A detailed pollen diagram from an East Anglian Lake, Old Buckenham Mere, registers vegetational changes from Late Glacial time to the present. When a chronology is projected upon it this allows the reconstruction of the effects of historic and prehistoric man upon local vegetation through the last 5,000 years. Neolithic influence was slight, but in the Late Bronze Age and pre-Roman Iron Age there was progressive forest clearance associated with pasturage. In Anglo-Saxon time arable cultivation extended greatly upon heavier soils. Secale (rye) was cultivated from the Roman time and from Early Anglo-Saxon time there is a substantial continuous curve for a pollen grain recognisable as that of Cannabis sativa (hemp), upon microscopic criteria newly defined. It was locally cultivated in large amount in Late Saxon and Norman time when the region was heavily populated and in Tudor time cultivation of this crop was enforceable by law.