T here were three main ceremonial centers-San Lorenzo, La Venta, and Tres Zapotes. The first of them, San Lorenzo, was built during the 12th century B.C. It took the form of a vast man-made mound built over a natural plateau, apparently in the form of a huge bird in flight. On it were set pyramids and courtyards, sprinkled with ponds and canals to provide water for ritual bathing. Eight gigantic stone heads were also put up in the complex. They were made from basalt, which was brought from the Tuxtla Mountains 50 miles (80km) away. It is thought that the stone heads were probably portraits of the Olmec rulers. They and the elite of Olmec society lived here, directing the efforts of the vast peasant labor force who constructed the monuments.