.4 Contribution to the Geologic History of the Floridian Plateau. 145 



The west coast of Florida strongly contrasts with the eastern; the 

 absence of prevalent alongshore currents is especially striking. Cape 

 Sable seems current-shaped by alongshore currents. From Cape Sable 

 to Cape Romano the coast line is minutely laciniate ; from Cape Romano 

 to Anclote Keys there is evidence of shore currents, keys with sounds 

 behind them paralleling the coast; from Anclote Keys to the mouth of 

 Ocklockonee River, just east of St. George Island, the coast is minutely 

 irregular. From the eastern end of St. George Island the coast is swept 

 by the countercurrent on the north side of the Gulf. 



The preceding account of the shore-line topography of Florida from 

 the standpoint of currents has an immediate bearing on contemporaneous 

 sedimentation and the building of such sand-spits as occur on the eastern 

 side of Biscayne Bay, and such keys as Virginia Key and Key Biscay ne. 

 Arenaceous material is swept southward by the ocean currents on the 

 outside of this spit and the two mentioned keys; while behind them Snake 

 Creek and Miami River are bringing their burdens of sand from the main- 

 land. The tendency of the process is to fill up Biscayne Bay and not only 

 to connect the spit and arenaceous keys to the mainland, but to join 

 them to the coral reef keys farther south. 



R.\.\'KS BEHIND KEYS. 



Sediment, mostly calcareous, is accumulating in the bays and sounds 

 behind the keys and is gradually filling them, although some is carried 

 to the outside. As has been stated, the tides run across the line of keys, 

 and the tidal currents have usually swept clean the channels between 

 them; but behind them are regions of slack water, and shoals are 

 built. The ridges and shoals behind Key Largo, Long Island, and the 

 Metacumbes are verj' instructive. No hard material at all was found 

 at any locality examined. 



Mangroves are an important factor in this work of construction. 

 When a shoal attains to about a foot of the surface of the water, the 

 floating pods of these plants catch on the soft bottom, take root, grow, 

 and develop root tangles below and tangles of branches above. They 

 catch and retain floating debris and convert the shoal into an island. 

 (See plates 9, 10, u, and 12, fig. a.) 



DELT.\S AT OUTER ENDS OF P.\SSAGES BETWEEN KEYS. 



Attention should also be called to the deltas forming at the seaward 

 end of some of the passages between ke^^s. Professor Shaler was the first 

 to remark on this phenomenon,' stating, "The volume of the material 

 can best be judged by the conditions exhibited by the deposits of limy 

 matter at the eastward end of the channel passing from Biscayne Bay 

 to the sea, known as Caesars Creek." This is not the only locality at which 

 such a delta is forming. The U. S. Coast Survey chart. No. 166, indicates 

 one at the eastern end of Bear Cut, off Cape Florida, and at the mouth 

 of Broad Creek, the last-mentioned bank being known as Old Rhodes 

 Bank (plate 3). There are probably other instances of this phenomenon. 



' Topography of Florida, Mus. Comp. Zool., Bull., vol. .xvi, p. 147, 1890. 

 10 



