n Atlantic, where it forms the southwestern end of the Bermuda Rise. Charts also show a number of siblings to Bermuda; seamounts, or submerged volcanoes that never broke the surface. Two of these seamounts lay just to the southwest of Bermuda. The Challenger and Plantagenet Banks top out at about 250 feet below the sea level, just like the volcanic part of Bermuda (see the accompanying illustration). The tops of other seamounts on the Bermuda rise are significantly deeper, and were largely unknown until hydrographic surveys began during and after World War II.
A larger-scale chart of Bermuda itself shows the only other obvious evidence of the volcanic past of the island. Castle Harbour, at the northern end of the island, has a broadly circular shape. One group of geologists has hypothesized that this reflects an underlying caldera, or vent, of a volcano. Calderas form from the explosive evacuation of a volcano, and leave roughly circular craters behind. The approximate rim is marked by St. David’s, Cooper’s, Nonsuch and Castle Islands, and the eastern end of Hamilton. A note to the curious: The majority of St. David’s Island, on the north side of Castle Harbor, is artificial fill, placed to permit the construction of the U.S. Naval Air Station. This fill rather neatly obscures the apparent rim of the crater in that area. Reefs and lagoons
An obvious question, given that all signs of the volcanic origins of Bermuda are buried, is just what has done all the burying. The answer, in plain sight of the arriving voyager, are the magnificent reefs that so effectively guard the shores of Bermuda. Indeed, whether one arrives by air or sea, it is likely that the boiler reefs along the northern edge of the Great Lagoon are the very first solid (if still wet) ground one sees on arrival. Well worth seeing they are.
These reefs are the northernmost coral reefs in the Atlantic Ocean, and close to being the most northerly in the world. Reefs are spectacular but fragile entities that thrive only under a narrow range of conditions. Most importantly, they require warm (always more than 64° F), well-lit water, free of pollutants and dirt. This list is ideally fulfilled by the waters surrounding Bermuda. To a large degree, the Atlantic Ocean in the neighborhood of Bermuda is insulated from the nutrients, dirt, and pollution washed into the sea from North America and Europe.
Bermuda sits near the middle of the Sargasso Sea, a gyre, or circulation system, within the North Atlantic. The Gulf Stream, which forms the western boundary of the gyre, is the best know of the four great currents that define the Sargasso Sea. Sediments washed to the ocean’s edge by rivers cannot cross these currents to enter the Sargasso Sea, granting Bermuda exceptionally clear, relatively unpolluted water. Without the life-giving nutrients provided by those sediments, the water column around Bermuda is largely barren of life, which also contributes to the clarity of the water.
Finally, the volcanic edifice provides a suitably shallow and well-lit substrate for the various plants and animals that together make up the reefs. Despite their fragility when exposed to a ship’s anchor or a diver’s hand, reefs thrive in the high-energy environment at the boundary between deep and shallow water. Here ocean currents and waves continually circulate fresh, nutrient-rich water to fuel growth of the reef, and organisms that can tolerate the often punishing mechanical force of breaking surf thrive in this area.
Behind the reef, in the area sheltered from direct wave action, lies the shallow back-reef lagoon. The reef front itself is formed by "boiler" reefs, small cup-shaped mounds of red algae and small, worm-like marine snails. These reefs lie at depths of less than 20 feet, and get their names from the boiling appearance of the seas breaking around them. Between the boiler reefs, corals and algae form more massive coral reefs.
A reef terrace, 60 to 100 feet down, lies seaward of the line of boiler and coral reefs. This terrace is punctuated by low-growing, long reefs that locally grow to within 40 feet of the surface.
Corals still dominate the reefs of Bermuda. Corals are a life-style as much as an organism. They are symbiotic associations of animals (closely related to sea anemones) and photosynthetic algae. Algae, both red and green, also exist on their own, independent of coralline hosts. That the reefs are still coral-dominated is a pleasant, if surprising, fact, given that the majority of reefs in the Caribbean are now dominated by algae. This transition is largely due to destruction of the fragile reef ecosystem by humans.
Marine ecologists at the Bermuda Biology Station for Research ascribe the continued dominance of corals in Bermuda to the banning of fish traps from the reefs.
Both algae and corals secrete calcium carbonate, or lime, to build structures and stalks as they grow. The aggregated ensemble of corals and algae forms a solid rock called limestone. Yellow, cream, or white, the limestones of Bermuda are essentially the same as those used in building stones the world over. A much older but essentially similar rock probably graces the facade of a bank or government building near you.
