At: we also find the following remarks:

360. Scholl, D. W. 1964. Recent sedimentary record in mangrove swamps and rise in sea level over the southwestern coast of Florida: Part 1. Marine Geol. 1:344-366.

"Beneath the shallowly submerged coastal mangrove forest of SW Florida  (ENP),  marine and
  brackish -water sediments of recent age overlie fresh-water peat  about 4000  years ago.  This
  sedimentary succession is thought to be  the  record  of  a  marine  inundation  of  the  western
  margin of the Everglades. The transgression sequence consists of a basal  unit of  autochthon-
 ous fibrous peat, an overlying allochthonous unit of peaty and calcareous  shell debris or shelly  
 quartz-rich sand and silt. The basal peat unit began to form 3,000-4,000 years ago, after cessa-
 tion of calcitic mud formation. The environmental  shift  from  fresh-water  to  brackish-water
 and marine conditions
came in response to a more or less steady rise in sea-level and marine
of former mainland paludal swamps.-Adapted from an  abstract  obtained  from a
 report on the geology of ENP."
(See reference no. 519.)  Emphasis Added
361. Scholl, D. W. 1962. Sedimentary record of the Holocene Transgression across the southwestern margin of the Everglades, southern Florida. Coas. & Shallow Water Res. Conf., p 670-673. (Abs.).
"A summary  of  studies  on  unconsolidated  sediments  in  the  mangrove  estuarine  swamps  of 
 southwest Florida is presented. The Holocene transgression  across the  southwestern margin of
 ENP is discussed. Unconsolidated calcareous-fine to very fine marine quartz sand  has  accumu-
lated on nearly flat bedrock in the Ten Thousand  Islands  are  (ENP).  Peat  found  immediately
 over bedrock and under sand dates 4,000 B.P.  (years  before present).  About 4 ft. of  Holocene
 sediments have accumulated in  the Ten Thousand  Islands  area,  while  only 2 ft.  are  found in
 Whitewater Bay. Sediments in  Whitewater Bay  consist  of  a  surface  layer  of  mangrove  peat
 containing marine and brackish molluscs, underlain by layers of marine, brackish, and fresh,
 with a basal layer of calcareous mud with fresh water gastropods only...
  Emphasis Added