the cave, if it be weak, may be broken. At certain seasons of the year, especially during the spring, waves make destructive use of the ice which is then breaking up, but it is only in high latitudes that sea-ice is of consequence in this way. In general, the effect of its presence in keeping down waves overbalances its effect as an agent of erosion.
Fig. 302.—Showing blocks similar to those of [Fig. 306], reduced and rounded by wave-action. Shore of Lake Champlain. The rock is Utica shale. (Perry.)
The direct effect of wave-erosion is restricted to a zone which is narrow both horizontally and vertically. There is no impact of breakers at levels lower than the troughs of the waves, though erosion may extend down to the limit of effective agitation ([p. 341]). The efficient impact of waves is limited upward by the level of the wave-crests, although the dash of the water produces feebler blows at higher levels. The rise and fall of the water during the flow and ebb of the tides gives the waves a greater vertical range than wind-waves alone would have. The vertical zone of direct wave-work is therefore limited above by the level of wave-crests, and below by the depth of wave-troughs (nearly). The indirect work of waves is limited only by the height of the shore, for as the zone of excavation is carried landward, masses higher up the slope are undermined and fall. The fallen rock temporarily protects the shore against the waves, but are themselves eventually broken up.
Fig. 303.—Diagram illustrating high sea-cliffs. It also shows a submerged terrace, due partly to wave-cutting (wave-cut terrace), and partly to building (wave-built terrace). (Gilbert.)
Fig. 304.—Diagram showing a low sea-cliff. (Gilbert.)
The pulsating current of the undertow ([p. 341]) has both an erosive and a transporting function. It carries the detritus of the shore to and fro, and dragging it over the bottom, continues downward the erosion initiated by the breakers. This downward erosion is the necessary concomitant of the shoreward progress of wave-erosion; for, if the land were merely planed away to the level of the wave-troughs, the incoming waves would break where shoal water was first reached, and become ineffective at the water margin. The rate of erosion by the undertow becomes less and less as the surface it affects is lowered. Littoral currents do little erosive work beyond that inflicted on the material which they transport.
The general result of wave-erosion is the advance of the sea on the land, the rate of advance being determined chiefly by the nature of the material attacked and the strength of the waves. Numerous as examples are of the retreat of coast-lines before the advance of the sea, it is not to be understood that the advance of the sea on the land is universal or uninterrupted. Numerous instances may be cited of the encroachment of the land on the sea. At Long Branch the advance of the sea, in spite of elaborate breakwaters, has been so rapid in recent years as to menace important buildings, while a few miles to the north and south, the land is advancing in the face of the waves. The low coast of the Middle Netherlands has retreated two miles or more in historic times,[160] but the opposite tendency is shown at other points in the same region. On the coast of England the sites of villages have disappeared by the advance of the sea within historic times,[161] but the coast of the same island affords illustrations of land advance. On the south side of Nantucket island, the sea-cliff has been known to retreat before the waves as much as six feet in a single year.[162] Almost every considerable stretch of coast affords illustrations both of the advance of the sea on the land and of land on the sea; but in the long run, the former must exceed the latter, diastrophic movements aside.
Fig. 305.—Steep cliff developed by waves. Allen Point, Grand Island, Lake Champlain. (Perry.)
Fig. 306.—Cliff in unconsolidated material (bowlder clay), with lake-beach in foreground. South Manitou Island, Lake Michigan. (Russell, U. S. Geol. Surv.)
Fig. 307.—Steep cliff in unconsolidated material, the result of rapid cutting. Southeast extremity of Grove Point, Md.
Topographic Features Developed by Wave-erosion.
Fig. 308.—Standing Rock. A wave-erosion monument. West shore of Random Sound, south of Clarenville, N. F. (Walcott, U. S. Geol. Surv.)
The sea-cliff.—The action of the waves, cutting as they do along a definite horizontal zone, has been compared to the action of a horizontal saw. As the waves cut into the shore at and near the water-level, the material above, being unsupported, falls, leaving a steep face above the line of cutting. This steep face is known as the sea-cliff ([Figs. 301 to 306]). The same term is sometimes applied to the cliffs of lakes. The principles involved in the development of the sea-cliff are applicable to any broad stretch of water.
