[chainsaw]

what the heck could that mean?

It means that sea shells were used for landfill.
The beaches in Florida use to be nothing but sea shells.

[CanaGuy]

chainsaw: Geologists believe that the Cenozoic sea levels in Florida have fluctuated from several hundred feet or more above the present level to more than several hundred feet below present sea level. 60% of Florida is currently less than 75 feet in elevation.

http://purl.fcla.edu/fcla/dl/UF00000124.pdf

http://commons.wikimedia.org/w/thumb.php?f=Florida topographic map-en.svg&width=2000px