Sand Dunes
Sand dunes (sandhills) are formed from windblown sand blown off exposed sandy beaches. The sand accumulates into ridges which originally lie parallel to the direction of the prevailing winds. Over time these ridges may break up and a more irregular arrangement of hills (dunes) and hollows (dune-slacks) forms. The formation of sand dunes is not continuous - those around the N. Ireland coast were formed thousands of years ago and little or no new dune formation now takes place. In addition, erosion by the sea can “bite off” portions of dune systems closest to the shore, a process likely to accelerate with the projected sea-level rise associated with rising global temperatures.
Sand dune systems are of great importance to biodiversity, and many are conserved. The most important sand dune systems in Northern Ireland are:
Co Down: the Dundrum Bay dunes, including Murlough Dunes National Nature Reserve, Ballykinler dunes, Tyrella dunes; also other, poorer quality, dunes along the outer coast of the Ards Peninsula eg at Ballywalter.
Co Antrim: Whitepark Bay, the Bushfoot dunes, Portrush and Whiterocks, Ballycastle Bay. Damaged dunes at Red Bay.
Co Londonderry: Portstewart and Castlerock dunes on either side of the Bann Estuary; the Magilligan-Umbra-Downhill dunes.
Nearest the sea shore, the dunes are usually dominated by marram grass (Ammophila arenaria) and the vegetation is ‘open’, ie with a lot of bare ground between the marram plants. Further back from the sea the vegetation is ‘closed’, the marram cannot survive, and the dune surface is covered by various other kinds of vegetation and plant community, many of which are grass-dominated (especially by Festuca rubra), while others are dominated by bryophytes, lichens (‘grey dunes’), or bracken (Pteridium aquilinum). The younger dunes are rich in calcium carbonate from seashells, allowing lime-loving species to flourish, but older parts of most dune systems are acidic, owing to the leaching effect of rain, and can support plants such as heathers (Erica spp. and Calluna vulgaris). The inland portions of many dune systems have been used as pine plantations.
Dune Slacks
The hollows between dunes are called slacks. Often, but not always, they are wet and marshy and many of the plants found in them are also characteristic of base-rich fens and lake-margins.
Blow-outs
Damage to the vegetation surface allows the wind to excavate hollows called blow-outs, which can eventually develop into new slacks.