PEBBLE PHOSPHATE DEPOSITS. 
39 
The sink at which this section is made is circular in outline and is possibly 
200 feet across. The depth of the sink to water level is 75 feet; the depth below 
water level has not been determined, although it is reported to be more than 
ioo feet. With the exception of the entrance of a small stream from the east 
the sides are almost perpendicular. This stream has cut its channel through to 
the hard ledge of stratum No. 4, over which it forms a waterfall. Strata num¬ 
bers one to four of the section stand vertical entirely around the sink. The sands 
of numbers five to eight weather brownish in color, forming a conspicuous band 
surrounding the sink, except where cut across by the stream. The sands of 
number nine and eleven slope slightly from the lack of supporting ledges. The 
marl, number twelve, stands vertical or nearly so. 
From its considerable thickness the phosphate pebble-bearing formation of 
this section may be expected to be found underlying a considerable area in this 
part of the State. 
In this connection should be mentioned the phosphatic marls and 
limestones found on Black Creek in Clay County. This marl is 
identical in appearance with the similar phosphatic marls which 
underlie the land pebble phosphates. These beds on Black Creek, 
however, are regarded as of Miocene age, and correlated from the 
invertebrate fossils with the Jacksonville formation. 
It may be of interest to note that the Black Creek phosphatic 
marls together with secondary pebble deposits derived from them 
were at one time worked to a limited extent by the late Governor 
N. B. Broward. The development of the deposits, however, was 
found not commercially practicable. 
The phosphatic beds at Hawthorne which served originally as 
the type locality of the Hawthorne formation appear with little 
doubt to represent the Alum Bluff formation.* These beds are also 
found capping the hills in central Marion County. 
In southern Florida the phosphatic marls believed to represent 
the Alum Biluff formation are found along Alafia River and thence 
east through Hillsboro and Polk Counties to Peace River, and 
probably lie comparatively near the surface as far east in Polk 
County as the margin of the lake region. In Manatee County these 
phosphatic marls are seen along M,anatee River and its tributaries. 
On Little Sarasota Bay they are exposed at White Beach and on 
small streams entering the Gulf. 
It is thus evident that the Alum Bluff formation underlies a 
large area in Florida. Its great thickness is shown by well records 
^Correlation of the Hawthorn Formation, by Thomas Wayland Vaughan 
and Charles Wythe Cooke, Journal of the Washington Academy of Sciences, 
Vol. IV, No. 10, pp. 250-253, May, 1914. 
