Blue Holes & Caves ::The Department of Environment and Coastal Resources::

(But first, a little about how our Islands came to be in the first place...)

All Limestone! All the Time!

The Chemistry…
Despite the fact that many stories and articles about our Islands assert that they are the remains of coral reefs, that’s really not the case at all. It’s not totally wrong, but it’s not totally right, either. There are a few places in the country that do have fossil reefs above the surface of the sea, most notably on West Caicos, but the vast majority of our Islands (and every other island in the Bahamian archipelago) were formed in a different way. We probably agree with the idea that all of our rocks are made up of coral because when we heft a piece of it, it looks and feels and weighs about the same as a piece of dead coral on the beach. But even though our rocks and our corals have a few things in common they are different things entirely.

The main thing they have in common is that they are both made of the very same mineral, calcium carbonate. That’s why they look and feel (and taste!) so much alike. Another thing they have in common, is that they both formed under the sea. Neither one could have formed without being under our beautiful warm ocean. But that’s about as far as it goes, I’m afraid.

In school these days, kids learn that it’s those little tiny coral animals, the polyps, that are responsible for building the massive reefs. The polyps extract a mineral called calcium carbonate from the seawater around them and turn it into their hard, rocky exo-skeletons. And as a matter of fact, all those hard-shelled animals in the sea, and many of the softer ones too, do the same thing. Conch shells, clamshells, sea urchin spines, crab and lobster shells, cuttlefish bones, octopus beaks, they are all made of calcium carbonate that the animals extract from the sea. (We, too, extract calcium carbonate from our environment, generally from the foods we eat, and like the polyps, it is the main component of our skeletons.) Fittingly, just as the skeletons of almost all the creatures from the sea that surrounds us are made of Calcium Carbonate, so too are the bones of our sea-born Islands.

This mineral, Calcium Carbonate or CaCO3, is dissolved in sea water to a very concentrated degree. Usually when we think of minerals in sea water, we only think of Sodium chloride, the mineral that gives sea water its salty taste. But many other minerals are dissolved in the sea, as well. After salt, the most common are magnesium, sulfur and calcium.

Solid from Liquid
When the water is warm and shallow, calcium carbonate has a tendency to precipitate out of the water as tiny particles all by itself. In almost the same way that the untended Salt Ponds on Grand Turk and Salt Cay still produce solid salt by evaporation and precipitation, so too do the broad shallow banks produce solid calcium carbonate.

The high concentration of CaCO3 dissolved in the water just seems to be looking for an excuse to turn into solid little particles and settle down. And that excuse is the slight warming of the seawater when it’s pushed up from the deeper part of the ocean on to the shallowest parts of the banks. Just a rise of a few degrees in temperature and the gentle agitation of the wind over the shallow banks will cause the CaCO3 to form tiny, little pellets suspended in the water. These pellets are called ooids. They are shaped like microscopic eggs and as they swirl around in the sea, they pick up layer upon layer of CaCO3 until they finally become just heavy enough to sink. They slowly sink to the floor of the bank, gathering more mineral as they go. Over the hundreds of thousands of years since our Islands started forming, vast accumulations of this oolitic sand have formed in the shallow waters of the Bahama and Turks Islands Banks. That fine, powdery sand on Grace Bay beach is almost all oolitic sand. Much of the sand that covers the wide Caicos Bank is oolitic sand.

