58 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



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 post- 

 Glacial age, despite continued statements by 

 many geologists that they "seem to be Recent." 

 R. W. Fairbridge and E. D. Gill of Australia " 

 think th at 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 '* finds no post-Pleistocene deposits above 

 a maturely developed soil, supposedly of post- 

 Pleistocene 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 

 down warped. 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 '° 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.— Shep&rd (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 re- 

 fer 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 con- 

 cept, we find that mature marine coasts have in 



" Letters of 1952. 

 " Letters of 1952. 



'• 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 sedi- 

 mentation, or a combination of processes. Hence, 

 the end I'esult of marine action on most types of 

 coasts is smoothing, though not always straighten- 

 ing, as smooth coasts may be curved. 



Processes. — Simplification of a coast may con- 

 sist of the reduction of projections by erosion, and 

 the deposition of beach and other deposits in re- 

 entrants. 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 bot- 

 tom contour, crossing the sites of entrenched 

 valleys on postentrenchment fill, while the main- 

 land 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 bar- 

 riers 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) seem- 

 ingly some smoothing of the front of parts of the 

 mangrove ridge (Sector 4.1) facing the Gulf, in 

 contrast with a possibly irregular original con- 

 figuration such as that of the Ten Thousand Is- 

 lands 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, Yucatdn peninsula) so that only small cuspate 

 points remain, (6) reduction by erosion of project- 

 ing 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). 



Signijicance . — The several degrees of shoreline 

 simplification evident in the preceding list, sug- 

 gest 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 