The height of the cliff depends on the height of the land on which the sea is advancing. Its slope may be steep or gentle (compare [Figs. 303 to 306]), according to the nature of the material of which it is composed and the rapidity of the cutting. Rapid cutting tends to produce steep cliffs and slow cutting gentle ones, for in the latter case weathering is more important relative to the cutting, and at sea-level (low altitudes) weathering generally tends to reduce the angle of slope. In general, the more resistant the material the steeper the slope of the cliff. Incoherent materials, such as sand and clay, are not likely to form steep cliffs; but if the cutting be very rapid, bold faces may be developed even in such materials ([Fig. 307]). If beds of slight resistance at sea-level underlie beds of greater resistance, the development of steep cliffs is favored. The structure of the cliff-rock also has an influence on the slope. The rock may be massive or bedded. If bedded, the beds may be horizontal, or they may dip at any angle, in any direction. The rock, whether stratified or not, may be abundantly or sparsely jointed. All these structures influence the slope and configuration of the sea-cliff (see [Figs. 305 to 308]).
Fig. 309.—“Old Man of Hoy.” (Geikie.)
Chimney-rocks, etc.—By working in along the joints of the rock, widening them and quarrying out the intervening blocks, pillars of rock (“chimney-rocks,” “pulpit-rocks”) or even considerable islets are sometimes isolated by the waves. This is most readily accomplished where the joints converge back from the shore. A well-known example of this sort is the “Old Man of Hoy” ([Fig. 309]) on the coast of the Orkneys. A pulpit-rock or other island, or any jutting point of rock may be pierced, giving an arch or bridge. La Roche Percée, a steep-faced isle near Gaspé Harbor, is an example.
Sea-caves.—Waves sometimes excavate caves at the bases of cliffs. This is especially likely to occur where the rock is much jointed and where the joints are not continued to the surface in a single plane. The bottom and roof of a sea-cave usually have a pronounced inclination landward. If the cliff be low, the cave may be extended landward until its roof is pierced. Through such an opening in the top of the cliff the water of the incoming waves may be forced in the form of spray. On the New England coast such holes are sometimes known as “spouting horns.” Similar openings may be made, as already pointed out, by the compression or rarefaction of the air in the cave as the wave enters or retreats. If the roof of the cave be partially destroyed, the portion which remains may form an arch or bridge. Such a bridge occurs on Santa Cruz Island, California ([Fig. 310]).
Fig. 310.—An arch developed by waves. Santa Cruz Island, Cal. (Law.)
The cave, the “spouting horn,” the “bridge,” the “pulpit-rock,” and other isolated islets, are all closely associated with the sea-cliff in origin.
The wave-cut terrace.—The bottom of the sea-cliff is bordered by a submerged platform over which the water is shallow. This platform, or at any rate its landward portion, represents the area over which the water has advanced as the result of wave-cutting, and is, therefore, known as the wave-cut terrace. From the method of cliff development it will be seen that the wave-cut terrace is its necessary accompaniment. Such a terrace has a gentle slope to seaward, for its outer and older edge has been degraded longer and more. Its slope is influenced by the strength of the waves, being greater where they are stronger. The outer edge of the wave-cut terrace is often marked by an abrupt descent. [Fig. 303] represents the wave-cut terrace in its relation to the sea-cliff above.
Fig. 311.—An elevated cliff above Great Salt Lake. In this case the water-level has been lowered. (Gilbert, U. S. Geol. Surv.)
So long as wave-cut terraces are submerged, they do not appear on topographic maps of the land, though they appear on the charts of the coasts; but if a coastal tract with wave-cut terraces be elevated, or if the sea-level be drawn down, the terraces become land. Elevated sea-cliffs and wave-cut terraces are among the best evidences of change of relative level between water and land ([Fig. 311]).
Wave-erosion and horizontal configuration.—The structure of the rock along shore has as much to do with the horizontal configuration of the wave-shaped coast, as with its relief. In general, waves develop reëntrants in the less resistant portions of the shore, leaving the more resistant parts as headlands (San Pedro Point and Devil’s Slide, [Pl. XX], Coast of California). It is to be noted that the resistance of rock to wave-erosion is not determined by its hardness alone. Every division plane, whether due to bedding, to jointing, or to irregular fracture, is a source of weakness to the rock, and rock of great hardness may be so broken as to offer relatively little resistance. Inequalities of resistance, whatever their cause, give origin to inequalities of coastal configuration where wave-erosion is in progress. Given a coast of marked regularity and equal exposure, but composed of unequally resistant material, the waves will make it irregular by cutting most where the material is least resistant. A regular coast of uniform material, but unequal exposure, will be made irregular by the greater cutting at the points of greater exposure. A coast of marked irregularity and homogeneous material will be made more regular by the cutting off of the projecting points, because they are most exposed. With a given set of conditions, waves tend to develop a certain sort of shore-line which, so far as its horizontal form is concerned, is relatively stable. Such a shore-line may be said to be mature[163] so far as wave-erosion is concerned. Since coastal lands are, in general, both heterogeneous and unequally exposed, a mature coast-line is somewhat irregular. Its maturity is attained when the lesser exposure in the reëntrants developed in the less resistant parts, balances the superior exposure of the projections of the more resistant portions.
