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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



positive indicator (Price 1939, Johnson 1938) of 

 emergence. Later work, here discussed, seems to 

 invahdate this criterion entirely as an indicator 

 of sea-level movement except where it may be 

 found wholly emerged or submerged. 



While not finding the concepts of submergence 

 and emergence as valuable for shoreline classifica- 

 tion as some others, we may well inquire what 

 features plainly indicate such items of shoreline 

 history. 



SUBMERGENT SHORELINE FEATURES 



Pleistocene entrenchment oj stream valleys. — It is 

 well established that the accumulation of lai-ge 

 amounts of ice in the arctic and circumarctic 

 regions several times during the Pleistocene, or 

 Great Ice Age, caused strong lowerings of sea 

 level. The latest well-established major lowering 

 occurred in the late Wisconsin or Wiirm glaciation 

 and amounted to about 450 feet in the Gulf of 

 Mexico (Fisk 1944, 1952) and the Gulf of Paria, 

 south of Trinidad, Venezuela." In some regions 

 the figure is set at between 240 and 350 feet 

 (Flint 1947). Fisk (1944, 1948, 1952) has shown 

 by borings cited in various reports of the Corps 

 of Engineers that the northwestern Gulf coast 

 had a large number of entrenched Pleistocene 

 stream valleys that have now been filled with 

 sediments. Configurations of branch estuaries 

 show that entrenchment was general in the Gulf. 

 Only on the hard shelf off peninsular Florida are 

 such valleys found submerged and fairly well 

 outlined by depressions. These valleys are marked 

 by depths of as much as 10 to 15 feet on the coast 

 charts off northern peninsular Florida. 



Embayed drowned valleys. — Incompletely filled 

 drowned valleys in the Gulf take at least two 

 forms, those in which the branching, dendritic 

 pattern of drowned tributaries is still prominent 

 (Baffin Bay in Texas) and those in which waves 

 and currents have broadened the valley at shallow 

 depth, producing oval, rounded or other equidi- 

 mensional shapes. The writer (Price 1947) has 

 shown that on the northwestern Gulf coast elon- 

 gated drowned valleys tend to become segmented 

 by spits and other obstructions, separately em- 

 bayed by segments and the bay bottoms made 

 flat under a dynamic equilibrium between erosion 

 and deposition. This equilibrium of basin shape 



» Personal communication, T. H. Van Andel. 



is actually the result of the formation of equilib- 

 rium bottom-profiles along most bay radii. 



Embayment of drowned streams is most 

 prominent in the Gulf on the compound Pleisto- 

 cene-to-Recent deltaic coast of Texas and south- 

 western Louisiana. There, the rivers are large 

 and the gradient of the Beaumont is not steep 

 (1 to 3 feet per mile). On the steeper plain of 

 Sector 1.2 (fig. 14), only one large, transverse 

 valley bay (Mobile Bay) occurs. Where the 

 plain is composed dominantly of active deltas or 

 hard rocks or has only relatively minor streams, 

 Sectors 1.11, 2.0 and 3.0, long broadly embayed 

 stream valleys are absent. The writer has further 

 considered local meteorological influences in the 

 shaping of the bays of the northwestern coast 

 (Price 1952). 



The harbor of Matanzas, Cuba, is thought to 

 be a drowned valley cut in a structural depression 

 or in a structurally weak zone. 



Submerged base of mangrove peat. — Davis' (1940) 

 conclusion that the mangrove swamps and peat 

 of Florida formed during a gradual, more or less 

 uninterrupted rise of sea level from about —8 feet 

 relative to mean sea level has been mentioned. 



Drowned lacustrine plain oj Florida Bay. — 

 Anaylsis of this unusual type of marine area needs 

 somewhat extended exposition. The entire water 

 area (Trask 1939, pp. 292, 293) is a honeycomb 

 of shallow, rimmed basins individually upwards 

 of 10 miles wide and 11 feet deep, the bottoms 

 bare with a cover of soft marl or shell sand. The 

 narrow rims are of marl and mangrove peat 

 (Davis 1940). The writer's interpretation is that 

 a rising sea moving up and across a very gently 

 sloping shoal surface carried with it a transgres- 

 sive shoreline zone of mangrove swamp. This 

 coastal swamp belt, the mangrove ridge, moved 

 slowly north and northeastward and is now present 

 along the north shore. This ridge is of irregular 

 shape and the marsh and swamp back of it to the 

 north are now and were probably at all times 

 honeycombed with lakes. Such lakes tend to 

 become enlarged by wind scour if the banks are 

 not encroached on too strongly by marsh and 

 swamp growth. The result is that some large 

 lakes occur among the innumerable small ones. 



It is further postulated that, as the Gulf waters 

 invaded the swamp, more and more deeply, 

 vegetation was slowly killed, and the lakes 

 gradually widened by drowning and wave erosion. 



