TCI’s cave system helps researchers understand local geology.

By Michael J. Lace, John E. Mylroie, Nancy A. Albury, and Joan R. Mylroie, Coastal Cave Survey

The Turks & Caicos Islands are well known for beautiful beaches and sparkling turquoise seas. Did you know that the Islands also have a unique and interesting underworld? It is made up of caves that contain beauty, history, and a fascinating geologic record.

The origins of the Turks & Caicos Islands

The Turks & Caicos Islands exist today because the rocks that form the Islands have been made by living organisms, both plant and animal, that precipitate calcium carbonate (CaCO₃), the compound that becomes limestone. The chemistry that allows the precipitation of calcium carbonate also allows it to be dissolved later by moving water. As a result, the bedrock surface becomes jagged and irregular, forming pits that carry water downward to create caves within the freshwater lenses that underlie the Islands. Each island has its own separate lens.

Stalactite-stalagmite column in Conch Bar Cave, Middle Caicos

The deposition of the limestone that is the TCI’s foundation and the subsequent dissolution of those rocks has been controlled over the last several million years by a series of natural climatic events known (in layman’s terms), as the Ice Ages (called glaciations by geologists). The time between glaciations, as currently exists in the world today, are called interglacials.

During a glaciation, snowfall in the high latitudes of Europe and North America fails to melt completely each summer, and that snow accumulates into thick piles of ice, which expand and move southward as ice sheets.  The natural trigger for this process is a subtle change in the earth’s orbit about the sun that leads to slightly cooler temperatures that promote snow survival, but also changing weather patterns that increase the amount of snow at high latitudes. These orbital changes, called the Milankovitch cycle, are the result of slight cyclical changes in how circular the earth’s orbit is, how much axial tilt exists, and how the precession of the earth’s axis changes the equinoxes. The end result of ice accumulation as glaciers is a fall in sea level as the evaporated ocean water is stored on the continents as ice. The drop in sea level has been as much as 125 m (410 ft) during a glaciation; during an interglacial slightly warmer than today, sea level has been as much as 6 m (20 ft) above today’s level as more ice had melted.

When a glaciation is occurring, and sea level is down by 100 meters or more, all the shallow banks of the TCI are exposed as dry land, with steep cliffs down to the sea. All the current islands on the Caicos Bank: West Caicos, Providenciales, North Caicos, Middle Caicos, East Caicos, South Caicos, the Ambergris Cays, and all the smaller islands and cays, become one large island. Grand Turk, Salt Cay, Sand Cay and associated islands and cays are on a separate bank and also become a single large island.  Glaciations last about 100,000 years, the interglacials last only about 10,000 years, but during that short time window (in the geologic sense), the geology of the TCI becomes established.

When sea level rises as ice sheets melt back, the tops of the banks become flooded with sea water. The lagoons form, and the corals, conchs, starfish, algae and other organisms begin to flourish and create calcium carbonate, primarily as reef structures and skeletons. Over time these skeletal features degrade as the organisms die, and the calcium carbonate is released, much of it as sand-sized particles that eventually wash landward to become beaches. Once on the beach, the wind can sweep up these sands to make dunes, called eolian calcarenites, which means literally “windblown calcium carbonate sand.”

Eolianite in road cuts in Middle Caicos and Long Bay, Providenciales

All land in the TCI above about 6 m (20 ft) is eolian calcarenite. In the lagoon, sands are deposited along with the coral reefs. When sea level is higher than today, as it was 120,000 years ago during the previous interglacial, these lagoon deposits were placed where we can see them now, as what was under shallow water then is above sea level today. Fossil reefs and related lagoon features are found all over the TCI a few meters above modern sea level. These all formed 120,000 years ago, a time called the last interglacial.

