State of Florida
Department of Natural Resources
Tom Gardner, Executive Director
Division of Resource Management
Jeremy Craft, Director
Florida Geological Survey
Walt Schmidt, State Geologist
Open File Report 27
The Geology of Hamilton County, Florida
Frank R. Rupert
Florida Geological Survey
Hamilton County is situated in the Northern Zone of
White(1970). In Hamilton County, the Northern Zone is divid-
ed into two major geomorphic provinces, the Northern High-
lands and the topographically lower Gulf Coastal Lowlands.
These adjacent provinces are separated by a persistent topo-
graphic break called the Cody Scarp (see Figure 1). The Cody
Scarp is believed to be a relict marine escarpment, originally
formed by high-standing Pleistocene seas. The surface eleva-
tion of the toe of the Cody Scarp varies between about 75 and
100 feet above MSL (mean sea level) in Hamilton County. In
some areas, dissolution of the underlying limestones has low-
ered the original escarpment elevation.
The Northern Highlands province spans north Florida from
the eastern edge of Bradford County, Florida, westward into
Alabama. The topographically high clayey sand hills compris-
ing this zone are thought to be dissected remnants of a much
more extensive highland plain which covered much of the Gulf
Coastal Plain (White, 1970). Within Hamilton County the
Northern Highlands occupy most of the eastern and northern
portions of the county. The topography is comprised of gently
rolling hills and large, flat swampy areas. Near surface sedi-
ments are predominantly sands and clayey sands. Dissolution
of the underlying limestones has modified the terrain in many
areas of the county, producing numerous depressions and low
swampy areas called "bays". Many of these large bays remain
wet during the rainy season. Land surface elevations vary from
approximately 70 feet MSL at the southern edge of the county
to about 160 feet MSL at the northern and northwestern edges
of the county.
The Gulf Coastal Lowlands comprise an area of southwest-
ern Hamilton County, south of the Cody Scarp (see Figure 1).
These topographic lowlands extend from the toe of the Cody
Scarp southward to the Gulf of Mexico. Land surface eleva-
tions range between 50 and 100 feet MSL. Much of the gener-
ally flat, sandy terrain of the lowlands has been modified by
Hamilton County is bounded on the west by the Withlacoo-
chee River, and on the southern and eastern edges by the
Suwannee River. The Alapaha River bisects western Hamilton
County, flowing southwestward, and forms a tributary to the
Suwannee River. The valleys of these rivers comprise a
geomorphic subzone of the Gulf Coastal Lowlands named the
River Valley Lowlands (Ceryak et al. 1983). These valleys are
typically incised, and are topographically lower than the sur-
rounding terrain. Elevations of the valley floors range from 50
feet MSL in the Suwannee River Valley to nearly 80 feet MSL
along the northern stretches of the Alapaha and Withlacoochee
River valleys. In some portions of these valleys, older geologic
formations crop out along the stream banks. The valley floor
sediments are comprised principally of Holocene age alluvial
clay and sand.
Large surface water features are rare in Hamilton County.
Lake Octahatchee, situated in the extreme northwestern corner
of Hamilton County, is the county's largest lake. A series of
small lakes, many formed in sinkhole collapse depressions, are
scattered throughout the county.
Hamilton County has several major springs, most situated
adjacent to the large rivers. Morgans Spring, Adams Spring,
Alapaha Rise and Holton Spring are located in the Gulf Coast-
al Lowlands and generally display variable flow rates, depend-
ent upon local precipitation. Bluff Cemetery Spring is situated
on the Alapaha River in northern Hamilton County, and
Louisa Spring, Wesson Iron Spring, and White Springs, once
the site of a sulfur spring health resort, are all located near the
Suwannee River in southeastern Hamilton County. Most of
these springs flow from conduits in the underlying Suwannee
Limestone and Ocala Group sediments.
-- -- -- -- -
EXPLANATION N' ,
CROSS SECTION LOCATION
D NORTHERN HIGHLANDS
I] GULF COASTAL LOWLANDS
Figure 1: Geomorphic and cross section location map.
St. Marks Formation
Hamilton County is underlain by hundreds of feet of marine
sands, clays, limestones and dolomites (Ceryak et al., 1983).
