LATER TERTIARY (MIOCENE AND PLIOCENE). 811 



"The Caloosahatchee marl occupies a region of such low altitude that it has only been 

 slightly dissected. In fact, with the exception of the vaUeys of the streams entering Charlotte 

 Harbor the surface of the region underlain by this formation is an almost imbroken plain. 

 While this is doubtless in part due to the later deposit of Pleistocene sand, it is doubtful if 

 the surface of the Caloosahatchee marl has ever been greatly eroded." 



The nonmarine Pliocene of Florida includes the Alachua clay and the Bone A'alley gravel. 

 The Alachua is thus described by Matson and Clapp : ^'* 



"The Alachua clay is known to occupy depressions such as sinks and gullies in the Oligo- 

 cene and probably also in the Miocene beds. Observations made by Dall along the banks of 

 Peace River have estabhshed the fact that a bone bed ('Peace Creek bone bed'), which he cor- 

 relates with the Alachua clay, rests upon older Pliocene beds. The relation between the Alachua 

 clay and this bed on Peace Creek has not been observed, but they are believed 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. It 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 fre- 

 quently more or less concretionary as a result of the aggregation of the iron oxide. The forma- 

 tion is nearly destitute of fossils except in a few localities where it is filled with vertebrate 

 remains." 



As the accumulations took place in depressions of the surface of the Oligocene beds, the 

 thickness is variable. In places it is 15 feet or more, and the average thickness is probably 

 not less than 10 feet. 



The Bone VaUey gravel, which comprises nearly all the pebble phosphates now being 

 mined in Florida, derives its name from a locality west of Bartow, where the beds are exploited 

 on a large scale. Eldridge designated this deposit land pebble phosphate, while Dall called 

 it simply pebble phosphate, both writers distinguishing between the Tertiary deposits of Plio- 

 cene age and the younger pebble phosphates, which vary in age from Pleistocene to Recent. 

 The deposits, according to Matson and Clapp, ^'"^ "are situated in the valleys of streams and vary 

 in altitude from less than 25 feet to nearly 100 feet above tide. Lithologically the formation 

 is composed of very poorly assorted materials, such as clay and phosphate pebbles, which 

 usually show some evidence of stratification. * * * The poor assortment of the materials 

 and the absence, locally, of distinct stratification point rather to fluviatile than marginal origin. 

 It is, however, probable that some of t'he deposits were made in the margin of estuaries. The 

 presence of such organic remains as bone fragments and shark's teeth is readily explained by 

 the fact that the phosphatic gravels were derived from older formations in which such materials 

 occurred. 



"The Bone VaUey gravel rests upon beds of Phocene or older rocks, and the relation is 

 probably unconformable, but the poor e^xposures make it impossible to observe contacts except 

 in a few localities. For this reason it may not be safe to postulate an unconformity between 

 the Bone VaUey gravel and the underlj-ing Pliocene marls, though the contact with the post- 

 Pliocene beds is certainly unconformable. * * * 



"While the exact correlation of this formation is somewhat uncertain, it is beheved to be 

 younger than the 'Arcadia marl' and older than the upper beds of the Caloosahatchee marl. 

 It is probably in part contemporaneous -with the Alachua clay. * * * 



"The exposures frequently reach a depth of more than 30 feet, without encountering 

 the underlying beds, and hence the maximum thickness may be safely fixed at more than 30 

 feet. The average thickness, however, is probably lower and it may not exceed 15 to 20 feet." 



West of Florida the presence of Pliocene rocks has not been definitely determined. It is 

 probable that a portion of the 700 feet of strata above the Rangia johnsoni bed in the Mobile 

 well is Pliocene. Berry examined outcrops and collected fossil plants in Alabama at a point 



