p 



GULF OF MEXICO 



55 



The outlines of the lakes and rimmed basins of 

 Florida Bay today show the characteristic coa- 

 lescing of small basins with each other and with 

 large ones, the intervening rims being removed. 

 The lacustrine plain, the so-called bay, with its 

 network of marl ridges prevents the development 

 of appreciable tidal flow and scour in and be- 

 tween basins except along the border of the bay 

 at the south. Here tidal channels have been 

 scoured through breaks in the line of the Florida 

 Keys, locally deepening the rimmed basins. 



Statistical study of the relation between width 

 and depth in the rimmed basins shows a rough 

 approximation to the progressive deepening with 

 increasing size characteristic of the bays of the 

 northwestern Gulf (Price 1947). In the Bay of 

 Florida this relation is modified on the southeast 

 by the limiting depth of the hard Miami oolite 

 and at the extreme west by an excess of sandy or 

 marly deposition in the relatively large basins 

 that there border the Gulf. Some of the western 

 basins are completely filled with sediment. 



Partly submerged eolian sand plain of Rio 

 Grande delta region. — Stretching inland across the 

 Pleistocene plains of this delta in Tamaulipas and 

 Texas to their inner erosional scarp is a plain of 

 eolian sand, or erg, with scattered dune fields. 

 All, except small blowout fans of bare sand (about 

 1 by 3 miles in size) and their fields of bare dunes, 

 is stabilized by grassy vegetation, thorny brush 

 and live oaks. The coastal lagoons now form 

 traps for eolian sand blowing inland from the 

 beaches of the barrier islands. Only in droughts is 

 some of this sand able to cross to the maiidand 

 over narrow flats that locally close the coastal 

 lagoon. This immense sand plain must have come 

 on shore before the barrier island was formed. 

 The simplest explanation follows that of Daly 

 (1934, pp. 197-201) that large amounts of sand 

 probably blew on some shores when the sea level 

 was low during one or more of the glacial periods. 

 Other possible explanations are that the sand 

 has come from the reworking of successive barrier 

 island sands and other beach deposits or from 

 sandy sediments in the walls and on the floors of 

 entrenched valleys. 



EMERGENT SHORELINE FEATURES 



Pocket harbors (emergent rimmed basins) of 

 Northwestern Cuba. — Several writers on Cuba (as 

 Hayes, Vaughan, and Spencer 1901) have referred 



to the purse-shaped or pocket harbors of Cuba. 

 Tliose of the sector from Havana to Bahia Honda 

 (fig. 12) on tiie northwest coast are of an unusual, 

 petal-shaped type. They lie in a plain from 3 

 to 5 feet above sea being upwards of 6 miles 

 long. A small stream usually enters one or 

 more of the several marginal indentations of the 

 small rounded-to-oval basin, not always in the 

 axial position. Other similar marginal indenta- 

 tions have either no appreciable inflowing drainage 

 or receive very slight drainage. Yet well-formed 

 submerged channels converge from all these in- 

 dentations toward a central channel of tidal type. 

 This channel may be as deep as 8 fathoms. Such 

 harbors do not seem to the writer to be explicable 

 as normal embayments of drowned stream courses 

 or of stream confluences, as some have suggested. 



If the coast of Cuba west of Bahia Honda (3.1, 

 fig. 14) is examined on the navigation charts, 

 basins similar to the pocket harbors and the 

 rimmed basins of the Bay of Florida, lying in 

 swampy terrain, will be seen here and there be- 

 hind the Colorados Barrier Reef, mostly clustering 

 toward the mainland shore. These basins have 

 axial or radial tidal channels draining to the Gulf 

 below sea through passes or breaches in the reef. 

 These small, rounded and rimmed basins of the 

 Colorados lagoon seem to be features of a present 

 mangrove-lined shoreline like those along the 

 north shore of Florida Bay. The writer inter- 

 prets the pocket harbors of the Havana type, 

 surrounded by a slightly elevated plain (Palmer 

 1945), as similar mangrove lakes scoured out at 

 sea level in the midst of a saline swamp and then 

 slightly elevated on the unstable young orogenic 

 coast (Sector 3) of Cuba. 



Barrier Island not an indicator of long-period 

 sea-level change. — Johnson (1919, 1925) thought 

 that his offshore bar, called barrier island by the 

 writer (Price 1951a, Shepard 1952), was a feature 

 predominantly of an emergent shoreline. He be- 

 lieved that the structure was formed by a semi- 

 permanent sea level change, a slight worldwide 

 lowering of sea level or an upwarping of the crust, 

 along an offshore bar formed originally as a sub- 

 marine feature. Fenneman (1938, p. 4), following 

 some early writers, believed, however, that a 

 barrier island was formed merely as an equilibrium 

 structure produced on a shallow shelving coast by 

 the balance between wave attack and bottom re- 

 sistance regardless of any history of sea level 