The oldest rock penetrated by water wells is limestone of the
Middle Eocene age (42 to 49 million years before present)
Avon Park Formation. Undifferentiated surficial sands and
clays of Pleistocene to Holocene age (1.8 million years old and
younger) are the youngest sediments present. The Avon Park
Formation and the younger overlying limestone units are
important freshwater aquifers, and the discussion of the geolo-
gy of Hamilton County will be confined to these Eocene age
and younger sediments. Figure 1 shows the geologic cross
section locations, and figures 2 and 3 are cross sections illus-
trating the stratigraphy of Hamilton County.
Avon Park Formation
The Avon Park Formation (Miller, 1986) is typically a
dense, tan to dark brown, porous dolomite, frequently inter-
bedded with tan, gray, or cream-colored limestones and dolom-
itic limestones of varying hardness (Ceryak et al., 1983).
Foraminifera are the dominant fossils present, although dolom-
itization has destroyed or altered many of the contained fossils.
The Avon Park Formation is a component of the Floridan
aquifer system and underlies Hamilton County at depths rang-
ing from 400 to 450 feet below land surface (Florida Geological
Survey in-house well data).
Upper Eocene (38-42 million years old) marine limestones
of the Ocala Group (Puri, 1957) unconformably overlie the
Avon Park Formation under all of Hamilton County (Ceryak et
al., 1983). The Ocala Group is comprised of three formations;
in ascending order, these are the Inglis Formation, the Willis-
ton Formation, and the Crystal River Formation. These forma-
tions are differentiated on the basis of lithology and fossil
content. Typically, the lithology of the Ocala Group grades
upward from alternating soft and hard, white to tan fossilifer-
ous limestone and dolomitic limestone of the Inglis and lower
Williston Formations into white to pale orange, abundantly
fossiliferous limestones of the upper Williston and Crystal
River Formations. Foraminifera, mollusks, bryozoans, and
echinoids are the dominant fossil types occurring in the Ocala
Group sediments. Thickness of the Ocala Group sediments
under Hamilton County averages about 200 to 300 feet. Depth
to the top of the Ocala Group is variable, ranging from about
100 feet to nearly 270 feet. The porous and cavernous nature
of the Ocala Group limestones make them important fresh-
water-bearing units of the Floridan aquifer system. Many
drinking water wells in Hamilton County withdraw water from
the Crystal River and Williston Formations.
The Suwannee Limestone (Cooke and Mansield,1936) is a
Lower Oligocene (33 to 38 million years old) pale orange to
white calcarenitic marine limestone overlying the Ocala Group
in most of Hamilton County. It typically contains abundant
fossils, including foraminifera, mollusks, and echinoids, and
may contain interbedded chert and dolomite beds. Depth to
the Suwannee Limestone ranges from about 20 feet to nearly
200 feet below land surface in Hamilton County. Thickness
varies from about 10 feet to 150 feet. It generally thins to the
east and southeast, ultimately pinching out in Columbia and
Baker Counties to the east. Except where the overlying St.
Marks Formation is present, the Suwannee Limestone is the
uppermost unit of the Floridan aquifer system in Hamilton
Scattered erosional remnants of the Lower Miocene (20 to
25 million years old) St. Marks Formation overlie the Suwan-
nee Limestone in western Hamilton County. The St. Marks is
a pale orange to white, sandy, silty, calcilutitic marine lime-
stone, occasionally containing mollusks and foraminifera. It is
known from only a few wells in and west of Jasper and along
portions of the Alapaha River in Hamilton County. It generally
occurs in discontinuous units less than 40 feet thick. Where
present, the St. Marks Formation is the uppermost unit of the
Floridan aquifer system.
The Hawthorn Group (Scott, 1988) is comprised of a series
of Lower and Middle Miocene (10 to 25 million years old)
generally phosphatic clays, sands, limestones and dolomites.
These sediments unconformably overlie the St. Marks Forma-
tion, where present, the Suwannee Limestone, or the Ocala
Group. Scott (1988) recognized four formations of the Haw-
thorn Group in Hamilton County. In ascending order, these
are the Penny Farms Formation, the Marks Head Formation,
the Coosawhatchee Formation, and the Statenville Formation.
