Relief throughout Ghana is generally low, with altitudes nowhere exceeding 3,000 feet (900 metres). The southwestern, northwestern, and extreme northern parts of the country consist of a dissected peneplain a land surface worn down by erosion to a nearly flat plain, later uplifted and again cut by erosion into hills and valleys or into flat uplands separated by valleys; it is made of Precambrian rocks (from 570 million to 3.8 billion years old). Most of the remainder of the country consists of Paleozoic deposits (from 245 to 570 million years old), which are thought to rest on older rocks. The Paleozoic sediments are composed mostly of beds of shales (laminated sediments consisting mostly of particles of clay) and sandstones in which strata of limestone occur in places. They occupy a large area called the Voltaian Basin in the north-central part of the country where the altitude rarely exceeds 500 feet. The basin is dominated by Lake Volta, an artificial lake that extends far into the central part of the country behind the Akosombo Dam and covers 3,275 square miles