134 
FLORIDA STATE GEOLOGICAL SURVEY. 
Terrestrial Fauna, the Alachua Clays:—Later in this essay I shall endeavor 
to indicate all that is known to date of writing of the marine Pliocene beds of 
Florida, but there is another formation to be spoken of which has been referred 
by some authorities to the Upper Miocene, though regarded by others as late 
Pliocene or even Pleistocene. This comprises the deposits of clay containing 
bones of extinct mammalia which, in my report to the Director of the U. S. 
Geological Survey in 1885, I termed the Alachua clays. 
The report mentioned above does not appear to have been pub¬ 
lished, and hence the quotation probably contains the first published 
reference to these clays. With these beds are included the “Peace 
Creek bone bed” of Dali , 1 which appears to be a local phase of the 
formation which may have been eroded and redeposited in its present 
position. 
Stratigraphic Position: —- The Alachua clay is known to occupy 
depressions such as sinks and gullies in the Oligocene and probably 
also in the Miocene beds. Observations made by Dali along the banks 
of Peace River have established the fact that a bone bed (“Peace 
Creek bone bed”) which he correlates with the Alachua clay, rests 
upon older Pliocene beds . 2 The relation between the Alachua clay 
and this bed on Peace Creek has not been observed, but they are be¬ 
lieved to be lacustrine or fluviatile deposits which may have been 
formed at about the same time. The Alachua clay is also thought to 
be contemporaneous with a part of the Caloosahatchee and Nashua 
marls. 
Lithologic CharacterThe Alachua clay consists of blue to gray 
sandy clay which weathers to light yellow or red from the presence 
of iron oxide. There is usually sufficient clay to give the material a 
distinct plasticity, and sand is commonly present in considerable 
quantities. The weathered material is frequently more or less con¬ 
cretionary as a result of the aggregation of the iron oxide. The for¬ 
mation is nearly destitute of fossils except in a few localities where 
it is filled with vertebrate remains. 
Thickness: — The Alachua clay represents accumulations in de¬ 
pressions of the surface of the Oligocene beds, and hence the thick¬ 
ness is variable. It is known to have a thickness of fifteen feet and 
the maximum may be much greater. It is difficult to estimate the 
average thickness, but it is probably not less than ten feet. 
Physiographic Expression:—The Alachua clay has no marked 
effect upon the topography; but from its occurrence in depressions in 
the underlying rocks it probably helps to diminish the relief. Locally 
1 Dali, Wm. H., Neocene of North America, U. S. Geol. Survey, Bull. 84, 
1892, pp 130-131. 
2 Dali, Wm. H., Neocene of North America, U. S. Geol. Survey, Bull. 84, 
1892, pp. 131-132. 