A relatively quiet lagoon lies between the boiler reefs and the islands themselves. In the North Lagoon one finds deposits of limey mud, formed from storm-tossed debris of the reef itself, as well as the remains of algae and the more fragile corals that grow in these areas. The lagoon typically has depths of 40 to 70 feet, but it is hardly navigable by those without local knowledge. Small, round patch reefs, their tops hidden just below sea level, pepper the lagoon as well, creating interesting hazards for the mariner. It is these lagoon sediments that make up the exposed rocks of on-shore Bermuda.
With minor exceptions, all of the rocks exposed on Bermuda are sand dunes, formed from pieces of the reef and lagoon brought to shore, blown inland, and naturally cemented into rock. Of course a mechanism that moves these fragments out of the ocean and onto land is needed, and this is where blue, tropical oceans meet blue, arctic glaciers.
Ice ages, glaciers, and Bermuda glaciers are important to Bermuda because they control sea level, which in turn controls where the reefs are. For the past two or three million years, sea level on Earth has oscillated over a total range of 350 to 400 feet. This is due, for the most part, to the coming and goings of the ice ages. Note the plural. At intervals of roughly 100,000 years, variations in Earth’s orbit, and a host of other, still poorly understood causes, rapidly drop the Earth into periods of profound cold. Soon afterward, glaciers spread across the continents, ushering in a new ice age.
During these ice ages, glaciers 3,000 feet thick extended to the latitude of Chicago and New York in North America. The water so abundantly frozen into that ice had to come from the oceans, and it is no surprise that sea level plummets during glaciations. Although estimates vary, most geologists would agree that during the height of the most recent Wisconsin Ice Age, some 20,000 to 25,000 years ago, sea level in Bermuda would have stood 250 feet below contemporary levels. Reefs are always near sea level, so they too would have been lower, and farther out along the edge of the platform.
Conversely, during the warm, interglacial periods between ice ages (such as the one humans have so profitably enjoyed for the past 11,000 years), glaciers melt and retreat toward the poles, and sea levels rise. During these times, sea level in Bermuda was perhaps 50 to 100 feet higher than it is now, much of now-dry Bermuda was submerged, and reefs and muds were deposited directly adjacent to the existing islands. Getting the coral fragments from the beach and onto the island is thus a simple matter of waiting for a large storm. During gales and hurricanes, those fragments of coral and limestone sand washed onto the beaches would be blown landward, forming low, lobate dunes along the shore. After many storms, the dunes would coalesce into miles-long ridges of sand parallel to the coast. Eventually these dunes would grind their way inland, rolling across and burying palm trees and grasses as they went.
Pinkish white sand dunes, 30 feet high with a sprinkling of palm trees and sand grass, would have been the dominant land form on the island. All that is needed to change these loose dunes into solid rock is a few thousands of years of rain. Rain water percolating through those fragments of coral and sand dissolve a bit of limestone, only to return it to the rock later. This redeposited limestone cements the grains of the dune together, converting the loose dune into firm rock. It is a slow, lengthy process, requiring hundreds of thousands of years. The dunes of Bermuda must be at least this old.
The oldest limestone rocks exposed on Bermuda, almost a million years old, form the central core of the island. Over succeeding cycles of sea level fall and rise, new rocks were layered on the top and sides of existing ones. Bermuda, like its namesake onion, has grown in successively larger and younger layers from the inside out. Unlike onion layers, each of the layers on Bermuda has a distinct age and properties. These distinctions have proven important to the people of Bermuda since their arrival.
The youngest layer of dune-turned-to-rock is a scant (geologically speaking) 85,000 years old. It’s barely rock at all, really, and in places is still sand, easily dug with the bare hand. An older group of layers, with ages of 125,000, 250,000, and 350,000 years (note the roughly 100,000-year periods) are well-cemented rocks, but can still be cut with a saw. The oldest layers, with ages 450,000 to greater than 900,000 years, are rock-hard.
There are two ways for the naturalist to estimate the age of any rock on Bermuda. The first is to hit the rock in question with a hammer. The youngest layer of rocks crumble, the moderately old ones dent, and the oldest ring like a bell. The other way to judge the age is to look at the nearest house. No doubt painted a improbable pastel, with a carefully whitewashed roof, there is a good chance that the house sits in a small quarry, with vertical rock faces on one, two or even three sides of the plot. If it looks like the plot was hewn from solid rock, it was. The moderately old rocks are an ideal building material. The nascent homeowner simply cuts out a suitable volume from the bedrock, shapes and cuts the resulting debris into building stone, and constructs the house in the now empty quarry. The youngest rocks provide building sand for the house, and the oldest rocks provide road aggregate for the driveway. On an island without wood or other building materials, such a range of materials seems almost miraculous.