Shallow Seas Become Land

Many, many years ago, during the time of the great glaciers, when sea levels dropped by as much as 400 feet, these shallow sandy plains became exposed to the air. As sea level fell, they became beaches, and as with all beaches, the dry fine sand was pushed around by the wind to form small dunes. Relatively quickly the top layers of these dunes hardened to become small ridges and hills. As sea level retreated even further, more sand was exposed and more dunes were formed, some of them right on top of the older ones. These too hardened and the ridges and hills became higher. Because the sand was so light, it was carried great distances by the wind and could be piled up very high. The soft stone produced in this way is limestone. The limestone made in this way, by the hardening (lithification) of wind borne sand dunes, is technically known as Aeolian limestone, but basically it’s the same old oolitic stuff that we’ve been talking about. Ask the folks who live on top of the Ridge in Grand Turk. Some of their houses are eighty feet above sea level and almost a quarter mile from the sea. And still they are constantly bedevilled by the never ending stream of fine sand that finds its way under their doors and into their houses, pushed up from the East side beach by the constant 15 mile an hour trade winds. The Ridge is still being built up in this way and parts of it probably grow by as much as a foot in a century.

These periods of lower sea level alternated with periods of higher sea levels, and so after several tens of thousands of years of dune building, there would be several tens of thousand of years of inundations, higher sea levels. The newly made dune ridges would be submerged into the shallow sea again. During these wet periods, more ooids would be produced in the new shallows, and at the next period of exposure, this newer sand would then be blown over the old dunes raising them still higher. There have been two of these major wet/dry periods over the past 200,000 years, and several minor wet/dry periods within each of those major stages. As for the present day, sea level is within about 45-60 feet of the highest it has ever been and we just don’t know if we are in a natural rising-water or falling-water phase.

Sand Dune Island...
A common site along many of the roads in Providenciales these days is a couple of big machines tearing and scratching away a rocky hill to turn it into a nice smooth road. We all want our roads to be smooth and straight and, if possible, without too many dips and curves in them, right?

But in order to get those roads relatively smooth, some of the high spots have to be shaved down and some of the low spots have to be raised up. On this rolling and furrowed Island then, it becomes necessary to take a little off the top of some of those high spots and fill up some of those low spots with what’s left over. And this noisy, heavy construction exercise leaves us with, not only a new (dusty and gritty) road but, an interesting geology lesson in how our Islands were built up in the first place.

Providenciales Road Cut shows Fossil Dune

One of the most easily accessible and best places to “read the rock” in this way happens to be in the parking lot of the Price Club grocery store, near the new Scotia Bank building on the Leeward Highway. (It’s also a convenient place to stop on this busy road without worrying about getting run over.) Look for the high, white, cut-away stone wall that is the eastern boundary of the parking lot, the one with the big reddish-brown stains right in the middle of it. We’ll get to the stains later.

Ignore the vertical scars made by the teeth of the heavy machines and take a look at all the skinny horizontal and diagonal bands that make up the rock face. They’re especially noticeable on the cut wall that runs along the Leeward Highway. Each one of these bands represents a period of time from a couple of decades to a couple of centuries when a layer of sand was blown up from the nearest beach onto the top of a previously formed dune. That layer quickly developed a hard crust so that it wouldn’t blow away. The crust could have been formed by a good soaking rain that packed the tiny particles together tightly. Or it might have been formed by a thin layer of beach plants, whose shallow root systems held the sand in place long enough for it to be compacted.

The lack of many obvious dark organic stains in the horizontal lines indicates that there probably wasn’t much actual plant life on these dunes over those many thousands of years. The white over white patterning tells us that most of this is just one windblown layer of sand over another. However, in several places on the parking lot side there is very interesting evidence that at least some plants grew on the dunes in the distant past, and they were bigger than just ankle high weeds. See that shallow hole with the long skinny rock sticking out of it? That skinny rock is a fossil plant root. It’s actually a root cast , not the petrified plant itself. When it was alive, that plant put a long root down into the sandy dune, looking for fresh water. The plant was then covered by a layer of sand and quickly rotted away, but it took a much longer time for the buried root to disappear. By the time it had, the sand around it had already turned into soft rock, so it left a long skinny hole. Rainwater, carrying a load of dissolved CaCO3 from the surface, seeped into the hole and deposited the dissolved mineral in it. That section of rock was a little harder than the surrounding rock, so today, as the soft wall quickly erodes from the effects of the wind and rain, it stands out plainly for us to see.

