For the most part, Bermuda’s limestones comprise aeolian (wind-blown) sand dunes. These dunes were built of bioclastic sand (broken skeletal remains of marine life) transported by the wind from beaches. Initially, the sand was trapped by vegetation in foredunes and stabilized at the back of beaches. However, despite the characterization of Bermuda dunes as immobile, beach-tied coastal ridges (Vacher, 1972), it seems that under certain conditions, sand was released from the back beach as “trangressive” dunes which migrated inland. Such processes have been observed in Bermuda in historic times, for example at Elbow Beach where clearing of coastal brush early in the 19th century caused dunes to climb as high as 45 metres (150ft) above sea level and to bury all but the chimney of a cottage. Dune migration, as opposed to simple vertical accumulation, is also implied by the significant inland extent of individual limestone dune formations (Geological Map, 1989), and by a bedding structure known as “cross bedding”, which is characteristic of Bermuda’s limestone dunes (see photo, below).
Internal structure of an aeolian dune at Barker’s Hill (photo above).
This large scale cross bedding is indicative of dune migration. The truncated slip-face beds (or foresets) represent the leeward face of a large dune, down which sand cascaded. The truncation surface represents a period of sediment starvation, wind erosion and forward advance of the dune. Subsequent low angle windward strata, on what is termed the “stoss” slope of the dune, are the product of a resurgence in sediment supply.
Direction of sand transport is from right to left (north to south).