FLORIDA GEOLOGICAL SURVEY


clayey where they thin over the crest of the anticline (fig. 8, A-A').
The deposits appear to increase in coarseness from the interior of
the Green Swamp area eastward to the Lake Wales Ridge. Much
of these deposits occur as cavity fill in the underlying limestones
especially in the ridge areas.
 The undifferentiated plastic deposits form the nonartesian
aquifer in the area. Generally, the deposits in the western part
of the Green Swamp area are thin or absent, low in permeability
and porosity; and therefore, they are of minor significance as an
aquifer.

 UNDIFFERENTIATED CLAY

 Undifferentiated clays of Miocene age underlie most of the area,
 except in the western part, and contain varying amounts of quartz,
 phosphatic sand, and interbedded limestone. The following
 general lithologic sequence (from younger to older) is indicated:
 (1) light gray-tan-blue-green, montmorillonitic clay with varying
 amounts of quartz, phosphatic sand, and interbedded limestone;
 (2) dark gray-green-blue phosphatic, silty clay with varying
 amounts of quartz pebbles, silt and mica flakes.
 The light-colored clay with interbedded limestone is part of the
 Hawthorn Formation of early and middle Miocene age. Generally,
 its occurrence is limited to the southeastern part of the Green
 Swamp area. It thickens eastward and southward and forms a
 secondary artesian aquifer which is a significant source of artesian
 water outside of the Green Swamp area. The dark, silty clay is
 probably equivalent to the Tampa Formation of early Miocene age
 (Carr, 1959). Generally, its occurrence is limited to the eastern
 part of the Green Swamp area where it forms an aquiclude.

 SUWANNEE LIMESTONE

 The Suwannee Limestone (Cooke and Mansfield, 1936) of
 Oligocene age is a white dense fossiliferous limestone. It is present
 in the southern and western parts of the Green Swamp area and
 crops out along the Withlacoochee River near Polk-Sumter-Pasco
 County line. The formation thickens southward in Polk and
 Hillsborough counties and westward in Pasco County. Many of the
 springs along the upper Hillsborough River flow from exposures
 of Suwannee Limestone. The Suwannee Limestone overlies the
 Crystal River Formation and it is overlain by either undifferen-
 tiated clay or undifferentiated plastic deposits.