94 
FLORIDA STATE GEOLOGICAL SURVEY. 
Toward the north, where the bluff is much lower and the Chesapeake thinned 
out to 5 or 6 feet in thickness, the sands below it are unfossiliferous and modified. 
The upper part is more exclusively sandy, and, lower down, the bed assumes 
the clayey compact greenish color of the oyster marl at Rock bluff, a few miles 
above, but here the green marl contained no fossils. 
The typical Alum Bluff formation consists of coarse, light-green¬ 
ish gray to white argillaceous sands, often showing cross-bedding and 
usually containing more or less interbedded clay and fullers earth. 
One of the most characteristic features of the sands is the presence of 
innumerable flakes of white mica—the “isinglass” of the well diggers. 
The change from the shell marls of the Chipola marl member is by a 
transition zone which contains some of the same species of shells 
which characterize the marls. This zone also contains nodules of cal¬ 
cium carbonate which often enclose fossils. The upper part of the 
sands is usually free from shells, but occasionally contains impressions 
of leaves and fragments of plants. Locally, the Alum Bluff formation 
contains some clay, and near Chattahoochee it consists of greenish, 
sticky marl. 
The fullers earth has the appearance of a dense, hard, fine-grained 
siliceous clay. It is thinly laminated, and commonly light gray to 
greenish in color. Sand partings sometimes occur, but they are com¬ 
paratively rare, the material usually being homogeneous. Beds of 
sand and clay are commonly associated with the fullers earth, the 
sections consisting of interbedded sand and clay. 
ThicknessThe aggregate thickness of the Alum Bluff formation 
is at least 135 feet, but the maximum thickness of a single section is 
scarcely one-half that amount. It is doubtful if the total thickness is 
represented in any single section. The thickness of the sands of the 
Alum Bluff formation at the type locality is about twenty to twenty- 
five feet, but farther north at Rock Bluff, Dali 1 reports a maximum of 
sixty-three feet. The fullers earth commonly occurs in beds of two to 
ten feet in thickness associated with several feet of clay and sand or 
sandstone. In some places two or more beds occur in the same sec¬ 
tion, separated by beds of sand and clay. The maximum observed 
thickness of fullers earth in a single section is about fifteen feet, and 
the aggregate thickness of the associated clays and sand which appear 
to belong to the same horizon is not less than twenty feet. 
Physiographic Expression: — The members of the Alum Bluff 
formation, with the exception of the Chipola marl member, exercise 
an influence on the topography of the northern part of the State. They 
are soft and easily eroded into deep valleys but are sufficiently resistant 
1 Dali, Wm. H., and Stanley-Brown, Joseph. Cenozoic Geology along the 
Apalachicola River, Geol. Soc. Am., Bull., vol. v, 1893, p. 166. 
