Interior Plains

a plains region in the central part of South America, from 10° to 39° S lat. It includes the natural areas the Mamoré Plains, the Pantano, Gran Chaco, and Entre Rios plains, and the Pampas. The region is basically made up of aggradation plains (with elevations of 50-70 m in the east to 500-600 m in the west), on the site of the tectonic trough between the Brazilian Highlands in the east, the Andes and the Pampas Sierras in the west, and Patagonia in the south. The plains are composed chiefly of a thick stratum of Cenozoic continental sediments characterized primarily by fluvial genesis in their upper portion. In the south loess is widely found. The plain is interrupted only by ancient folded-block mountains located between 18° and 20° S lat. (up to 1,425 m) and in the southern Pampas (up to 1,243 m). There is a characteristic sequential shift of regional landscapes from the subequatorial savanna forests and savannas in the north, through the tropical sparse forests in the center, to the subtropical savannas, prairies, and steppes in the south. Along the Paraguay and Paraná rivers there is a belt of marshy forests.