above present sea level in the much cored and studied post-Pleistocene alluvial fill of Mississippi River in the Atchafalaya river basin, Louisiana (Fisk 1952).

No unquestionable evidence seems yet to have been offered that elevated, unwarped (eustatic) shorelines below +25 feet are of Recent or postGlacial age, despite continued statements by many geologists that they "seem to be Recent." R. W. Fairbridge and E. D. Gill of Australia 17 think that the materials of the shorelines of Australia below 10 feet are not sufficiently weathered and leached to have been formed before the last major sea level lowering. On Chesapeake Bay, G. F. Carter 18 finds no post-Pleistocene deposits above a maturely developed soil, supposedly of postPleistocene age, which dips beneath bay sediments and has been cored into off-shore. We do not know that the shores of the Chesapeake have been downwarped. The Pamlico terrace is reported as running level along this coast from Maryland to Florida.

The only dated shoreline deposits above sea level that are thought to be of historical or earlier Recent Age of which the writer has been able to learn, come from young orogenic coasts, as that of Tripoli 19 in Lebanon (Wetzel and Haller 1945) and on the Pacific coast of South America. These coasts must be suspected of having had crustal movements going on at any time, even in recent millenia. Thus, Jerico, 175 miles southwest of Tripoli, was once destroyed by an earthquake and 200 historical shocks are reported for the area of Israel (Ball and Ball, 1953).

SHORELINE CHANGES AND PROCESSES

SHORELINE SIMPLIFICATION

Terminology.-Shepard (1937a; 1948, pp. 70-73) says that "as numerous coasts and shorelines have undergone little modification since the sea level and the land came to rest, it seemed logical to refer to these as Primary . . . and to . . . those which have been considerably modified by the waves and currents as Secondary . . .” In his tables he calls "primary" shorelines youthful and "secondary" coasts mature. Following this concept, we find that mature marine coasts have in

17 Letters of 1952.

18 Letters of 1952.

19 At 2 to 3 meters above sea 600 m. inland and possibly 3,000 to 4,000 years old.

general become simplified in contour, with their irregularities reduced by erosion, solution or sedimentation, or a combination of processes. Hence, the end result of marine action on most types of coasts is smoothing, though not always straightening, as smooth coasts may be curved.

Processes. Simplification of a coast may consist of the reduction of projections by erosion, and the deposition of beach and other deposits in reentrants. It may also be brought about by the formation offshore in shallow water of a barrier island or barrier spits (Price 1951a, Shepard 1952). Such inorganic barriers tend to follow along a bottom contour, crossing the sites of entrenched valleys on postentrenchment fill, while the mainland shoreline is deeply indented by the shallow embayments of the former valleys. Thus, the new marine shoreline is smooth and shorter than the mainland shoreline off which it is built.

Examples. Simplification of Gulf shorelines is shown by (1) extensive development of sandy barriers where there are or were irregularities of the mainland shoreline, chiefly between the convexities of deltas (Sector 1), (2) the gradual filling of coastal lagoons (as east of Galveston Bay, sector 1.2), (3) the incipient smoothing of projections along some sectors of the drowned karst coast (2.1), (4) seemingly some smoothing of the front of parts of the mangrove ridge (Sector 4.1) facing the Gulf, in contrast with a possibly irregular original configuration such as that of the Ten Thousand Islands or the north shore of the Bay of Florida, (5) smoothing of the karst irregularities of the elevated Champoton-Campeche fault-block (Sector 2.2, Yucatán peninsula) so that only small cuspate points remain, (6) reduction by erosion of projecting folded limestone rock (northern Sector 3.1) and of the ends of narrow tongues of lava solidified to rock extending into the Gulf from the active volcanic salients of the young orogenic coast of Mexico (southern Sector 3.1).

Significance.-The several degrees of shoreline simplification evident in the preceding list, suggest a considerable quantitative range in the effective application of marine energy to shoreline modification during the 3,000 to 5,000 years of essential stillstand of the Gulf. Just as we find variation in simplification related to the hardness and resistance of the shoreline materials, rocks or soft sediments, so we may suspect that there have been differences in the amounts of energy available

for shoreline work. This supposition is justified by (1) the consideration that erosion at the shoreline has a vertical as well as a horizontal component, (2) comparison of variations in the form and offshore gradients of the bottom of the continental shelf on various sectors of the Gulf, and (3) inspection of the charts of resultant winds along the shorelines of the Gulf (U. S. Weather Bureau, 1938). These factors indicate that it may be feasible, from the partly quantitative, partly qualitative data presented or referred to here, to set up a preliminary energy classification of the coasts and shorelines of the Gulf of Mexico. This is attempted in the tabulation.

Extensive Marine Modification of Coasts of Gulf.-A summary of prominent shoreline conditions that indicate the degree of coastal modification is shown in tabular form below. The simplified coasts (the secondary or mature coasts of Shepard) greatly dominate in linear distribution, indicating that the sea has been at about the same level for a substantial period of time in relation to the resistance of most of the coastal materials to shoreline modification.

shoreface grades into a nearly smooth plain, here called the ramp, the gradient of which flattens slowly in an offshore direction for varying distances, commonly to 30 fathoms or more. The profiles drawn on this section of the shelf are mathematically of the hyperbolic or asymptotic type, the so-called logarithmic or exponential curves.

