146 
FLORIDA STATE GEOLOGICAL SURVEY. 
deposits farther north. The name Recent will be restricted to deposits 
which are clearly of late geologic age, such as sand dunes, alluvial 
sands, etc. In general, it is considered safe to draw the line between 
the two periods at the final emergence of the low lands from beneath 
the sea; and, as this movement appears to be the most marked physical 
change since the early Quaternary, it probably forms the best line of 
demarcation between Pleistocene and Recent. While theoretically the 
line may be drawn with considerable exactness, the discrimination of 
Pleistocene and Recent deposits often presents many practical dif¬ 
ficulties because the changes in conditions governing deposition were 
gradual and the materials for the Recent deposits were often derived 
directly from the beds of late Pleistocene age. 
PLEISTOCENE. 
In Florida, the beds of Pleistocene age include both marine and 
non-marine deposits, and the non-marine beds may be separated, on 
the basis of origin, into residual, lacustrine, fluviatile and aeolian. 
FOSSILIFEROUS MARLS. 
General:—The fossiliferous marine Pleistocene appears to be one 
of the oldest Quaternary formations. It comprises a series of sands 
and shell marls with a sandy matrix. From a deposit of fossiliferous 
sand on North Creek, a tributary of Little Sarasota Bay, Dali 1 de¬ 
scribes a large number of fossils. The shell bed according to this 
writer consists of sand, darkened by organic matter, and the total 
thickness is less than two feet. 
During the progress of the field work for this report collections 
were obtained which indicate that the fossiliferous Pleistocene is much 
more extensively developed than was formerly believed. The beds 
containing fossils of this age present a great uniformity in character, 
being composed of white or light gray sands, usually coarse-grained 
and sometimes containing layers of marine or fresh water shells. 
Rarely beds of clay occur interstratified with the sand, and sometimes 
the material contains scattered argillaceous material. The fossils are 
usually found near the base of the beds and the fauna sometimes- 
changes from marine near the base of the exposures to non-marine 
above, indicating a freshening of the water. In many localities the 
fossils are confined to the lower layers of sand. In such cases, the 
absence of fossils in the upper part of the sand may have been caused 
by partial emergence of the beds during deposition. Sharks’ teeth, 
1 Dali, Wm. H., Wagner Free Inst. Sci., Trans., vol. iii, pt. 6, 1903, pp. 1615- 
1616. 