The predominant lithologies in the different formations are
light gray to olive gray to yellowish gray interbedded sands,
clays, and dolostones, containing varying quantities of phos-
phate grains. For the purposes of the present report, these
formations are collectively included in the Hawthorn Group
sediments, as shown on Figures 2 and 3. Hawthorn Group
sediments are exposed along the banks of the larger streams
and rivers bounding the county, particularly the Suwannee
River. Throughout the county, the top of the Hawthorn Group
is usually less than 50 feet below land surface. Thickness of the
Hawthorn Group varies locally between approximately 30 feet
south of the Cody Scarp and 150 feet in the northern and
eastern portions of the county.
The clays of the Hawthorn Group comprise an intermediate
confining unit which overlies the Floridan aquifer system. At
the same time, the carbonate units within the Hawthorn Group
may serve locally as an intermediate aquifer system.
Pleistocene and Holocene Series
Overlying the Hawthorn Group sediments are a series of
Pleistocene and Holocene age (1.8 million years old and young-
er) undifferentiated quartz sands and clayey quartz sands.
Most of these deposits represent relict Pleistocene marine
sands and Holocene aeolian and alluvial deposits. These undif-
ferentiated sands are generally less than 50 feet thick in Hamil-
Ground water is water that fills the pore spaces in subsur-
face rocks and sediments. This water is derived principally
from precipitation within Hamilton County and adjoining
counties. The bulk of Hamilton County's consumptive water is
withdrawn from ground water aquifers. Three main aquifers
are present under Hamilton County; in order of increasing
depth, these are the surficial aquifer system, the intermediate
aquifer system, and the Floridan aquifer system.
The surficial aquifer system
The surficial aquifer system is the uppermost freshwater
aquifer in Hamilton County. This non-artesian aquifer is
contained within the undifferentiated Pleistocene and Holo-
cene sands in eastern and northwestern Hamilton County. The
areal extent of the surficial aquifer system approximately corre-
sponds to the occurrence of relict Okeefenokee Terrace sands,
generally situated between 100 and 170 feet MSL (Ceryak et al.,
1983). The surficial aquifer system is unconfined, and its upper
surface is the water table. In general, the water table elevation
fluctuates with precipitation rate and conforms to the topogra-
phy of the land surface. Recharge to this aquifer is largely
through rainfall percolating through the loose surficial sedi-
ments, and to a lesser extent, by upward seepage from the
underlying intermediate aquifer system. Water naturally dis-
charges from the aquifer by evaporation, transpiration, spring
flow, lateral discharge along the Cody Scarp and into topo-
graphically low areas, and by downward seepage into the
intermediate aquifer system (Ceryak et al., 1983).
Figure 2: Cross section A A'
0 1 2 3 4 5
S.-200 I '
0 2 4 6 8
-75- --250 KILOMETERS
VERTICAL EXAGGERATION 160 TIMES TRUE SCALE
WELL NUMBERS ARE FLORIDA GEOLOGICAL SURVEY ACCESSION NUMBERS.
SAND AND CLAY 7
ON PARK FORMATION
Figure 3: Cross section B B'
VERTICAL EXAGGERATION 160 TIMES TRUE SCALE
WELL NUMBERS ARE FLORIDA GEOLOGICAL SURVEY ACCESSION NUMBERS.
The intermediate aquifer system
The intermediate aquifer system underlies the surficial
aquifer system in Hamilton County, and is largely contained
within the sand and carbonate units of the Miocene age
Hawthorn Group sediments. Permeable beds within the in-
termediate aquifer system vary considerably in thickness over
the areal extent of the aquifer. When clayey confining beds are
absent in the upper Hawthorn Group sediments, the interme-
diate aquifer system may be recharged by downward seepage
from the surficial aquifer system. Likewise, if clay beds or the
typical dolomitized limestone beds in the lower Hawthorn
Group sediment section are absent, the intermediate aquifer
system may be in hydrologic continuity with the underlying
Floridan aquifer system. Lateral discharge from the intermedi-
ate aquifer system may occur along the Cody Scarp or in areas
where streams have cut down into the sediments comprising
In general, the thickness of the intermediate aquifer system
corresponds to the thickness of the Hawthorn Group sedi-
ments, ranging from approximately 30 to 150 feet. The in-
termediate aquifer system is not used extensively for consump-
tive water in Hamilton County. Most wells draw from the
deeper Suwannee Limestone and Ocala Group sediments of
the Floridan aquifer system (Ceryak et al., 1983).