On the exposed land existing today, and across the entire exposed banks when sea level is low during a glaciation, soils form. The TCI is a 100% calcium carbonate environment (NaCl, halite, and CaSO₄•2H₂O, gypsum, called evaporate minerals, are sometimes found). The soils are red, brown or tan because they contain iron oxides, plus other oxides such as Al₂O₃, that arrive on the bank as wind-blown dust all the way from the Sahara Desert. When these soils become fossilized, they turn into a very hard and dense crust called a terra rossa paleosol.  These paleosols cover all the rocks that formed on interglacials older than today. The young eolian calcarenites that have formed in the past few thousand years of the current interglacial are too young to have collected significant Saharan dust, and so lack a thick red soil. The coastal rocks of the cays from Little Water Cay north to Parrot Cay are all eolian calcarenites and related rocks less than 5,000 years old.

The geology of the TCI is simple. When sea level is high and the bank tops are flooded, a variety of limestones are made. When sea level is low, for a duration ten times longer than for sea level highstands, the main geological events are erosion and soil formation. There have been ten or more cycles of glaciation and interglacials during the last two million years, and the rocks produced by the last three or four cycles are preserved as dry land in the TCI today. The only fossil reefs and related lagoon deposits found today formed 120,000 years ago. Such deposits from earlier high sea levels have not been found.  Either the sea level wasn’t above modern, the platform has subsided slightly, or those deposits have eroded away.

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.

Fossil coral and conch shells found on the Crossing Place Trail, Middle Caicos

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 water of the freshwater lens by diffuse flow to the coast is unable to conduct water efficiently. Tubular cave conduits develop, much like water pipes, to carry the flow. These tubes can be quite large, up to 10 m (33 ft) or more in diameter. They can become large enough that their roofs become unstable at certain locations, and collapse occurs. This collapse can work its way upward until it breaches the surface of the bank. When sea level rises at the end of a glaciation, these collapses become flooded to form blue holes. Cave divers can go into these blue holes, and sometimes are able to enter the large, horizontal caves that developed during a lower sea level.

When sea level was higher than today during the last interglacial, the TCI was a collection of islands much smaller than today, as only the eolian calcarenite dune ridges were above sea level. In such small islands, diffuse flow easily carries the water of the lens to the island margin, and only flank margin caves develop. Even Conch Bar Cave, with its huge length, is only a series of connected chambers that run along the edge of the ridge enclosing the cave; it is not a conduit cave system.

These flank margin caves form without entrances; years later, erosion of the hillside, or collapse of the roof, may create an entrance. Conch Bar Cave has both types of entrances. Indian Cave on Middle Caicos is highly altered by collapse and the roof remains as a series of arches. There may be flank margin caves still intact inside hillsides today in a completely natural state, unentered by humans or bats.

What do TCI’s caves tell us? 

First, the mere existence of the caves tells us that sea level was once higher. They allow geology to be seen from the inside, which includes the nature of the rock, and how that rock has been altered through time. The caves contain mineral displays, the most common being those made of the mineral calcite, such as stalactites that hang from the ceiling, and stalagmites that rise up from the ground, as well as slopes of calcite called flowstone.  A variety of exotic minerals can also be found. Stalagmites are especially important, as they grow from drip water in thin layers, and their interior looks much like the growth rings found in the trunk of a tree. These layers can be dated, and their isotopes examined, to study how climate changed in the past.

Along the coasts of the TCI, especially the Crossing Place Trail of Middle Caicos, a number of flank margin caves have been exposed by wave erosion of the high dune ridges. The caves are present in sea cliffs, and present an interesting problem, as sea cliffs are exactly where one would expect to find sea caves produced by the mechanical energy of waves and not by dissolving the rock.

One useful line of evidence is where the cave exists above the tide level, above the intertidal notch. Such a cave could not form by wave energy under current conditions. These caves also contain eroded stalagmites, stalactites, and flowstone, which can only originate inside caves. The morphology of the cave walls is also a clue; intricate rock erosion occurs by rock dissolution, wave energy tends to make smooth walls by brute force.  Juniper Hole, on Middle Caicos, has been described as the largest sea cave in the TCI, but it is actually a flank margin cave that has been over-printed by wave action in the last few thousand years.

