112 
FLORIDA STATE GEOLOGICAL SURVEY. 
ance since its deposition. The exposures are found bordering those 
of the rocks belonging to the Apalachicola group and there is a gentle 
dip seaward. The Miocene beds were probably affected by the gen¬ 
eral arching of the strata of the State, though presumably the initial 
movement took place before their deposition. 
Local Details:—On Black Creek the Jacksonville formation is ex¬ 
posed at intervals for a distance of several miles, but the thickness of 
single exposures seldom exceed six feet. On the north bank of 
Black Creek, about five miles above the Atlantic Coast Line Railroad, 
there is a section showing five feet of the limestone overlain by six to 
eight feet of light gray sand and sandy loam. At this locality, the 
rock consists of massive beds of the typical light gray porous lime¬ 
stone containing a considerable admixture of sand and clay. The 
weathered surface varies in color from pale yellow to buff, and owing 
to the solution and removal of the lime, the weathered rock often ap¬ 
pears to be a calcareous sandstone. Exposures of the limestone occur 
at intervals for a distance of about two miles above this locality; and 
then give place to Pleistocene and alluvial sands. Fossils are abund¬ 
ant in the form of casts and moulds which are often very beautifully 
preserved, but are difficult to procure because of the friable character 
of the matrix. The following fossils 1 were collected at this locality: 
LocalityFive miles west of railroad bridge, Black Creek, Fla. 
Pecten madisonius Say. (Scales on ribs fine, suggesting var. sayanus Dali.) 
Carditamera cf. arata Conrad. 
Cardium, probably robustum Solander. 
Venus rileyi Conrad. 
About two miles above Middleburg, on the north bank of the 
creek, the Jacksonville formation again appears in a bluff about 
twenty-five feet high. 
Section about 2 miles above Middleburg on north bank of Black Creek. 
Light gray sandy loam.. 6-8 inches. 
White Pleistocene sand ..... 2 feet. 
Erosion unconformity .... 
Red sandy clay ... 4 feet. 
Erosion unconformity ... •... ... • 
Dark blue to brown sandy clay plastic when wet, granular when dry 4 feet. 
Unconformity . 
Soft porous light gray, siliceous and arenaceous limestone, very fos- 
s’iliferous with casts chiefly of bivalve shells....'... 10 feet. 
Soft dense light gray limestone similar to above to water.. 4 feet. 
The Miocene age of the limestone is shown by the following list 
of fossils, 2 collected from the upper part of the beds: 
1 Identifications by T. Wayland Vaughan. 
2 Identifications by T. Wayland Vaughan. 
