The Somerset Levels (or Somerset Levels and Moors as they are less commonly but more correctly known) are a sparsely populated wetland area of central Somerset, between the Quantock and Mendip hills. They consist of marine clay levels along the coast, and the inland (often peat based) moors. The Levels are divided into two by the Polden Hills; land to the south is drained by the River Parrett while land to the north is drained by the River Axe and the River Brue. The total area of the Levels amounts to about 647.5 square kilometres (160,000 acres) and broadly corresponds to the administrative district of Sedgemoor but also includes the south west of Mendip district. Approximately 70% of the area is grassland and 30% is arable. Stretching about 32 kilometres (20 mi) inland, this expanse of flat land barely rises above sea level. Before it was drained, much of the land was under a shallow brackish sea in winter and was marsh land in summer. Drainage began with the Romans, and was restarted at various times: by the Anglo-Saxons; in the Middle Ages by the Glastonbury Abbey, from 1400–1770; and during the Second World War, with the construction of the Huntspill River. Pumping and management of water levels still continues.