The very young eolianites found from Little Water Cay north to Parrot Cay have numerous small sea caves formed right at current tide level. These eolianites, as noted earlier, are very young, too young to contain flank margin caves from a past interglacial.

Flank margin caves that have opened naturally often contain colonies of bats. Thick piles of bat excretement, called guano, cover the floor of many caves. This guano is a powerful fertilizer, as it contains nitrates and phosphates needed by plants. Throughout the TCI, these deposits were mined for both local use, and for export to neighboring countries, primarily the USA in the late 1800s. The industry collapsed after World War I because the Haber process, a way to make nitrate fertilizer from the nitrogen gas in the atmosphere, was commercially developed. Bats are critically important in the ecology as they eat insects, and transfer pollen between plants, both important to people.

Why are caves important?

Caves contain an unusual collection of animals that have adapted to life underground. Some of these animals lack eyes and pigment, unnecessary energy expenditures in a lightless environment (the human eye has the highest metabolic rate of any organ in the body). The food chain is based on the organic material that enters the cave. Sometimes that food is very fine particulate matter that flows into the cave with the drip water, other times it is larger pieces of vegetation that fall down a hole in the ceiling, but most of the organic energy comes from the bat guano and the molds, fungi, and invertebrates that live on that bat waste. While invertebrate cave life is abundant in the TCI, the only vertebrates are cave fish and bats.

Throughout the history of humans, caves have been sites for living and for conducting ceremony. Cave paintings from the Pyrenees of Europe have been dated to 30,000 years ago. Archeological materials indicate that the Lucayan-Taino culture first emerged in the TCI after AD 700. Some caves were used as shelters, burial sites or ceremonial spaces by native Lucayans well before Columbus reached the Islands. Even today, caves are used as hurricane shelters, and in the case of Conch Bar Cave, as a tourist destination.

The caves are also very important sites for instruction about how the natural environment works, and how geological and biological processes function in concert to create an important ecosystem. Conch Bar Cave is one example of how carefully selected cave sites can serve as ecotourism destinations with a delicate balance of access and resource preservation.

Just as caves provide clues to revealing the geologic past of the Islands, so too can caves offer insights into how coastal landscapes may change in the future. Caves, coastal land stabilities and water quality are all intimately connected within a karst landscape, shaping the sustainability of future land uses and long-term quality of life in the islands.

Preserving TCI’s cave environments

One cannot protect what one does not know of. Caves are incredibly fragile, and human visitation can be detrimental to the caves if not done properly. Some caves show evidence of vandalism, either as graffiti, or breaking of rock and cave formations. Other caves have suffered from well-meaning explorers who disturbed bats during nursery season, or trod over organisms living in the cave soil.

Unlike hills, beaches and lagoons, which are landforms visible to all, caves are cryptic and can be hidden from any but the most intensive investigations. They can be inadvertently destroyed because they were not known to exist. The major problem with managing caves as a resource is not knowing what caves exist, and having a map and resource inventory of those caves. The preservation and management of these fragile resources depends on defining not only their geologic origins but also documenting the inherent biodiversity and cultural significance associated with each cave site.

The authors of this article are part of the Coastal Cave Survey (CCS), an independent volunteer group that has traveled the world to locate, map, and document caves found in islands and other coastal settings. These data are given to the resource managers of each area to allow them to create the best management plan for their resources.

In The Bahamas, CCS has mapped over 400 caves, with the data being delivered to the Bahamas National Trust. Through DEMA, they have begun to examine the cave resources of the Turks & Caicos Islands. The caves of the TCI are fascinating, with unique features not seen elsewhere, but there is much more to be learned. For more information, contact mylroie@geosci.msstate.edu.