GEOLOGY BETWEEN APALACHICOLA AND OCKLOCKNEE RIVERS. 39 
tainecl from this formation. The fossil plants are found at Alum 
Bluff, on the Apalachicola river.* The vertebrates from this for- 
mation have been obtained at the fullers earth mines at Midway 
and at Quincy, and have been described in the Eighth Annual Re- 
port of this Survey, pp. 82-92, 1916. The fossils indicate that the 
formation is of Miocene age. 
ft 
STRUCTURE. 
The numerous exposures of this formation make it possible to 
use it to supplement the data on structure obtained from the Chat- 
tahoochee formation. On the Ocklocknee river this formation re- 
mains above water level to tidewater, exposures having been noted 
as far down stream as Sanborn Ferry. On the Apalachicola river 
the exposures below Alum Bluff are not numerous. At Estiffa- 
nulga, about 10 miles in a direct line below Blountstown, there is 
exposed on the river bluff about 20 feet of prevailingly coarse 
cross-bedded sands which have the lithologic characteristics of 
the upper part of the Alum Bluff formation. Aside from petri- 
fied wood, no fossils were found at this exposure. The elevation 
at the top of this bluff is probably 45 or 50 feet above sea. Little 
river flows on or cuts its channel into this formation from the 
State line to its union with the Ocklocknee river, the gradient 
of this stream being approximately equivalent to the dip of this 
formation. Many of the tributaries of the Apalachicola, Ock- 
locknee and Little rivers cut across and expose the strata of this 
formation. 
Perhaps the most convenient horizon to use in determining the 
elevation of exposures within this formation is that of the fullers 
earth deposits. The fullers earth strata are. not continuous, but 
are found, so far as observed, at a definite horizon within the for- 
mation. When typically developed the formation contains two 
layers of fullers earth separated by a stratum of sandstone. The 
fullers earth layers vary in thickness from 2 to 3 to 7 or 8 feet, 
while the intervening sandstone is from 1 to 3 feet thick. The 
fullers earth itself varies from light earth of commercial value to 
heavy earth that is not adapted for commercial use. 
* The Physical Condition and Age Indicated by the Flora of the Alum Bluff 
Formation. By Edward Wilber Berry, U. S. Geol. Surv., Prof. Paper 98-E, 1916. 
