The history of the Fens as a wetland landscape began around 10,000 years ago when melting glaciers caused sea levels to rise and Britain became an island.
Marine and estuarine clays and silts were deposited as the sea underwent a succession of advances and retreats. These formed the ‘Silt Fens’.
In other areas where the sea had a less direct influence, high sea levels and the deposition of marine sediments had the affect of slowing the movement of water off the land and into the Wash. This caused the rivers to flood and create boggy areas. It was under these conditions that the deep peat soils of the “Black Fens” were formed. Even today the remains of the ancient oaks that preceded the wetlands are occasionally dug out of the peat and sold on as “bog oaks”.
The single obvious factor uniting the Fens is the low lying, level terrain. Much of the land is below sea level, relying on pumped drainage and the control of sluices at high and low tides to maintain its agricultural viability.
The underlying geology of the Fens is a combination of post glacial alluvium and freshwater clays and post Roman marine clays. The exception being elevated islands of Jurassic clay at the inner margins of the area, for example at Ely.
The soils over the central and coastal fens are rich, fertile, stoneless, calcareous, silty soils while inland are swathes of dark, friable, fen peat. The original courses of the rivers meandered slowly across the level fens causing widespread seasonal waterlogging by river water and high tides. In some locations ‘roddons’, inland silt banks, mark the former course of old river beds and now lie like stranded sea serpents up to 2-3 m above the dark peat soils which have subsequently shrunk due to continuous cultivation, drainage and wind erosion of the peat.
Although at first acquaintance the Fens can appear to be similar, there are marked variations and graduations as one moves from fen to fen and, more noticeably, between areas with differing lengths of settlement history. There are three broad distinctions within the Fens.
The ‘Settled fens’ or ‘Townlands’ run in a broad arc inland from the Wash, between Kings Lynn and Boston. An ancient smallscale landscape of sinuous lanes and relative intimacy with a higher density of settlements, some fine churches and remnant grasslands.
The extensive ‘Peaty Fens’ or ‘Black Fens’. This area has broad rectilinear fields and straight roads. The only consistent relief to the level landform are the ‘negative’ notches of the drainage ditches and the raised berms and banks of the artificial drainage channels. Within the broad area of peat fens lie a few isolated islands of higher ground, most notably the Isle of Ely.
The fens of southeast Lincolnshire between the Townlands and the Wolds. The last area to be drained, completed by 1820, having been accelerated in the agricultural and industrial revolution. The drainage here was so thorough that scarcely a vestige remains of what had been one of Britain’s richest wildlife habitats. This is an open productive landscape with a strongly rectilinear form.