62 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



shore structure along much of the western shores 

 of the limestone peninsulas of Florida and Yuca- 

 tan, and the low gradients prevailing there off- 

 shore, led the writer to investigate these regions 

 for examples of the beachless and breakerless 

 coasts. More information is available for Florida 

 than for Yucatan. 



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 Yucatan, where 

 the bottom gradient is low and the shoreline and 

 bottom essentially unmodified by marine forces. 



Comparison of the theoretical "breakerless bot- 

 tom" curve of Keulegan and Krumbein (1949), 

 described as profile 7 (p. 60 and fig. 15) with the 

 actual rolHng 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 ero- 

 sion 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." 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 south- 

 westward, that is, counterclockwise,^ from Apa- 

 lachicola 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), 



w 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). 



^ 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 (27G°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. ^^ Split ends indicate a clockwise 

 drift (to the west) on the north and northwest 

 coasts of Yucatan 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 ocean- 

 ographic data that there is a Coriolis effect ^' 

 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 oc- 

 curs 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. W., 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. 



" 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 Yucatin current (N. to NNE. through Yucatan Channel; Leipper 

 p. 121, fig. 34). 



>* 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. 



