Grazing marsh was created from Medieval times by building sea walls (earth banks) across tidal mudflats and saltmarsh to make polders (although the term "polder" is little used in Britain). Polders in Britain are mostly drained by gravity, rather than by active pumping – the original tidal drainage channels were augmented by new ditches, and flap valves in the sea walls allow water to drain out at low tide, but prevent the sea or tidal river from entering at high tide. The process of constructing polders in this way is called inning or reclaiming from the sea.
Grazing marsh has 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 marsh has also been made on open coasts where these are very low-lying.
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. The land levels either side of a counter wall often differ by several metres – paradoxically the lower side is the land inned earlier, because sediment could continue to build up on the side which remained tidal.
Wintering wildfowl are characteristic of grazing marshes, often including large flocks of Wigeon, Brent Goose, White-fronted Goose and Bewick's Swan. Many of these birds are hunted by predators such as Peregrine and Marsh Harrier.
The ditches often have a range of salinity depending on how close to the sea wall they are. In the more saline ditches live specialist brackish-water plants and animals. These include for example the rare brackish amphipod Gammarus insensibilis and sea club-rush (Bolboschoenus maritimus). Fresher ditches may support rare animals such as the great silver water beetle (Hydrophilus piceus) and the great raft spider (Dolomedes plantarius), and a wide range of pondweeds (Potamogeton and relatives).
The grassland vegetation usually has a fairly small number of species, but those which do occur are often scarce elsewhere, such as sea arrowgrass (Triglochin maritimum), divided sedge (Carex divisa) and strawberry clover Trifolium fragiferum.
Many grazing marshes have been converted into arable land, often using pumped drainage to lower the water levels enough to allow crops to be grown. The low ditch levels and agricultural runoff combine to remove much of the aquatic wildlife, although the arable fields may still be used by some wintering wildfowl.
Some areas of grazing marsh and other polder land have been used to recreate tidal habitats by a process of managed retreat.