62 
FLORIDA GEOLOGICAL SURVEY-I4TH ANNUAL REPORT 
TAMPA FORMATION 
The name Tampa formation has been applied to what is probably in 
a large part the equivalent of the Chattahoochee formation on the gulf 
side of southern Florida. This rock is found at the surface in Hernando, 
Hillsborough and northern Manatee Counties. It is well exposed on 
the Hillsborough River, where it is seen to be a relatively hard lime¬ 
stone in places, as on Tampa Bay, containing flint and chalcedony. The 
fossils of the Tampa formation have been described by W. H. Dali in 
Bulletin 90 of the U. S. National Museum, 1915. 
MIOCENE 
alum bluff formation 
The Alum Bluff, the oldest of the Miocene formations, is perhaps the 
most widespread of the formations seen at the surface in Florida. The 
materials of this formation include more or less calcareous sands and 
sandstones, becoming in places sandy limestones. The formation con¬ 
tains likewise sandy and calcareous clays, as well as non-calcareous 
clays of the variety of fuller’s earth. The fossils of the formation are 
in part such as are found under shallow water marine conditions and are 
in part land and fresh-water animals and plants, the sediments having 
evidently accumulated near shore. The thickness of the formation may 
in places reach 200 feet, although it is as a rule much thinner. 
This formation is found extensively developed in peninsular Florida 
and in west Florida,, as indicated on the geologic map. The westward 
extension of this formation contains extensive marine shell beds or 
marls. From the Apalachicola River south through peninsular Florida, 
the formation contains more or less phosphatic material, either in the 
form of small brown, black or light-colored concretions, or as a finely 
disseminated phosphate in a matrix of sand and clay. The phosphatic 
phase of this formation, however, is not to be confused with the much 
richer phosphate deposits which form the phosphates of commercial 
value. The phosphates now being mined, although representing con¬ 
centrates from the Alum Bluff formation, are in their present form of 
much later date, probably Pliocene. 
