GULF OF MEXICO 



53 



etation, slowly advances seaward by the consoli- 

 dation of peaty growth and by trapping fine- 

 grained inorganic sediment (Vaughan 1909). 

 Shells are added by the accumulation of small 

 species of Mollusca on the roots (Davis 1940). It 

 may be assumed that the maximum of accumula- 

 tion of marl, clay, and silt takes place always 

 somewhat forward, that is, gulfward, in the slowly 

 advancing swamp. Storm waters and tidal os- 

 cillations combine to permit the up-building of 

 the accumulating swamp materials by those inor- 

 ganic sediments which, in turn, may promote, at 

 and above high tide levels, a denser undergrowth 

 of the less aquatic plants. 



As the zone of maximum arrest and accumula- 

 tion of inorganic sediment advances Gulfward, 

 lack of accretion or a decrease in rate of accretion 

 in the zone nearer the original mainland permits 

 normal compaction of peat and marl to show in 

 an invasion of groundwater and brackish marine 

 waters. The swamp is abundantly penetrated all 

 along the western peninsula coast by transverse 

 tidal scour channels, permitting Gulf waters to 

 enter the rear zone. 



If the foregoing processes and results depict the 

 true history of the formation of the mangrove 

 barrier ridge and lagoon on the western coast of 

 peninsular Florida during the stillstand for the 

 3,000 to 5,000 years of Fisk's (1944) determin- 

 ations, then the considerable width of 5 to 10 

 nautical miles of the mangrove belt is a product 

 of the extended period of time during which ap- 

 proximate stillstand has persisted. If minor 

 oscillations have occurred during this period, then 

 some of the alternations of peat and of peat marl 

 and in the types of peat reported by Davis may 

 have been related to changes of sea level. An 

 end condition of seaward advance may be found 

 where the bottom slopes too steeply, or the growth 

 has finally reached a zone where the processes 

 outlined no longer produce bottom offshore that 

 I is sufficiently shoal to support mangroves. Under 

 I this hypothesis, we may understand why the man- 

 i grove growth on the southern part of the Gulf 

 i shore of Florida is exceptionally wide, as the 

 ! combination of conditions required for the full 

 I formation of a mangrove ridge and lagoon are 

 exceptional. Under this hypothesis the rate of 

 Gulfward advance is the ratio between the width 

 of the ridge, 30,000 to 50,000 feet, and the dura- 

 tion of stillstand, 3,000 to 5,000 years. Using the 



figure 5,000 from Fisk's estimate, we find that the 

 net outward advance of the mangrove forest has 

 been between 6 and 10 feet per year. This is a 

 measurable quantity- 

 It has been said that Davis (1940) finds the 

 present swamp forest to be resting on and rooted 

 through a surficial zone of marl a few feet thick 

 without appreciable peat deposition in it. Hence, 

 his interpretation that the accumulation of the 

 average of 7.5 feet of buried peat and marl took 

 place mainly during a rise of sea level seems not to 

 conflict with the present writer's hypothesis for 

 forward growth during stillstand. Accumulating 

 datings by the deterioration of radiocarbon may 

 permit a rate of upward growth, less compaction, 

 to be made, Davis having found no means of 

 doing so. 



The mangrove barrier ridge and coastal lagoon 

 are similar, in accomplishing appreciable shoreline 

 prograding, to the other barriers known, the barrier 

 island of sand, the barrier coraline reef and the 

 rare barrier oyster reef, noted in the Gulf of 

 Mexico where a large reef of Crassosfrea virginica 

 forms a bay barrier 25 miles long across the mouth 

 of Atchafalaya Bay. 



EMERGENT AND SUBMERGENT 

 SHORELINES OF THE GULF 



USE OF TERMS 



Johnson (1919) laid much stress, in his shoreline 

 studies, on the determination of whether the 

 features of a shoreline were dominantly those of 

 the relative submergence of a land surface or the 

 relative emergence of a sea bottom. His interest 

 centered in the immediate history of sea level and 

 its effect on shorelines. Others have found it 

 impracticable to discriminate entirely on many 

 coasts between the exact form of the present 

 shoreline and the topography of the coastal zone 

 which has determined major features of both 

 coast and shoreline (Shepard 1937a, Price 1939). 

 This distinction involves the difficulty in deter- 

 mining for each sector whether the several sub- 

 mergences or emergences of the coasts during 

 the Pleistocene have produced its dominant 

 features. 



A major shortcoming of the Johnson classifica- 

 tion or the way in which it came to be applied was 

 its use of the common and widely developed 

 barrier island as either a major criterion or a 



