Times of the Islands Summer 2016

green pages newsletter of the department of environment & maritime affairs
How TCI’s caves were formed
So what about the caves? They are tied to the history of
sea levels as well. The caves in the TCI form by three
different methods. The dissolution of a soluble bedrock
produces unique landforms and underground flow systems
called karst. The TCI is made entirely of limestone,
a soluble rock, so the TCI is what geologists call a karst
landscape, and caves are a common feature.
Other geological forces, such as waves and wind,
can also attack rock and produce small caves that mimic
caves produced by dissolution. Such features are called
pseudokarst. In the TCI, many coastal cliffs have been
attacked by waves to make sea caves (also known as littoral
caves). Other cliffs have spherical pockets known as
tafoni, produced by wind separating the grains of rock so
that they fall out of the cliff. Tafoni can form quickly in
the poorly cemented eolian calcarenites found in the TCI,
and the walls of quarries, road cuts, and even the eolianite
blocks that make some of the older buildings contain
small tafoni. Both sea caves and tafoni are pseudokarst,
and form simple chambers.
The karst caves of the TCI form by dissolution in the
freshwater lens that underlies the Islands. During the last
interglacial (MIS 5e) 120,000 years ago, when sea level
was higher, the freshwater lens was also higher in elevation.
Caves dissolved in the limestone at that time are
drained and air-filled as sea level is lower today by about
6 m (20 ft). The freshwater lens is called a lens because it
is thick under the center of an island, and thins towards
the island margin.
Along the lens margin, where it discharges to the sea,
fresh water and marine water mix. This mixing process
creates unsaturated water that can dissolve limestone,
even if both the fresh and marine water were saturated
before mixing. The process is called mixing dissolution,
and it has created the vast majority of caves found in the
TCI today. Because these caves form at the margin of the
lens, under the flank of the enclosing landmass, they are
called flank margin caves. Conch Bar Cave, on Middle
Caicos, is one of the largest flank margin caves in the
world with 2.4 km (1.5 miles) of mapped passages.
When sea level is low, as for much of the last 2
million years, the Caicos Bank, as noted earlier, is one
large island. Large islands behave differently than small
islands, because as islands get larger, their area increases
faster than does the perimeter and the discharge of the
Top: This is a sea cave produced by wave erosion, found in southwest
Providenciales. Bottom: This tafoni, a void produced by wind erosion,
appears in a road cut in Long Bay, Providenciales.
This typical flank margin cave passage, found in Providenciales,
includes the presence of a bat.
Times of the Islands Summer 2016 23