The ramp grades, usually far offshore, into a usually smooth convex section, here called the "camber," the gradient of which usually increases rapidly to the top of the irregular, steep, continental shelf slope. The sparse soundings available for the shelf of the young orogenic coast of Mexico (3, fig. 14), suggest that, except where a beach or barrier is present, this coast may lack a ramp, the camber beginning at or near the base of whatever shore cliff or shoreface is present. The so-called shelf break (Dietz and Menard, 1951) should be the junction between ramp and camber.

Data showing the locations and ramp slopes of the profiles (curves) of figure 15 and the sectors on which the curves are located are given in a tabulation following the illustration.

Location of profiles in figure 15.-All profiles measured perpendicular to shoreline from navigation charts U. S. Coast and Geodetic Survey.

(1) Off old Corpus Christi Pass and Padre Island barrier island 27°35' N. Lat., 97°13′ W. Long. Chart 1286, 1922 edition. A profile at same place from original survey sheet (1880) shows only minor irregularities and smoothly asymptotic curvature to 90-foot depth. Beach. Sand and clay bottom.

(2) Off Padre Island at Baffin Bay mouth, 27°18' and 97°20'. Chart 1286, beach sector: "Little Shell." Beach. Sand and clay bottom. (3) Off Matagorda Peninsula barrier island, off mouth Trespalacious Bay, 28°00', 96°10'. Chart 1284, 1945 edition. Beach. Sand and clay bottom. Fathogram off Galveston shows ramp as smooth as curves 1-3.

(4) Off barrier island on Florida peninsula 10 miles north of Cape Romano, 26°03' and 81°48'. Chart 1254, 1931 edition. Beach. Sand inshore. Rock bottom (limestone) with some sand and shells.

(5) Off Pine Islands-Key West shoals (Miami oölite with mangrove swamp deposits above), Florida, at Johnson Keys, 24°42', 81°36'. Chart 1251, 1940 edition. Profile begins at -8 feet. Add 8 to all depths for this curve in figure 15.

Sedimentation and the profiles.-The shoreface, ramp and camber of the normal coastal plain shelf, as exhibited on the Gulf of Mexico, seem to have specific characteristics as to sedimentation (map, fig. 16, p. 79) and erosion. From meager data, it seems that sand and shifting bars characterize the shoreface. Contemporary sands, relict deltas and barriers of former sea levels, with some contemporary clay deposition, characterize the ramp. Except when the entire profile is migrating landward, transportation probably dominates the ramp after any relict elevations have been removed from the part under consideration. Fine-grained sediments, mostly land-derived clays, and presumably the process of deposition, characterize the camber. Off the mouths of large deltas, little or no coarse sand reaches the Gulf and the charts show "mud" beginning near shore. Where sand is present it usually extends to 5 to 10 fathoms (Bates 1953; Lohse 1952).

Dietz and Menard (1951) have lately advanced evidence and argument for the belief that, at the level of the passage of the shelf from the steep concavity into the gentler slope, in present terminology, where the shoreface joins the ramp, is found the depth of maximum wave action on the bottom. They term it the depth of maximum abrasion, replacing the older concept of "wave base."

If the Gulf has remained essentially at the same level for the past 3,000 to 5,000 years, as previously suggested, it is evident that, on bottoms closely approximating the hyperbolic curve the shelf bottom must be in equilibrium. This should be true especially in coastal materials of slight resistance and where large amounts of marine

energy have been effectively applied. That the topography of the bottom is a simple mathematical surface with a hyperbolic bottom profile, is believed to indicate that the forces are in equilibrium. Where the bottoms are of hard rock and largely retain a subaerial topography, it may be concluded that the marine forces have inherited a surface produced under different conditions which they have been unable to destroy or to which they happen to be more or less adjusted.

The equilibrium profile of the coastal plain shelf is in a state of dynamic, not static, equilibrium. In dynamic equilibrium, variations of temporary, short-term value are to be expected. Thus, heavy storm waves are known to shift offshore bars 20 temporarily as much as a half-mile from their previous positions on the shoreface. Variation of the equilibrium will be about the mean. Marked departures from the mean are caused only by forces external to those in equilibrium. The shift of a river mouth, the coming of a lava flow, or the warping of the earth's crust, would be external forces or conditions which might upset a previously existing equilibrium on the shelf.

Usefulness of equilibrium profile.-Despite some pessimism (Kuenen 1950, p. 302) as to the value of the profile in geologic studies and much misconception on the part of writers as to the difference between static and dynamic equilibria in nature, the present writer finds that the profile of equilibrium is a suitable index of the response of a continental shelf bottom to the application of marine energy for a significant period of time. If, as some think, there have been several oscillations of sea level of as much as 10 to 20 feet during the past 4,000 years or so, a proposition that remains unproved, then the interpretation of the modification of the shorelines and shelf by marine energy is less clear than as here tentatively presented.

