The coastal region, extending about 16–48 km (10–30 mi) inland, consists of sand dunes, flat pasture land, and polders (land reclaimed from the sea and protected by dikes), and attains a maximum of 15 m (50 ft) above sea level. Eastward, this region gradually gives way to a gently rolling central plain, whose many fertile valleys are irrigated by an extensive network of canals and waterways. Altitudes in this region are about 60–180 m (200– 600 ft). The Ardennes, a heavily wooded plateau, is located in southeast Belgium and continues into France. It has an average altitude of about 460 m (1,500 ft) and reaches a maximum of 694 m (2,277 ft) at the Signal de Botrange, the country's highest point. Chief rivers are the Schelde (Scheldt, Escaut) and the Meuse (Maas), both of which rise in France, flow through Belgium, pass through the Netherlands, and empty into the North Sea.