GEOLOGY CHOCTAWHATCHEE AND APALACHICOLA RIVERS. 8 7 
the Choctawhatchee. from the east is blocked in the same way, form- 
ing a lake several square miles in area. 
The following account of the origin of the lakes of the Chipola 
river has been given in volume 27 of the bulletins of the Geological 
Society of America.* 
The Chipola river in Florida affords a good illustration of a tributary 
stream ponded by the deposition of sediment in the valley of the main stream. 
This river, which originates entirely within the coastal plain, flows for a con- 
siderable part of its course across limestones and is fed very largely by clear 
water limestone springs. It is therefore a clear-water stream, carrying only a 
limited amount of sediment. The Apalachicola river, of which the Chipola is 
a tributary, heads, on the contrary, in the mountains of northern Georgia and 
receives a heavy load of sediment, which is deposited, as the river becomes 
overloaded, near the Gulf. The Chipola enters the Apalachicola about 25 miles, 
by land, from the Gulf of Mexico. In this lower part of its course the Apa- 
lachicola is rapidly aggrading its valley, while the Chipola, owing to the limited 
amount of sediment which it carries, is building its valley much more slowly. 
This condition results in flooding the valley of the Chipola river. The lake 
thus formed, known as the Dead Lake of the Chipola river, derives an added 
interest from the fact that it has come into existence so recently that the 
cypress timber of the former river swamp, now mostly dead, is still standing, 
although the water in the lake has reached a depth of from 10 to 20 feet, 
while the lake itself is 10 or 12 miles long and from 1 to 2 miles wide. The 
channel of the river may still be followed in its winding course through the 
lake. 
GEOLOGY. 
The geologic formations found at the surface, within this area 
are chiefly those of the Eocene, Oligocene, and Miocene periods, 
altho more recent deposits may be present near the Gulf Coast. 
As in the case of the other formations of Florida the materials of 
which these formations are made include chiefly sands, clays, lime- 
stones and shell marls. The limestones have been divided on the 
basis of the kinds of fossils which they contain into three forma- 
tions, known as the Ocala, the Marianna, and Chattahoochee 
formations. The shell marls likewise are readily divided into at 
least two formations differing in age and in the kinds of fossil 
shells which they contain. These are known as the Alum Bluff and 
the Choctawhatchee formations. 
* Dead lake's of the Chipola river, Florida. By E. H. Sellards, Bull. Geol. 
Soc. Amer., Vol. 27, p. 109, 1916. 
