Page 217 - Sedimentology and Stratigraphy

204 Clastic Coasts and Estuaries

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Fig. 13.7 A wave-dominated coastline with a coastal plain bordered by a sandy beach: chenier ridges are relics of former
beach strand plains.

The seaward margin of a barrier island has a beach The conditions required for a barrier to form are as
and commonly a beach dune ridge where aeolian follows. First, an abundant supply of sand or gravel-
processes rework the sand. Vegetation helps to stabi- sized sediment is required and this must be sufficient
lise the dunes. On the landward side of the island the to match or exceed any losses of sediment by erosion.
layers of sand deposited during storms pinch out into The supply of the sediment is commonly by wave-
the muddy marshes of the edge of the lagoon. During driven longshore drift from the mouth of a river at
storm surges seawater may locally overtop the beach some other point along the coast and there may also
ridge and deposit washovers of sediment that has be some reworking of material from the sea bed
been reworked from the barrier and deposited in the further offshore. Second, the tidal range must be
lagoon (Fig. 13.8). Washover deposits are low-angle small. In macrotidal settings the exchange of water
cones of stratified sands dipping landwards from the between a lagoon and the sea during each tidal cycle
barrier into the lagoon. would prevent the formation of a barrier because a

Fig. 13.8 A wave-dominated coastline with a beach-barrier bar protecting a lagoon.