Late Devensian river deposits beneath the Floodplain Terrace of the River Thames at Abingdon, Berkshire, England

Abstract

An organic channel-fill deposit in gravel and sand exposed at Thrupp House Farm quarry is described. The gravels and sands were laid down in a bed-load dominated river under a cold climate regime. Two radiocarbon dates of 13,260 ± 180 BP and 13,580 ± 120 BP were obtained and indicate that the deposit accumulated during the Late Devensian substage. However, the abundance of certain members of the insect fauna, particularly Pycnoglypta lurida and Olophrum assimile suggest that the deposits may date from the latter half of the Windermere Interstadial, i.e. the C14 dates may be 1000 yr. too old. The discrepancy is attributed to inwashed, older carbonaceous material. This is supported by the find of Pinus bark, radiocarbon-dated at ℂ q 2 . The organic deposit accumulated in a floodplain channel with a variable water level. Initially, shallow water supported a rich aquatic and marsh vegetation with a diverse beetle fauna. Later, fluctuating water level led to a decline in species diversity, meadow vegetation invaded the pool and was occasionally damaged by fire. Finally, slightly higher water levels returned and semi-aquatic vegetation colonised the pool area with a range of beetle species present. Little can be judged of the regional vegetation. New Devensian plant records include Sagittaria natans Pall., Apium inundatum (L) Reichb. F., Naumburgia ( Lysimachia ) thyrsiflora (L) and Ranunculus platanifolius L. The climate is thought to have deteriorated during deposition of the organic bed. The relation of the sediments to those previously described from the Floodplain gravels of the Upper Thames Valley is discussed and their significance in dating the terrace aggradation within the Thames Valley as a whole is considered.