Though much of the Amazon basin is covered in tropical forest there are also large areas of grassland, which are often flooded for much of the year. Scattered over some of these, such as the Llanos de Mojos in the south-west, are the remains of ancient field systems.
Long drainage ditches were dug and the soil from them mounded up in long ridges on which the crops were grown. Often this brought to the surface soils more fertile than the topsoil. Raised above the level of the floods, the crops had a longer growing season. Silts dug out of the drainage ditches when they were cleaned were added to the ridges, fertilizing the soil.
The ditches provided a reservoir of water during the dry portion of the year and would also support fish and other aquatic resources, providing an invaluable source of protein. In some areas these raised fields grew maize and beans, in others manioc (a starchy rootcrop).
Although they are hard to date exactly, most of these field systems were probably built around AD 500. Among the fields other raised areas were mounds supporting villages and their cemeteries, and causeways giving access to fields and settlements.
