114 
FLORIDA STATE GEOLOGICAL SURVEY. 
the Miocene which here has a thickness of 114 feet may rest directly 
upon the Vicksburg group. 
At Rock Springs, the original Miocene exposure, the rock con¬ 
sists of a light gray to white marly limestone containing a number of 
characteristic fossils. At this locality, Smith 1 collected Pecten madi- 
sonius, Venus alveata, Venericardia granulata, Carditamera arata, 
Mytiloconcha incurva, Cardium sublineatumf and Oliva literataf 
According to’ Dali, Miocene fossils were also obtained in a boring 
at Lake Worth on the east coast. 
CHOCTAWHATCHEE MARL. 
The Choctawhatchee marl includes the “Ecphora bed ” 2 and “alum¬ 
inous clay ” 3 of Dali. It comprises a grayish sandy shell marl and 
gray plastic sandy clay of Miocene age. It lies stratigraphically be¬ 
tween the Oligocene and Pliocene beds, and contains characteristic 
species of Miocene fossils. 
Stratigraphic Position:—The Choctawhatchee marl rests uncon- 
formably upon the Alum Bluff formation at Alum Bluff , 4 where the 
contact shows a wavy surface marked by shallow channels due to 
erosion. At this locality there is also a sharp change from the coarse 
light gray sands of the Alum Bluff formation, which contain few fos¬ 
sils, to the bluish-gray shell marl of the Choctawhatchee, with its 
abundant fauna. Several years ago Dr. Vaughan noted similar evi¬ 
dence of an unconformity between the Oligocene and Miocene at 
Jackson’s Bluff on the Ocklocknee River. This fact is well shown 
by his section 5 at that locality: 
Section at Jackson’s Bluff, Ocklocknee River. Thickness. 
Feet. Inches . 
10. Sandy soil, thickness not determined. 
9. Unexposed in slope to top of bluff face, about. 16 
8. Yellow sands unconformably overlying the Choctawhatchee 
marl . 5 
7. Yellow sandy fossiliferous Choctawhatchee marl, in places 
where unweathered of a greenish color . 6 6 
(There are about 3 feet additional under the yellow sands. 
This bed is extremely fossiliferous and from it I obtained 
the species listed by Dr. Dali in his discussion of the 
Florida Miocene.) 
1 Smith, E. A., Am. Jour. Sci., 3rd ser., 1881, vol. xxi, p. 302. 
"Dali, Wm. H., Neocene of North America U. S. Geol. Survey, Bull. 84,. 
1902, p. 124. 
3 Dali, Wm. H., and Stanley-Brown, Joseph, Cenezoic Geology Along the 
Apalachicola River, Geol. Soc. Am., Bull. V, 1894, pp. 168-169. 
4 Vaughan, T. Wayland. Unpublished notes. 
5 Vaughan, T. Wayland. Unpublished notes. 
