Geology Site Account

Site description

Excavation of the cutting for the M25 Motorway at Belhus Park, Aveley, between 1979 and 1981, cut through sediments dating from a previously unrecognised interglacial stage now informally called the Purfleet Interglacial (Marine Isotope Stage 9) after the interglacial sediments at Purfleet Chalk Pits SSSI. It is one of four interglacials that have occurred in Britain since the retreat of the ice sheet at the end of the Anglian glacial stage. The interglacial sediments were found to consist of a bed of black organic clay and organic detritus resting on terrace gravels, and overlain by a variable sequence of sand, loam and clay, with seams of sandy gravel (Ward 1987). They appeared to occupy a broad channel, part of a former course of the River Thames about 300,000 years ago.

In the vicinity of Belhus Park bridge the organic seam contained abundant seeds of dogwood and cones of alder, underlain by shelly gravel with freshwater molluscs separated from the terrace gravel by a thin impervious seam of clay that had prevented the shells being dissolved by acid groundwater. The species present indicate a substantial river with clear, moving water and a rushy bank. Other inter-esting finds included fruits of water-chestnut, a perfectly preserved Scots pine cone, bones of pond-tortoise and a red squirrel molar (then only the third record from the Pleistocene of the British Isles), as well as species of freshwater fish similar to those found in rivers today. Laboratory sieving of bulk samples collected from the organic clay and associated shelly gravel has resulted in the recovery of at least 74 species of beetle as well as shells of small land snails washed into the sediment in times of flood.

The variable sediments overlying the organic clay yielded several flint implements, some in very sharp condition, that prove the presence of humans here during this interglacial stage. The largest hand-axe, found in situ in the west face of the cutting by Graham Ward in April 1981, is believed to have been the fifth largest yet found in Britain. The sediments containing the flint implements may date from the begin-ning of the subsequent cold stage (Marine Iso-tope Stage 8) (Bridgland 1994 p. 227 and Wymer 1999 p. 69)

Collecting fossils in the cutting of the M25 motorway at Belhus Park, Aveley when it was under construction in 1981. In this photograph the larger shells and seeds are being collected from the shelly sand from beneath the organic bed by wet-sieving. Bulk samples were then taken back to the laboratory where smaller shells, seeds and teeth of fish and rodents could be recovered. Photo © G.R.Ward.