WITHIN historic times, the English fenland stretched over the greater part of the area to the west and south of the Wash, extending as far north as Lincoln and as far south as Huntingdon and Cambridge. On the seaward side, the surface deposits are semi-marine silts, laid down, and afterwards occupied, during the Romano-British period. On the landward side, the upper layers are peat produced by discharge of the flood waters of the Rivers Witham, Welland, Nene and Ouse into the extensive shallow basin of the fens. The fen peats are alkaline and therefore support a vegetation of the true 'fen' type. Very little of the original vegetation of the peat fen remains, however, since the whole area has been drained and brought under extensive cultivation. Its present characteristics are the black peaty soil, uniform flatness and deep ditches full of reeds (Phragmites communis) which separate fields of cereals, potatoes and sugar beet.