Theoretical breakerless curve fits Florida.-Keulegan and Krumbein (1949) made a theoretical study of the critical steepest bottom slope in shallow water on a shelf across which waves from deep oceanic waters may move but be constantly deformed and constantly lose energy so that they arrive at and near shore without enough height or energy to break or to develop shore structures, such as beaches or cliffs. The absence of such

20 The true underwater feature, not the barrier island. This occured at Galveston, Texas.

shore structure along much of the western shores of the limestone peninsulas of Florida and Yucatán, and the low gradients prevailing there offshore, led the writer to investigate these regions for examples of the beachless and breakerless coasts. More information is available for Florida than for Yucatán.

It was found that the requisite combination of (1) unmodified or little-modified shorelines, (2) gentle offshore slope and (3) essential absence of breakers (Corps of Engineers, U. S. Army 1940) exists on long stretches (Sectors 2.1 and 4.1, fig. 14) of the Gulf shoreline of peninsular Florida." By analogy, similar conditions are believed to exist on more than half the lengths of the western peninsular coasts of Florida and Yucatán, where the bottom gradient is low and the shoreline and bottom essentially unmodified by marine forces.

Comparison of the theoretical "breakerless bottom" curve of Keulegan and Krumbein (1949), described as profile 7 (p. 60 and fig. 15) with the actual rolling bottom profile of the drowned karst shelf of peninsular Florida (profile 6, fig. 15), shows that the two curves closely superimpose and are identical in over-all gradient. But the drowned karst profile has not been fully smoothed by erosion and deposition and is not yet a marine profile or equilibrium, although slight modifications of it indicate that such a development is going on.

DIRECTIONS OF LONGSHORE DRIFT

In the northwestern Gulf of Mexico, where a strong longshore sediment drift occurs, and wherever a barrier spit terminates, the dominant drift of the year is in the direction of the elongated, pointed barrier ends.22 These criteria agree there with the known histories of inlet migration, although there is a weaker summer drift to the northeast. Using spit criteria, the dominant longshore drift is seen to be westward and southwestward, that is, counterclockwise,23 from Apalachicola delta, Florida, to the poorly mapped volcanic sectors (Sector 3, fig. 14). Where sandy beaches and barriers occur on peninsular Florida,

"The data on waves and swell are being studied at the Agricultural and Mechanical College of Texas by Charles Bretschneider (Bretschneider and Reid 1953).

"The so-called Gulliver's rule (Johnson 1919, p. 376) cannot be applied here successfully in all cases from chart data and is of doubtful validity in any case. See Bullard (1942) and Price (1952).

23 With reference to the center of the Gulf.

longshore drift occurs. A northward drift exists for 20 nautical miles from the headland at Indian Rocks (270°52′ N. Lat.) to Anclote Keys. A much stronger south-southeastward drift exists from Indian Rocks to Cape Romano and its large underwater bars, a distance of 75 nautical miles. Southeastward drift again appears south of Cape Sable, where fine-grained sediments have been carried into the northwestern part of Florida Bay. Colorados barrier reef at the western end of Cuba diverges from the shoreline to the west, suggesting a clockwise drift.24 Split ends indicate a clockwise drift (to the west) on the north and northwest coasts of Yucatán to the Laguna de Terminos (Sector 1.11, fig. 14).

The unmodified and slightly modified drowned karst and mangrove ridge shorelines do not show appreciable longshore drift, judging by their irregular shorelines and dominantly transverse tidal channels. Convergence areas exist at the cuspate delta of the Apalachicola and the cuspate foreland of Cape Sable, Florida. The cuspate foreland of Cabo Rojo (fig. 12; Sector 3, fig. 14), is asymmetrical, showing that the counterclockwise drift persists across it despite convergence.

Bates (1953) shows from photographs and oceanographic data that there is a Coriolis effect 25 turning Mississippi River water westward along shore. This coincides in direction with a weak, westward-moving wind-powered drift. Together there is formed a dominant counterclockwise drift (to the right). Distribution of sediments along the delta front agrees well with this drift. Air photographs show that the Coriolis drift occurs also at the mouths of the other rivers of the northwestern Gulf coast. It is not operative, however, in equatorial and near-equatorial waters such as the southern Gulf of Mexico.

REFERENCES

BALL, M. We, and DOUGLAS, BALL.

1953. Oil prospects of Israel. Bull. Am. Assn. Petrol. Geols. 37 (1): 1–113.

BARTON, D. C.

1930. Deltaic coastal plain of southeastern Texas. Bull. Geol. Soc. Am. 41 (3): 359–382.

24 Observations of drift in this direction have been recorded. The drift seems to be powered by a clockwise eddy developing off the right flank of the Yucatán current (N. to NNE. through Yucatán Channel; Leipper p. 121, fig. 34).

25 Relative right hand turning of flows because of the rotating coordinates of the revolving earth. The turn is to the left in the southern hemisphere.