The Minas Basin is a semi-enclosed inlet of the Bay of Fundy. It consists of a triangular area of tidal mudflats and sandflats, surrounded by patches of salt marsh, most of which have been diked and drained for agriculture. Twice a day, the sea fills and empties the basin, rising and falling by over 40 ft (12 m), which is the largest tidal range in the world. No other coastal marine area has such a large proportion of its floor exposed at low tide. Sediments in the basin, which are brought in and deposited by tides, range from coarse sand to fine silt and clay. The tidal flats formed by these sediments contain high densities of a marine amphipod, the Bay of Fundy mud-shrimp, which provides food for huge numbers of migrating shorebirds, including sandpipers and plovers. The numbers peak from July to October, and for some species exceed 1 percent of the world population.