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grazing marsh
Grazing marsh is a British Isles term for flat, marshy grassland in polders. It consists of large grass fields separated by fresh or brackish ditches, and is often important for its wildlife.
==History==
Grazing marshes were created from Medieval times by building sea walls (earth banks) across tidal mudflats and salt marsh to make polders (though the term "polder" is little used in Britain). Polders in Britain are mostly drained by gravity, rather than active pumping. The original tidal drainage channels were augmented by new ditches, and flap valves in the sea walls let water drain out at low tide and prevent the sea or tidal river from entering at high tide. Constructing polders in this way is called ''inning'' or ''reclaiming'' from the sea.
Grazing marshes have been made in most lowland estuaries in Britain, often leaving only the river channel and the lowest part of the estuary tidal. In a few cases (such as Newtown Harbour on the Isle of Wight, and Pagham Harbour in West Sussex) the sea walls have been breached, and the estuaries have returned to a tidal state. Grazing marshes have also been made on low-lying open coasts.
Many grazing marshes were inned in stages, and the old sea walls (called ''counter walls'') may be found marooned far from the current sea wall. Land levels on either side of a counter wall often differ by several metres. Paradoxically, the lower side is the land inned earlier, because sediment continued to build up on the side that remained tidal.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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