Since the conditions of erosion along coasts are constantly, even if slowly, changing, maturity is constantly being approached, but rarely reached. Other forces and processes, such as those of aggradation, vulcanism, and diastrophism, are in operation along coasts, and their results are sometimes antagonistic to those of the waves. The horizontal configuration of coasts is, therefore, the result of many coöperating forces, of which waves are but one. It is, nevertheless, important to note the goal to which the waves are working, even though they are continually defeated in their attempt to reach it. Their immediate goal is an equilibrium of erosion-rate and maturity of configuration; their final goal is the destruction of the land and the deposition of its substance in the sea, that is, in a position nearer the center of gravity of the earth.
Transportation by Waves.
The material eroded from the shore by the waves in the shaping of the cliff and terrace is carried away by the joint action of the waves, undertow, and shore-currents.
The in-coming wave begins to shift material where it begins to drag bottom, that is, a little outside the line of breakers. From the line where transportation begins, to the line of breakers, bottom detritus is shifted shoreward by the waves, while the undertow tends to carry it back again. Between the breakers and the shore there is also a tendency for the on-shore movement to carry débris to the water’s edge, and for the ebbing wave to carry it back again. The result of these opposed tendencies is to keep sediment in transit between the shore and the line of breakers. If the in-coming waves have a direction normal to the shore, the advance and recoil of the water move particles toward and from the shore, but effect no transfer along the shore; but the results which waves normal to the shore would achieve are always modified by other waves and by littoral currents.
If the in-coming wave is oblique to the shore, it shifts material in its own direction. The transfer by undertow, taken alone, would be sensibly normal to the shore, but the effect of the oblique waves is to slightly modify this direction. There is thus a slow transportation along shore, even in the absence of steady currents. A great amount of transportation would be effected in this way, though it would be carried on at a slow rate. Oblique waves also tend to develop a definite shore-current ([p. 342]) which affects both the amount and direction of the transportation. Any particle in suspension, or in motion on the bottom as the result of the wave or undertow, is shifted along shore by the littoral current, which affects the same water ([Fig. 300]). By the coöperation of wave- and shore-current, more and heavier material can be moved than by either alone, and the direction of movement is more nearly parallel to the shore than that of the wave. Similarly, by the coöperation of undertow and shore-current, more and heavier material can be moved than by either alone. The direction of movement is readily inferred from [Fig. 300]. The direction in which débris is shifted by wave- and shore-current is modified by the undertow, and the direction which would result from undertow and current is modified by the wave. It is often the waves of storms, rather than those of the prevailing winds, which determine the direction of greatest shore transportation.
The waves, the undertow, and the littoral currents work together in assorting the detritus of the shore. The coarsest parts may be beyond the power of all but the strongest waves. They accumulate where agitation is great. Less coarse parts are shifted farther from the site of greatest agitation, but no materials which are classed as coarse are carried beyond the depth of sensible movement. The coarse material which covers the bottom where the agitation of the water at the bottom is effective, constitutes shore drift.
Shore drift is not all derived from the shore by the cutting of the waves. A part of it is brought to the sea by streams and mingled with that eroded from the cliffs. The material which is fine enough to be held in suspension is measurably independent of depth. This is shown during storms when the water becomes turbid far beyond the line of breakers, and clears only after the waves have died away.
This sorting of shore drift, effected while it is in transportation, is often very perfect. The conditions favoring assortment are (1) vigorous wave-action, (2) prolonged transportation, and (3) a moderate volume of sediment.[164] The effect of these several conditions will be readily understood.
Extensive transportation of shore drift of a given degree of coarseness is favored by (1) strong waves and undertow, (2) continuous currents, and (3) shallow water, deepening but gradually off shore.
Deposition by Waves, Undertow, and Shore-currents.[165]
Fig. 312.—Cross-section of the beach. (Gilbert.)
