Geology Reference
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oblong beachrock can be found throughout the
Bahamas. A large area of identical pillow-shape
rocks can be seen in 3-5 m of water off the east
side of Andros Island near Nichols Town. Many
of the rocks there have provided a foundation for
reef initiation, but for the most part, the rocks are
visible and easy to access by swimming off the
beach. Little (2006) believes these stones, like
those at Bimini, are anthropogenic. Other per-
sonal observations have been made in Abaco and
the Exuma Islands in the Bahamas and off the
northwest coast of Vieques Island, Puerto Rico.
Hurricanes Charley in 2004 and Katrina in 2005
exposed and undermined beachrock with identi-
cal shapes as those at Bimini on the east side of
Loggerhead Key at Dry Tortugas (Fig. 5). These
stones were in the intertidal zone when fi rst
observed by the author in the early 1970s. Because
of undermining by storm waves, they are now
in water approximately 1 m deep. Similar large
beachrock slabs are present at nearby Hospital
Key in water depths up to 3 m. These observa-
tions provide support for the conclusion of Shinn
(1978) that the Bimini stones were submerged by
both erosion of underlying sand and rising sea
level. Based on the bulk 14 C ages of around 3 ka,
they are too deep to be explained by sea-level
rise alone.
Ginsburg (1953) conducted his research on
beachrock that was mostly buried on the west
side of Loggerhead Key. Recent erosion of over-
lying sand has exposed the rock and revealed a
10- to 15-cm diameter iron pipe extending several
metres perpendicular to the beach. The pipe, now
a part of the rock, is believed to have been a sew-
age pipe laid in sand for the Carnegie Institution
Marine Laboratory that fi rst opened in 1905
(Shinn & Jaap, 2005). Alfred G. Mayor, the fi rst
director of the laboratory, reported to R.A. Daly
that sand deposited at Loggerhead Key by a hur-
ricane in 1919 became beachrock in one year
(Daly, 1924).
One of the more interesting recently discov-
ered beachrock sites is at Pulley Ridge in 90 m
of water on the west Florida shelf. Pulley Ridge
is a drowned barrier island as detailed by recent
multibeam surveys (Jarrett et al . 2005). Pulley
Ridge is for the most part capped by a veneer
of live coral but in places the underlying rock
is exposed, revealing pillow-shape limestone
blocks with dimensions identical to the stones
off Bimini (Fig. 6). Pulley Ridge has a well-defi ned
fi shhook or J shape at its northern end (Fig. 7).
Such J shapes are typical features of barrier
(a)
200 m
(b)
Fig. 4. (a) A recent Google Earth image (rotated to produce
an oblique view) of accreting hook-shape sand spits on the
northeast coast of Qatar in the Persian Gulf (Shinn, 1973).
Arrow shows approximate location of forming beachrock
shown in photograph (b) taken in 2005. Soft drink can
(circled) provides scale. The spit indicated by arrow is
the second spit to form since the original study by Shinn
(1973) that was conducted in 1967. Beachrock occurs on
all the spits and increases in hardness in the older spits.
stones, the so-called 'inverted J', is often cited as
evidence of human construction. Observations
of beachrock formation on curved spits in the
Persian Gulf indicate that similar processes prob-
ably have produced the 'inverted J' off Bimini.
Even at Bimini, a similar present-day 'J' prob-
ably would be evident at the entrance into Bimini
Harbour were it not for human intervention. That
area has been extensively modifi ed by a seawall
and a small boat channel.
Other areas of submerged beachrock
The 'road' of beachrock stones is not restricted to
the Bimini area. Similar road-like alignments of
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