SECOND ANNUAL REPORT-SOUTHERN EEORIDA. 217 
given by Hunt, but at Indian Key Channel the thickness may be about 
sixty-nine feet. 
Physiographic Expression:—The causes determining the outline 
of the great arc of the reef have already been discussed. The islands 
form a discontinuous low wall separating the waters of the Atlantic 
from those of Biscayne Bay, the Bay of Florida, and Blackwater 
and Card Sounds. Mention has already been made of the average 
and maximum elevation of the rock surface, the depths of the sur¬ 
rounding water, the outlines of the keys, and the weathering of the 
rock. 
The work of water is shown by the numerous vertical holes form¬ 
ing natural wells, or, as they are called, springs, and the many shallow 
hollows or potholes. These potholes seldom reach much below sea 
level. Occasionally a hole may run down twenty feet, and cases have 
been noted of free openings or cavities filled with soft material that 
extended thirty feet below sea level. Yet, though the rock is full 
of holes, it is not cavernous. Most potholes end in crevices or pas¬ 
sages filled with loose material, without free openings of any consid¬ 
erable size. Holes a foot in diameter extending more than a very 
short distance from a surface opening are rare. 
The reef rock along beaches presents above mean high tide mark 
a curiously rough and jagged look, due to the solvent effect of spray. 
Between tide levels the faces of the low ledges are undercut by the 
waves. Often a loose slab looks as though it had been thrown up by 
the sea, or an apparent heavy bedding slopes toward the water, giving 
the appearance of beach rock. Examination shows that the loose slab 
and the apparent bedding are erosion effects, due to solution and 
cementation. The look of a spray-worn beach is shown in the accom¬ 
panying illustrations. (PI. x, fig. 1.) 
Paleontologic Characteristics:—The Key Largo limestone shows a 
rather small variety of fossils recognizable to the naked eye, though 
heads of corals standing as they grew are frequent, and occasionally, 
an overturned head such as may be seen on the present reef is visible. 
All the corals are of living species. Determinable shells are not espe¬ 
cially common, though aggregates of shell fragments are found from 
place to place. Among the most common molluscan relics are the 
holes made by various lithodomes in the coral. In addition to mol- 
lusks, foraminifers, coralline algae and echinoids have contributed 
to the formation of the rock. The fossils indicate the post-Pliocene 
age of the reef. 
Areal Distribution:—The total length of this Pleistocene coral reef 
is still undetermined. Its continuity from Soldier Key to Bahia 
1 Hunt, E. B. Op. cit. p. 205. 