How The Holes Got There

Banana Hole

Now, remember that big, brown stain I mentioned earlier. That’s a solution hole, or as commonly called in the Islands, a “banana hole”. And there’s another, older one next to it. Our limestone is very soft (for a rock) and is easily affected by the elements. Wind-born dust acts just like sandpaper and erodes the exposed surface very quickly, in a matter of a few years. Rainwater falling on the thin soil and leaf litter among the bushes picks up chemicals from the decaying plants and becomes slightly acidic. This acidic water easily etches small holes in the soft rock that underlies the thin soil.

The small holes catch and hold more rainwater and become bigger and deeper. Where the rock is particularly soft or along natural cracks, the acidic water enlarges the holes making them deeper and wider. The holes quickly fill up with organic material from the surrounding forest; leaves, bugs, lizards, creating protected pockets of very fertile earth. It’s only a matter of time until a seed catches in the sheltered cleft and a combination of the chemical weathering and pressure from fast root growth act to extend and deepen the banana hole even further.

To The Caves At Last
Caves take shape in the soft limestone in almost the same way that the Banana Holes form, through the action of slightly acidic water dissolving the alkaline rock.

Everybody in the Islands has heard of the Conch Bar Caves and many of us have even made the trip to Middle Caicos to see them. But we always return with questions. How were they formed? How old are they? How deep are they? Are there new caves to be found?

When first looking at the immense rooms and caverns at the openings of the Conch Bar Caves, many people get the idea that they were formed by the rising and falling of the earth itself. But, in fact, just the opposite is true. The great limestone platforms that make up the Turks and Caicos Banks are exceptionally stable and no serious tectonic, earth shaking movements have occurred in this area for millennia. Rather, the caves have been formed by the action of water: rainwater soaking into the rock from above and slowly finding its way to sea level. Also involved in the building of our caves has been the rise and fall of the oceans due to the advances and retreats of the great glaciers tens of thousands of years ago.

Beneath many of our larger islands, saturating the porous limestone, is a layer, or lens, of fresh water. When rain falls on a limestone island, it quickly sinks into the porous rock, picking up a mineral content from the organic material, soil and rock that it passes through. This water accumulates within the rock, continuing it’s downward path. Eventually, though, it meets the layer of salt water that permeates our limestone platforms at sea level. Because fresh water is less dense than salt water, the lens of fresh water will float on top of the saline groundwater. In many areas, this lens is very thin.

Yikes! More Chemistry…

Where the descending fresh water meets the saline groundwater some mixing occurs and produces a chemically complex zone of brackish water. Depending upon the size of the lens and the complexity of the rock, this zone can be a very thin or quite broad band.

This brackish water is chemically very aggressive and capable of dissolving limestone at a much greater rate than either fresh or salt water alone. This brackish zone is called the halocline, and where the halocline comes in contact with the rock, cave development can begin.

A change in sea level will change the level of the freshwater lens within the rock and therefore, the level of the halocline as well, stopping cave development in areas that have drained and starting it at the new level of the halocline. This explains how caves can form at different paleo-sea levels and eventually join up to make a very elaborate, multi-level system of passages and rooms.

The upper and lower passages in the Conch Bar Caves are a good example of this process. These different sections, formed thousands of years apart, have connected into one large cave system.

Tropical Caves from the Ice Ages...
Water evaporates from the world’s seas and oceans to produce clouds. Rain falls to earth and that water quickly returns to the sea. In northern latitudes, precipitation in the form of snow also flows to the sea when it melts in the springtime. As an Ice Age begins, though, more and more of that water is removed from the system and trapped in the form of great spreading sheets of ice in the high latitudes. In the warmer latitudes, though, water still evaporates from the sea causing the sea level to drop lower and lower as more water becomes bound up in ice.

