Professor Sir Clifford Darby’s county, regional and national maps of a range of data drawn from the Domesday Book revolutionized scholarship on the social and economic history of late Anglo-Saxon England (e.g. 1935, 1936a, 1936b, 1971, 1977). While this paper does not seek to challenge Darby’s general conclusions, a case study re-examination of the inter-relationship between population density and physical geography in the Cambridgeshire fenland in 1086 suggests the regional usefulness of methodological adjustments to his mapping. It indicates that the population density of the peat and silt fens in the late eleventh century may have been significantly higher than that shown in Darby’s original maps, with implications for the contemporary social and economic history of eastern England.