When the sea again rose and became warmer this pro- 

 vided the base upon which the present reefs have grown. 

 The reefs are comparatively weak, and probably able 

 to exist only by virtue of the influence of the warm 

 Gulf Stream water. 



Florida possesses bank reefs which, like these of 

 Bermuda, rarely reach the surface. They are built upon 

 a broad shallow platform. This was originally, 50 mil- 

 lion years ago, at the bottom of an extensive shallow 

 sea which covered the southern portion of the South- 

 eastern United States. It was then raised above the 

 surface and worn by wind and rain action. Subsequently 

 the eroded land surface was again submerged and re- 

 mained so during what is known as the Tertiary period. 

 A number of reefs were formed over a considerable 

 part of the area at this stage. 



At a much later time, within the last million years, 

 the land was intermittently exposed and flooded as a 

 result of the withdrawal of water during glacial periods, 

 and its release during interglacial times. The Pleisto- 

 cene reefs which were formed during this series of 

 changes formed rock 1 00 feet thick. At the time of the 

 last glacial epoch, while the sea level was low a platform 

 was cut into the land. As the ice caps finally melted 

 the platform was once more flooded and today it forms 

 the base upon which grow the coral reefs off the Florida 

 Keys. As a result of independent earth movements, 

 however, a portion of the old Pleistocene reefs still 

 remains above water to form the line of keys extending 

 from Miami to Key West and Dry Tortugas. 



The present day reefs have grown a mile or two 

 offshore from the Keys. Between the reefs and the Keys 

 a process of chemical and mechanical sedimentation is 



38 