Floridan aquifer system
The Floridan aquifer system is comprised of hundreds of
feet of Eocene through Miocene age marine limestones, includ-
ing the Avon Park Formation the Ocala Group, the Suwannee
Limestone, and where present, the St. Marks Formation. It is
the principal source of drinking water in Hamilton County.
The Floridan aquifer system occurs as an artesian aquifer in
the portion of Hamilton County corresponding to the Northern
Highlands geomorphic zone. In the Gulf Coastal Lowlands,
where the Hawthorn Group sediments are absent or less than
about 50 feet thick, the Floridan is generally under non-arte-
sian conditions (Ceryak et al., 1983). Stewart(1980) showed the
Gulf Coastal Lowlands to be a recharge area for the Floridan
aquifer system. Depth to the Floridan aquifer system in
Hamilton County ranges from about 20 feet in southern Hamil-
ton County to about 250 feet below land surface in the north-
ern part of the county. Total thickness of the potable water
within the Floridan aquifer system is estimated to be about
1000 feet (Ceryak et al., 1983).
The Floridan aquifer system is recharged in part from
seepage from the overlying intermediate aquifer system, locally
through sinks and limestone exposures along rivers, and largely
from hydrologic continuity with the principal aquifer system to
the north in Georgia. Water leaves the Floridan aquifer system
through natural movement down-gradient and subsequent
discharge from springs and through seeps along the River
The principal mineral commodities occurring in Hamilton
County are sand, clay, limestone, and phosphate. The follow-
ing discussion summarizes the current mining status and poten-
tial for each commodity in the county.
A number of shallow private pits in Hamilton County are
worked for local fill sand and aggregate. Pleistocene sand
deposits occur as thin veneers over Hawthorn Group sediments
in much of the region. Since there is insufficient local demand
for sand products, the potential for commercial mining is low at
Localized deposits of clay and sandy clay are associated with
the Pleistocene and Holocene marine terrace deposits and
alluvium and in the Hawthorn Group sediments. Most of these
deposits are contained in and interbedded with other sedi-
ments, and as a result are relatively impure. Reserve estimates
of the clay deposits in Hamilton County have not been made,
and future exploitation will depend largely on more extensive
exploration and testing, as well as market demand.
Limestone deposits occur at or near the surface in south-
western Hamilton County in the Gulf Coastal Lowlands and
adjacent to the Suwannee River. Most near surface limestone
is associated with the Suwannee Limestone and Ocala Group.
These deposits are not mined, and are largely untested as to
purity and commercial potential. Future exploitation will be
largely dependent upon local demand for limestone products
such as roadbase.
Phosphate is currently mined in southeastern Hamilton
County. Occidental Chemical Corporation operates in two
mining areas in Hamilton County, the Swift Creek Mine north-
west of White Springs and the Suwannee River Mine, northeast
of White Springs.
The phosphate ore is contained in the Miocene age Haw-
thorn Group sediments, and is extracted with the sand and
clayey sand matrix by large draglines. Beneficiation of the
phosphate is accomplished through a flotation process, which
separates the quartz matrix sands and clays from the phosphate
ore. The uses for phosphate are primarily as agricultural ferti-
Potential for this resource remains high in southeastern
Hamilton County. Continued mining of phosphate will be
dependent on market demand worldwide.
Ceryak, R, Knapp, M., and Burnson, T., 1983, The geology and
water resources of the upper Suwannee River basin,
Florida:Florida Bureau of Geology Report of Investigation
no. 97, 166p.
Cooke, C., and Mansfield, W., 1936, Suwannee Limestone of
Florida (abstract): Geological Society of America Proceed
ings, 1935, p. 71-72.
Miller, J., 1986, Hvdrogeologic framework of the Floridan
aquifer system in Florida and in parts of Georgia. Alabama,
and South Carolina: U.S. Geological Survey Professional
Paper 1403-B, p. 25 27.
Puri, H., 1957, Stratigraphy and zonation of the Ocala
Grou:.Florida Geological Survey Bulletin no. 38, 248 p.
Scott, T., 1988, The lithostratigraphv of the Hawthorn Group
(Miocene) of Florida: Florida Geological Survey Bulletin
no. 59, 148 p.
Stewart, J., 1980, Areas of natural recharge to the Floridan
aquifer in Florida: Florida Bureau of Geology Map Series
White, W., 1970, The geomorphology of the Florida
veninsula:Florida Bureau of Geology Bulletin no. 51, 164 p.