With this sea level drop, and consequent freshwater lens drop, the older caves drain of water and become air filled. It is only then that stalagmites and stalactites (generally called speleothems) start to grow.

So, how old are these Caves?
Cave formation and speleothem growth take a long time, but dating these features in a general way is fairly easy. As we have seen, the caverns themselves are formed when that elevation is under water, or more correctly, at sea level. In order to date sub-aerial, or above-water caves, we have to look back to when most of this area was covered by the sea.

As most rocks in the Bahamian archipelago have been found to be no more than 150,000 years old, our caves must be younger than that. Remember, caves form inside rock: rocks don’t form around caves. Researchers have determined a period between 131,000 and 119,000 years ago when sea level stood about six meters above where it stands today. This would be about the oldest our caves could be and almost all “dry” caves in the Bahamas today stand only between two and ten meters above sea level.

More recently, there were shorter periods of glacial melting when sea level was higher than it is today, but, of course, during the long periods when glaciers covered most of the Northern hemisphere, sea level was much lower. It has been estimated that sea level has been at or above its present elevation for no more than 25,000 of the past 150,000 years. There are, then, many cave systems in our Islands that are now far underwater.

Blue Holes
The Blue Hole, on the Caicos Bank just south of Middle Caicos, is the opening of one of these deep caves. Two hundred feet below sea level, the roof of a large cavern collapsed, leaving a hole on the surface ¼ mile in diameter. At the bottom of the hole, caverns spread out in several directions, just like a cave on land.

In North Caicos, a small solution hole formed on the surface and eventually got deep enough to break through to another deep cavern. Cottage Pond is the above ground entrance to another deep cave system. Divers have found water-formed side channels 75 feet below the surface and speleothem formations at over 120 feet deep, proving that these water filled depths were once above sea level.

All of these caves are still forming in the hills of our larger islands. Wherever the freshwater lens meets the saline groundwater, new caverns are being made. Wherever rain falls on a rocky hill top, new holes and passages are being fashioned.

Glossary
Aeolian – means “wind-borne”. Aeolus was the ancient Greek god of the winds. Aeolian limestone was formed by being deposited bit by bit by the wind.

Calcium carbonate (CaCO3) – One of the most abundant minerals on the earth. Not only found on land, it is one of the many different minerals that is dissolved in the world’s oceans. It is also a very necessary mineral to our own health and well being. Various forms of calcium carbonate are called limestone, calcite, travertine, aragonite or caliche depending on how they were deposited.

Exo-skeleton - “Exo- “ is a Greek prefix that means outside. Unlike us, some creatures wear their skeletons on the outside of their bodies, usually for defence. Crabs, lobsters, shrimp, corals and most insects have hard armour shells that protect and support their soft bodies from the outside. All of us mammals have “endo(inside)-skeletons”.

Halocline - Where freshwater and salt water layers meet and mix. This brackish water layer is chemically very reactive.

Ooid, oolite, oolitic- Ooid is another Greek word that means “egg”, so little egg-shaped particles are called ooids. “Lite” of “Lith” is yet another Greek word part meaning stone. So a large mass of ooids stuck together is oolitic stone. And “to lithify” means “to turn to stone”. The most common form of limestone in our Islands is called oolite. The purest form of oolite is called “aragonite”.
(Boy! These science types sure like those Greeks words, don’t they?)

Precipitation – In a chemical sense, precipitation is the deposition of a solid in any form from a solution. When a solution has more of any substance dissolved in it than it can easily hold, some of that substance will turn back into a solid again.

speleothem - the general term used by geologists to describe a cave formation made of re-deposited calcium carbonate, calcite. Stalagmites (go up), stalactites (go down), columns, flowstone are all speleothems.

Sub-aerial - means, in cave talk, above water. The Conch Bar Caves of Middle Caicos are the largest sub-aerial cave system in the Bahamian archipelago. Passages above the water that an explorer can access add up to almost 5 kilometers. The length of passages below the water is unknown for now.