Studies of the post-glacial history of British vegetation XVI. Flandrian deposits of the fenland margin at Holme Fen and Whittlesey Mere, Hunts

Stratigraphic survey, pollen analysis, micro- and macrofossil plant identification and radiocarbon dating have been employed to elucidate the origin and character of the deposits of the western margin of the East Anglian Fenland within the area of Holme Fen and the former Whittlesey and Trundle Meres, Huntingdonshire. As a consequence of waterlogging, alder fen-woods became established upon the Jurassic Clay fen floor some 6000 years ago; they were succeeded in turn by eutrophic Cladium-dominated fen which gave place to floating ‘scraw bog’ and ultimately to acidic raised bog, that persisted in Holme Fen and round Whittlesey Mere until after A.D. 1850. A considerable number of acidicolous flowering plants and mosses are now recorded sub-fossil from these deposits. Pollen analyses disclose the characteristic 'elm-decline’ attributable to early Neolithic activity and much more extensive woodland clearance and agriculture in the middle Bronze Age when thin clay layers intrusive into the raised bogs and radiocarbon dated to 1400 B.C., are interpreted as the consequences of soil erosion and flooding from the adjacent uplands. Failure of these clay bands to enter Whittlesey Mere basin is attributed to the presence of the high dome of the surrounding raised bog. Still more extensive agricultural activity is indicated by the pollen analyses in the later periods. The growth of raised bog was interrupted in the Whittlesey Mere basin by the sudden incursion of clays and silts deposited in salt or brackish water, the landwards facies of the ‘Fen Clay’ of all the South Level area of the Fens. Again presumably because of the barrier of high raised bog round Whittlesey Mere these minerogenic deposits did not extend either into Holme Fen or into the basin of Trundle Mere. After an interval of marine retrogression, referable to a period embracing the Bronze Age-Iron Age transition and during which freshwater peat formed over the clays and silts in Whittlesey Mere basin, renewed flooding caused the deposition of the fresh-water shell marl whose outline represents the extent of the former Trundle and Whittlesey Meres. It seems clear that this was the consequence of backing up of fresh calcareous water from the uplands against the banks of the natural channel of the River Nene raised by tidal action in the renewed marine transgression of the Iron Age and Romano-British period. Historical and physiographic evidence indicates that the river formed the northern bank of Whittlesey Mere, and that the mere was related to the natural river in the same way as Red Mere in the South Level and Ugg, Brick, Ramsey and Benwick Meres in the Middle Level. The effects of artificial drainage (especially that of Whittlesey Mere since ca. 1850) are followed: the great and well documented loss in surface height is attributed to initial compaction and subsequent wastage, and the northern half of the bed of Whittlesey Mere is shown to be at a higher level than the southern half on account of the clays and silts that underlie only the northern part. Development in the Whittlesey-Holme area is tentatively correlated with that already established for the Woodwalton Fen area of the Fenland margin contiguous to the south and with the South Level as a whole.