The beach.—The zone occupied by the shore drift in transit is the beach. The lower margin is beneath the water, a little beyond the line where the great storm-waves break. Its upper margin is at the level reached by storm-waves, and is usually a few feet above the level of still water. To the beach, material is brought from seaward by the in-coming waves, and from it detritus is carried out by the undertow. The cross-section of a beach is shown in [Fig. 312]. In horizontal position the beach follows the general boundary between water and land, though it does not conform to its minor irregularities ([Fig. 313]). The beach or barrier ridge often causes the deflection of the lower courses of streams descending to it ([Pl. XXI]).
Fig. 313.—A lake-beach (barrier); Griffin’s Bay, Lake Ontario.
PLATE XXI.
U. S. Geol. Surv.
Scale, 1 mile per inch.
NEW JERSEY.
PLATE XXII.
U. S. Geol. Surv.
Scale, 1 mile per inch.
Fig. 1. PORTION OF SOUTH COAST OF MARTHAS VINEYARD, MASSACHUSETTS.
U. S. Geol. Surv.
Scale, 1 mile per inch.
Fig. 2. PORTION OF THE CALIFORNIA COAST NEAR TAMALPAIS.
Fig. 314.—Section of a barrier. (Gilbert.)
The barrier.—When the agitation of the water along shore becomes insufficient to carry the material, it is dropped. In its deposition it assumes various forms. Where the bottom of the lake or sea near shore has a very gentle inclination, the in-coming waves break some distance from the shore-line, and it is here that the most violent agitation occurs when the waves are strong. To this line of breakers, material is shifted from both directions: from shore by undertow, and from seaward by the waves. Accumulating here, it builds up a low ridge. This is a barrier ([Fig. 314]). If it is built up above the surface of the water by storm-waves, it may shut in a lagoon behind it, and this may ultimately be filled by sediment washed down from the land. At one stage in the filling, the lagoon becomes a marsh.[166] In the part which the barrier plays in the history of a coast, it is identical with the beach.
Fig. 315.—A recurved spit. Dutch Point, Grand Traverse Bay, Lake Michigan.
Fig. 316.—Cross-section of a bar. (Gilbert.)
The spit, the bar, and the loop.—The disposition of shore-deposits depends largely on the currents at and near shore. If the coast-line is deeply indented, the littoral current usually fails to follow the reëntrants. In holding its course across the mouth of a small bay, a shore-current usually passes into deeper water. Here its velocity is checked because its motion is communicated to the water beneath it, and a larger amount of water being involved in the motion, the motion of each part is diminished. If sediment was being moved along its bottom before the current was checked, some part of it is dropped when and where the current is slackened. It follows that deposition commonly takes place beneath a littoral current as it crosses the mouth of a bay. The belt of deposition is often narrow, and the result is the construction of a ridge beneath the water in the direction of the current. The current would never build the embankment up to the water-level, but when its surface approaches the level of effective agitation, the waves may begin to work on it, as on a barrier, and may build it up to, and even above, the surface of the water. So long as the end of such an embankment is free, it is a spit ([Fig. 315] and [Pl. XXI]). If the spit be lengthened until it crosses, or nearly crosses, the bay, shutting it off from the open water, it becomes a bar. Bars have shut in lakes (ponds) on the coast of Martha’s Vineyard, Mass. ([Fig. 1, Pl. XXII]), and lakes and lagoons at numerous points both on the Atlantic and the Pacific coasts ([Fig. 2, Pl. XXII], Rodeo lagoon). The same phenomena are to be seen along many lake shores. Bars sometimes tie islands to the mainland ([Pl. XXIII, Fig. 1], Nahant, Mass.; [Fig. 2], near Biddeford, Me.). The structure of a bar as seen in cross-section is shown in [Fig. 316].
The construction of a spit has been aptly compared to the construction of a railway embankment across a depression. The material is first carried out from the bordering upland (shallow water) and dumped where the slope to the depression (deep water) begins. The embankment thus begun is extended by the carrying out of new material, which is left at the end of the dump already made.
If the bay across which the bar is built receives abundant drainage from the land, the outflow from the bay may be sufficient to prevent the completion of the bar ([Fig. 2, Pl. XXII]), for when the growth of the spit has sufficiently narrowed the outlet of the bay, the sediment brought to the end of the spit by the littoral current will be swept out beyond the spit by the current setting out from the bay.
The completion of a bar may be interfered with by tidal currents, even without land-drainage. Currents generated by the tides may sweep in or out of the bay with increased force as the entrance is narrowed, carrying in or out the sediment which the littoral current would have left at the end of the spit. The scour of the tides often insures deep entrances (inlets) to bays, and maintains definite channels or “thorofares” in the lagoon marshes behind barriers and spits. The sediment brought down from the land, as well as that washed in by tidal currents and waves, tends to fill up the lagoon behind a barrier, a spit, or a bar, converting it into land ([Fig. 317]).
