The Fens - Formation and Geography

Formation and Geography

At the end of the most recent glacial period, known in Britain as the Devensian, ten thousand years ago, Britain and continental Europe were joined by the ridge between Friesland and Norfolk. The topography of the bed of the North Sea indicates that the rivers of the southern part of eastern England flowed into the Rhine, hence through the English Channel. From the Fens northward along the modern coast, the drainage flowed into the northern North Sea basin. As the ice melted, the rising sea level drowned the lower lands, leading ultimately to the present coastline.

These rising sea levels flooded the previously inland woodland of the Fenland basin, and over the next few thousand years led to extensive development of both saltwater and freshwater wetlands. Silt and clay soils were deposited by marine floods in the saltwater areas and along the beds of tidal rivers, while organic soils, or peats, developed in the freshwater marshes. Fenland water levels peaked in the Iron Age; earlier Bronze and Neolithic settlements were covered by peat deposits, and have only recently been found. During the Roman period, water levels fell once again, and settlements were possible on the new silt soils deposited near the coast. Though water levels rose once again in the early medieval period, by this time artificial banks protected the coastal settlements and the interior from further deposits of marine silts, though peats continued to develop in the freshwater wetlands of the interior fens.

The wetlands of the fens have historically included:

• Washes: these are places such as tidal mud flats and braided rivers, which are sometimes exposed and at other times covered with water.
• Salt marsh: this is the higher part of a tidal wash, on which salt-adapted plants grew.
• Fen: this is a broad expanse of nutrient-rich shallow water in which dead plants do not fully decay, resulting in a flora of emergent plants growing in saturated peat.
• Moor: this developed where the peat grew above the reach of the land water which carried the nutrients to the fen. Its development was enabled where the fen was watered directly by rainfall. The slightly acidic rain washed the hydroxide ions out of the peat, making it more suitable for acid-loving plants, notably Sphagnum species. This is exactly the same as bog, but the word moor was applied to this acid peatland occurring on hills. These moors disappeared in the nineteenth century, and it had been thought that the Fenland did not have this kind of peat, until archeological and documentary evidence demonstrated that it did until the early nineteenth century.
• Waters: these have included:
  • tidal creeks, which reached from the sea into the marsh, the Townlands and in some places, the fen, and seem to have been named only if big enough to be regarded as havens;
  • meres, or shallow lakes which were more or less static but aerated by wind action;
  • many rivers, both natural and (from Roman times on) artificial.

Major areas for settlement were:

• the Townlands, a broad bank of silt (the remains of the huge creek levees that developed naturally during the Bronze and Iron Ages), on which the settlers built their homes and grew their vegetables;
• the fen islands: areas of higher land which were never covered by the growing peat;
• the fen edges: uplands surrounding the fens.

In general, of the three principal soil types found in the Fenland today, the mineral-based silt resulted from the energetic marine environment of the creeks, the clay was deposited in tidal mud-flats and salt-marsh, while the peat grew in the fen and bog. The peat produces black soils, which are directly comparable to the American muck soils. A roddon, the dried raised bed of a watercourse, is more suitable for building than the less stable peat.

Since the 19th century, all of the acid peat in the Fens has disappeared; drying and wastage of peats has greatly reduced the depth of the alkaline peat soils and reduced the overall elevation of large areas of the peat fens.