Geology of Eleuthera Island
Taken from The Ephemeral Islands, by David Campbell
The genesis of the modern Bahamas Islands was choreographed by events which took place thousands of miles away in the polar extremes of the planet. Four times during the Pleistocene Epoch, which began one million years ago, the ice caps expanded and glaciers scraped over the temperate regions of the northern hemisphere. The final Ice Age, called the Wisconsin, was a relatively recent event in terms of geological time; it ended about 10,000 years ago.
In retreat, the Ice Ages left many legacies. The massive glacial moraines of the American Midwest, the Great Lakes, and the scoured granitic shield of eastern Canada are all remnants of the Wisconsin Era. So are, indirectly, the modern Bahama Islands. For although the glaciers came no closer to the islands than 1,000 miles, they lowered the sea levels sufficiently to foster the conditions that led, in this interglacial period of high sea levels, to the modern-day Bahamas.
Triassic Period (200 million years ago):
- Reptiles dominated the land and sea, the modern-day continents were united into one super continent, Pangaea.
- Volcanic rifts separated the continents into massive blocks. These were prototypes of modern-day continents which drifted apart on great, jointed sheets of the Earth’s crust, like parts of a jigsaw puzzle.
- The rift valleys were filled with by the sea and eventually the Atlantic Ocean was born.
- In the shallow, newborn sea of the Atlantic, algae, mollusks, and corals lived and died. After their death, their lime skeletons and shells drifted to the sea floor and were added to, layer by layer, by chemical precipitation of more lime dissolved in seawater. By this process, oolites, nucleated kernels of almost pure calcium, were created.
- Over tens of millions of years the weight of the sediments increased, the sea floor subsided, but the blooming waters above kept up the pace.
- Today, the Bahamas platform is still covered by a shallow see but the layer of calcium carbonate is many times deeper than the waters above (approx 3.5 miles deep). The result, 700 islands and cays of the Bahamas.
- Ancient river beds and sea floor faulting are two of the main theories behind several extremely deep basins, such as the two mile deep Tongue of the Ocean, and Exuma Sound.
- Southeastern Bahamas – crooked and Acklins, Mayaguana, the Turks, Caicos, Silver, Mouchois, and Navidad Banks – stand like lonely mountain tops in the Atlantic, unlike the western and northern Bahamas, which are joined in vase banks of carbon sediments. Geologist have theorized that these islands have a more recent history which began as an arc of volcanic islands stretching about 500 miles. Since the end of the volcanism, the mountain tops have been covered by a slab of carbonates, and so ostensibly resemble the rest of the Bahamas.
- The Bahamas islands have never been mush higher than sea level. The endearing lesson of Bahamas geology is that over the millennia the sea level has not been constant. There is no permanency here, no certainty, for these are the ephemeral islands. The land will eventually return to the sea and no doubt later be resurrected.
- The most recent Ice Age, the Wisconsin, which at its greatest extent, 16,000 years ago, bound up as much as 1/5 of the worlds water and lowered sea levels worldwide to approximately 300 feet below what they are today.
- Due to the extremely low water levels, the entire Bahamas Bank formed one island, Palaeoprovidence, which included Andros, the Biminis, the Berry’s, New Providence, Eleuthera, the Exumas, Cat Island, Long Island, and Ragged Island, a total land mass that rivaled in size present-day Hispaniola.
- Winds racing over Paleoprovidence created long and narrow sand dunes, particularly along the Atlantic shore. When the sea returned, the weathered dune tops in many areas were all that remained above sea level; modern-day Eleuthera, Cat Island, Longs Island are these windward remnants.