Fig. 317.—Sketch of a portion of the New Jersey coast. The dotted belt next the sea is the barrier, modified by the wind. The area marked by the diagonal lines is the mainland. In the marshy area between, there are numerous channels or “thorofares” kept open by the currents. The figures show the depths of water in feet. Scale about ⅜ inch = 1 mile.
Since spits and bars are built only where there is shore-drift in transit, they are always built out from a beach or barrier. The distal end of the bar may also join a beach or barrier. Traced back to its source, the beach from which a spit leads out is often found to terminate in the cliff from which the material of the beach and the spit were derived ([Pl. XX] and [Fig. 2, Pl. XXII]). In such cases the sediment of the beach has been shifted but a short distance; but in other cases it has traveled far.
Fig. 318.—Map of shore-terraces, largely wave-built. Lake Bonneville. (Gilbert.)
Fig. 319.—A portion of the Texas coast showing the tendency of shore-deposition to simplify the coast-line. The deposits (the narrow necks of land parallel to the coast) shut in the bays. (From chart of C. and G. Surv.)
Fig. 320.—Map showing that in the early stages of the simplification of a shore-line the irregularities are increased. The numbers indicate the depth of water in fathoms. (From chart of C. and G. Surv.)
The spit is usually either straight or in conformity with the general course of the shore-current, but since the littoral current itself is subject to alteration as the result of shifting winds, the spit may depart from straightness. Winds which simply reverse the direction of the littoral current retard its construction, but may not otherwise affect it; but if a strong current be made to flow past the end of a spit, it may cut away its extremity and rebuild the materials into a smaller spit, joining the main one at an angle. This gives rise to a hook ([Fig. 315]). Successive storms may develop successive hooks along the side of a growing spit. The end of a hook may be so extended as to join the mainland, when it becomes a loop.
Wave-built terraces.—Under the influence of off-shore currents, littoral currents may be drawn from the coast-line. If such a current continues as a well-defined surface-current, it builds a spit, but if it spreads, it tends to build a terrace. The accumulation then is not at the end of a beach, as in the case of a spit, but on its side, and the result of the deposition is to carry the beach seaward. The undertow abets the process. The widened beach is a wave-built terrace. The wave-built terrace often borders the wave-cut terrace along its seaward margin (Figs. [303] and [318]). With the help of waves, the surface of the terrace may be built up into land by the expansion of the crest of the beach. Terrace-cutting and terrace-building are both involved in the development of the continental shelves.
Beach ridges, spits, bars, etc., like sea-cliffs and wave-cut terraces, are often preserved after the relative level of sea and land has changed. If the shore has risen, relatively or absolutely, these features are relied on as evidences of the change. If shore features be submerged instead of elevated, they furnish less accessible, though not less real, evidence of the change of level. Similar features about lakes have a like significance, but in this case it is often demonstrable that it is the water rather than the land which has changed its level.
Effect of Shore-deposition on Coastal Configuration.
The tendency of shore-deposition is to cut off bays and to straighten and simplify the shore-lines. This is abundantly illustrated along the Atlantic and Gulf coasts of the United States (see [Fig. 319] and [Pl. XXII]). It is to be noted, however, that in the simplification of the shore-line through deposition, the initial stages often result in great irregularity ([Fig. 320] and [Pl. XXIII]). In some cases, the irregularities are not temporary. Thus deltas ([p. 198]), though not wholly the work of sea- (or lake-) water, often constitute irregularities of a more or less permanent nature. This is the case where they project beyond the general trend of the coast-line. Where, on the other hand, they are built at the heads of bays, they tend to simplify the coast-line by obliterating the indentation. The delta at the head of the Gulf of California is an example. So too is the delta of the Mississippi, the real head of which is far above the present debouchure of the stream. The form of the delta in ground-plan depends on the horizontal configuration of the coast where it is developed, on the strength of the waves and shore-currents, and on their relation to the amount of detritus contributed by the stream concerned. Good illustrations are furnished by the Gulf of Mexico where the deltas of the Mississippi and Rio Grande are in contrast.
So far as concerns the vertical configuration of coasts, erosion and deposition are in contrast, for while the former tends to develop steep, irregular, and often high slopes ([p. 349]) from the land to the sea, the latter tends to develop gentle, regular, and low ones. A partial exception to the latter part of this general statement comes about through the building of dunes, the material for which is furnished by the waves.