small, stationary, artificial island built on a freshwater lake for agricultural purposes. Chinampan was the ancient name for the southwestern region of the Valley of Mexico, the region of Xochimilco, and it was there that the technique was—and is still—most widely used. It consists in building up a number of narrow islands, each averaging some 6 to 10 metres (20 to 35 feet) wide and some 100 to 200 metres (325 to 650 feet) long, using layers of vegetation, dirt, and mud. The lake provides the chinampa with moisture laden with decomposing organic wastes that irrigate and fertilize the island's soil, supporting an intensive and highly productive form of cultivation.