Page 251 - The British Big Four

he stereotypical cenotes often resemble that the entire platform has subsided under
T small circular ponds, measuring some
tens of meters in diameter with sheer drops
at the edge. The infiltrating rainwater floats
on top of higher density saline water intrud-
ing from the coastal margins. Where a blue
hole, or the flooded cave to which it is an
opening, provides deep enough access the
interface between the fresh and saline water
may be reached. The density interface be-
tween the fresh and saline waters is a halo-
cline, which means a sharp change in salt
concentration over a small change in depth.
The mixing of the fresh and saline water re-
sults in a blurry swirling effect caused by re- a small fraction of their prehistoric extent. both Stalactite and Stalagmite are provided
fraction between the different densities of
fresh and saline waters.
T he limestone that comprises the Baha-
ma Banks has been accumulating since
at least the Cretaceous period, and per-
haps as early as the Jurassic; today the to-
tal thickness under the Great Bahama Bank
is over 4500 meters. As its limestones were
deposited in shallow water, the only way to
explain this massive column is to estimate
water that filters through the rocks. During
its own weight at a rate of roughly 3.6 cen-
timeters per 1,000 years. Not all blue holes
look like a hole in the ground with many
looking like a pond. The tunnels are formed
by water rushing through and dissolving
the soft limestone. The waters of the Ba-
hama Banks are very shallow; on the Great
Bahama Bank they are generally no deeper
than 25 meters. The slopes around them
however, such as the border of the Tongue
of the Ocean in the Great Bahama Bank, are
very steep. The Banks were dry land during
past ice ages, when sea level was as much as
120 meters lower than at present; the area
of the Bahamas today thus represents only
When they were exposed to the atmosphere,
their limestones were subjected to chemical
weathering that created the caves and sink-
holes common to karst terrain, resulting in
structures like blue holes.
M any of these caves have Stalactites
hanging from the ceiling resulting
from mineral deposits transported by the
the last Ice Age water levels were lower and
many of the current blue holes and caves
were dry and that is when the Stalactites
formed. When it rains the water peculated
through the limestone and during the pro-
cess dissolved some of the limestone and
carried it away. The water then dripped from
the roof of the cave and over thousands of
years limestone was slowly deposited and
formed the icicle shaped Stalactite. The
same water drops that fall from the tip of
the Stalactite deposits lime on the ground
creating a Stalagmite over time. A variety of
shapes result from factors such as rhythm
of the drop and the height of the fall. When
enough time to grow and under the right
conditions they can join together and form
a column that can measure up to several me-
ters tall. I At the end of the Ice Age the sea
levels rose and this raised the ground level
table of the waters of the Bahamas.