Is Old English for "Marsh" or "Marshy Ground", a term that applied to a large area of southern Lincolnshire, almost all of Cambridgeshire and parts of Norfolk and Northamptonshire. To quote the East Anglian Archaeology Society, "The Fenland region of eastern England was once the largest wetland in Britain." Although there are some hills and high ground in the area, many parts were virtual islands (Old Scandinavian holmr) at high tide or in the rainy season and the rest spent most of the year under water.
The Fens is a large area, which slowly drains towards the Wash, England's largest tidal estuary. The area abuts a number of other character areas. To the east lie North West Norfolk and the Breckland. To the southeast rises the gently rolling East Anglian Chalk and to the southwest are the undulating Bedfordshire and Cambridgeshire Clay lands. To the west, rise the gently shelving slopes of the Kesteven Uplands and the southern Lincolnshire Edge. To the northwest, the woods and gravel workings of the Central Lincolnshire Clay Vale gradually slope down to the Lincolnshire Fens, while due north the Lincolnshire Wolds rise to create a dominant 'Upland' horizon. The Steeping river marks the quieter northeast boundary to the Lincolnshire Coast and Marshes. The land boundary of the Fens is typically drawn along a series of catch water drains, dykes, canalised rivers and lodes.
The Fens Natural Area is a low-lying, level terrain, which rarely reaches 10 metres above sea level, except for fen 'islands' such as the Isle of Ely. The land is predominantly cultivated with little natural or semi-natural habitat remaining. Rich soils and varied intensive agricultural use emphasise the scale and geometry of the land and produce strong seasonal colour changes within the landscape. Woodland cover is very sparse with the majority of trees found lining roads and villages and shelterbelts. Marshes, swamps and fens add a distinct character to the area and provide outstanding habitats such as swamps, fen meadow and neutral and improved grasslands.
All the fens have artificial watercourses essential to drainage. These canalised features run in straight lines for miles exerting a strong influence in the landscape. Canals and dykes from the Roman period are still visible in some places.
The vegetation is often low growing, dominated by sedges, rushes and occasionally bog mosses.