Prehistoric wooden trackways of the Somerset Levels: their construction, age and relation to climatic change

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Abstract
The Somerset Levels are a submerged landscape. The valleys between the Mendip and the Polden Hills extend to 90 feet (27 metres) below Ordnance Datum, and they are filled to approximately the height of present sea-level by a soft blue-grey clay deposited in brackish water. Radiocarbon dating of material from Burnham-on-Sea shows that the marine transgression had been almost completed by 4300 B.C. (Q 134), and a date from Tealham Moor places the final transition to fresh-water conditions about 3500 B.C. (Q 120) (Godwin and Willis, 1959). As fresh water accumulated upon the almost level surface of the recent clay, reed-swamp dominated by Phragmites communis and Cladium mariscus occupied the whole region, building up coarse, loose-textured peat to a thickness of 1 or 2 metres. In the normal course of hydrarch succession (possibly enhanced by a climatic turn towards dryness), the reed swamps were invaded by fen-woods of alder and birch. These were extremely widespread at a time given by the first tentative radiocarbon dates as within the third millennium B.C.About 2000 B.C. there now began to develop within these fen woodlands an entirely different vegetation type, primarily composed of Sphagnum moss, cottongrass (Eriophorum spp.), ling (Calluna vulgaris), associated ericoid plants, and deer-grass (Trichophorum caespitosum). These communities quickly built up a complex of large raised bogs such as are now familiar around the Solway and the Central Irish plain, and of which Flander's Moss is a good Scottish example. These are gently domed structures rising a few metres above the level of the immediately surrounding land and relying almost entirely upon precipitation for their water supply.