Above sea level, the Bermuda islands are for the most part built of wind-blown sand dunes, known as aeolianites, which have been cemented into limestone rock Only a small minority of the limestones exposed at the land surface were originally deposited by the sea, for example as beaches.
Below sea level, the limestones extend to depths of approximately 50 meters where they rest on a truncated volcanic seamount, which is thought to have been largely built during at least two eruptions between 45 and 33 million years ago.
The limestones are primarily of Pleistocene age (less than 1.7 million years old) and were deposited in response to dramatic cycles of glacial and inter-glacial conditions during this epoch. Bermuda, alternately, emerged and was submerged by the 300 ft (150 m), or so, fluctuations of sea level. Dune building was most active when the high, inter-glacial seas flooded onto the Bermuda Platform (see below). During the glacial episodes, however, when the island was fully emerged and at its maximum size, this limestone deposition was replaced by vegetation growth and the development of soil. The end result was Bermuda’s characteristic geology of sandy, limestone aeolianites interlayered with palaeosols (fossil soils).
Under the inter-glacial conditions of today, Bermuda is reduced to a relatively narrow strip of reef-protected islands confined to the southern rim of the Bermuda Platform (see below).