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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



change. The writer's study of bottom profiles in 

 the Gulf (fig. 15) indicates that barrier islands are 

 (1) associated with well-developed equilibrium 

 profiles, (2) on a shallow coast where the bottom 

 is now at least 15 to 45 feet deep within one to 

 two miles of shore and (3) thereafter slopes out- 

 ward between about 2.0 and 5.0 feet per mile, (4) 

 where sand, gravel or cobble are abundant along 

 shore, and (5) where onshore wave attack is strong. 

 These observations tend to confirm Fenneman's 

 interpretation. Other observations, briefly stated, 

 indicate that the barrier island does not require a 

 worldwide or other semipermanent fall of sea level 

 to bring it above sea, but that the only change in 

 level needed is a local, short-period change be- 

 tween storm levels and normal sea levels taking 

 place during periods of a few hours or days. 



A series of aerial photographs taken at intervals 

 of several years over the period 1934 to 1949 

 (Bates 1953) shows that a bar formed just below 

 the intertidal zone off a new mouth of Brazos 

 River remained submerged until a hurricane 

 had occurred, after which it became a typical 

 emergent barrier island of cuspate outline. A 

 second bar then formed off a breach in this bar- 

 rier, after which other hurricanes occurred before 

 the second bar was, in turn, raised above sea to 

 form a second line of emergent barriers. The 

 inference is strong that, in each case, a pre-existing 

 submarine bar was built higher during a hurricane, 

 so that during the storm it bore the same height 

 relation to the elevated storm sea level as it had 

 formerly borne to the normal level of the Gulf. 

 The bars emerged as barrier islands after the 

 subsidence of the temporarily high sea levels. 



On October 3, 1948, a hurricane passed about 

 100 miles off the coast of southwestern Texas, 

 causing a high sea level or storm tide of some 3 or 

 4 feet for two days or more along the barrier 

 islands. A week later, the writer found that the 

 summit of the beach, the beach ridge, in front of 

 the shore dunes had been built up and remade by 

 the storm and was slightly farther inland than its 

 former alignment. The shift in position was evi- 

 denced by erosion of dune faces. The convexly 

 rounded beach ridge then rested where the front 

 part of the dunes had been. 



The raising of the beach ridge, previously 

 described, to an elevation above its position 

 during normal times was shown by the rapid 

 mass-wasting that had affected it in a single 



week. The beach ridge on this island formerly 

 had, in places, a fairly well developed pavement 

 of shell, but now the pavement had just begun 

 to be formed on the newly made ridge. The 

 pavement was formed from disseminated shell by 

 the washing and blowing away of sand, according 

 to a well-established process. It was evident that 

 this ridge had lost some 6 inches of its height and 

 would lose another foot or a foot-and-a-half before 

 a pavement would be formed to protect it. The 

 former paved beach ridges had evidently lost 

 similar heights. 



Reports and illustrations of hmricane damage 

 to New England beaches (Brown 1939, Howard 

 1939) show that the beach ridges were remade at 

 higher levels, moved inland from their forvier 

 positions and their axes rotated slightly by the « 

 hurricane waters. 



These observations indicate clearly that the 

 summit ridge of a barrier island functions briefly 

 during storm tides as an underwater offshore bar 

 and thereafter emerges as a barrier island. 



Evans (1942) found that waves operating at a 

 steady sea level tend to modify the slopes and 

 positions of underwater bars, but not to build 

 them up above water. 



The great development of active barrier islands 

 on the Gulf coast, dominating the shorelines of 

 the alluvial sectors (1, fig. 14), does not then, in 

 the writer's opinion, tell a story of permanent or 

 semipermanent sea level change, or mark either a 

 submergent or an emergent shoreline condition. 



The question of the source of the supply of 

 material for the barrier, long thought to be a 

 critical factor, is found to be secondary. Thus, 

 barriers occur in the Gulf where longshore sedi- 

 ment drift is prominent (Sectors 1.12, 1.2, fig. 14) 

 the sand derived largely from rivers (Bullard 1942), 

 also where a longshore drift from a land connec- 

 tion (Chandeleur Islands, La.) is absent and 

 where no land-derived sediment is present but 

 onshore waves are strong and the barrier is built 

 of broken shells from the adjacent bottoms, as on 

 the north shore of Yucatan (2.2, fig. 14). 



Emergent shoreline terraces and notches. — The 

 lowest well-established elevated shoreline is that 

 of the Pamlico of the Atlantic coast and Florida, 

 standing at about 25 feet above mean sea level. 

 This shoreline is marked in many regions by a 

 well-cut and well-preserved terrace or by a broad 

 elevated lagoon flat with a barrier island. Less 