The curious voyager can easily find beautiful and quite apparent examples of all these rocks across the island. Many quarries contain spectacular exposures (Government quarry, near St. George’s, is particularly recommended), as do the grottoes and caves across the island. Town Cut channel offers spectacular views of the youngest layers, but then the Cut is hardly the place for careful examination of anything, except maritime traffic.
Perhaps it is best to wait for landfall, and then tour the quay at St. George’s. Begin the tour by walking westward along the dock. The cliff to the north of the dock is composed of countless grains of broken corals, snails and other mollusks. Take the time to trace one’s finger along the sloping lines within the rock, and realize that, some 250,000 years ago, this was the top of a 20 foot high dune of white coral sand, blown in from the sea to your left.
Continuing along the dock, one comes to narrow, and busy, Wellington Street. Follow this to the right until one sees, on the northern side of the road, a bright reddish-gray streak of sand cutting diagonally across the rock. This strip of sand, examined closely, contains red-stained grains of quartz sand, thousands of shells of small land snails (used by naturalist Stephen J. Gould in his investigations of the nature and style of evolution) and large, blocky pieces of limestone. It looks and feels like the dirt one might find back home, and indeed it is. In fact, it may be from home. The shells and the limestone are native to Bermuda, but the dirt is notit has blown all the way from the Sahara and the central plains of North America. Deserts and soil
Bermuda is a lush, fragrant, and beautiful place. Indeed, arriving from sea, one can often smell the island before one sees it. Given the poor soil in Bermuda, its lushness is all the more remarkable.
The limestones are pure, and when weathered they leave little residue behind. This doesn’t leave much for making productive soil, and without rich topsoil, many plants (crops, for example) will fail to grow or thrive. Indeed, topsoil is imported into Bermuda from the mainland for use in backyard gardens. Recall this when buying provisions for the voyage home; the stiff price one pays is for both the vegetable and the boat which brought it over.
Bermuda, however, does have some home-grown soil. During inter-glacial times, high sea levels allow formation of new rock layers on the island. At other times, when sea level is low, the rocks of Bermuda are exposed to the atmosphere. Rain water slowly erodes the rocks, often dissolving them into craggy, cavity ridden masses, and plant roots dig their way into the rock, fracturing it even more. In truth, the effects of erosion on the island are easy to find: simply sit on any rock surface and count the holes in your clothes.
This erosion is accompanied by a process so remarkable that it needs a disclaimer: it is true. At 32° N, Bermuda is on the edge of two of the great atmospheric cells on earth. Somewhere south of 30° N, the Hadley Cell produces the northeast trade winds, essentially transporting air masses from Europe and Africa toward the southwest. North of 30° N, the Ferrel Cell (another Hadley-like cell) produces the Westerlies, which transport air masses from North America to the northeast. Bermudaunique againbenefits from both. During summer, the Bermuda High steers North African air toward Bermuda, while during the winter the air masses are North American. These air masses transport dust as well as air. Dust from Saharan dust storms is routinely collected in Miami, and can be followed on images from satellites. Dust from storms in Texas and New Mexico has also been followed across the Atlantic.
During glacial periods, the cooler, drier climate produces even more dust in these areas, dust that can easily travel between continents before settling to the surface. That red soil exposed on Wellington Street, and indeed all of the naturally occurring soils on Bermuda, are formed from dust blown to the island from the Sahara Desert, and from the (then) dry steppes of North America. Perhaps it is fair to say that all soil on Bermuda is imported, either naturally or artificially.
If this process seems unlikely, earlier alternatives are worse. In the early 1930’s, the presence of obviously foreign soils on Bermuda was variously ascribed to transport in the guts of migrating birds and the tentacles of jellyfish. But this transport is slow, even glacial, in tempo. The dust settles out at a rate of about 1 mm per thousand years, or about 10 cm worth during the 100,000-year glacial cycle. The repeating cycles of glacial and interglacial periods produces a similar cycle of limestones, soils, limestones and so on.
This cycle is splayed across at the entrance to (beautiful) Rock Hill Park, about three quarters of a mile west of St. George’s Town on Wellington Road.
Here the youngest limestone, the Southampton, was excavated down to the next limestone to allow a flat roadbed. On the vertical walls of the road cut, an arc of red soil curves down to the east. A single finger placed on this ancient soil touches grains of dust from two continents. It’s a sort of voyaging on the cheap!