Cromerian interglacial deposits at Sugworth, near Oxford, England, and their relation to the Plateau Drift of the Cotswolds and the terrace sequence of the Upper and Middle Thames

Abstract

Excavation for the Abingdon bypass road revealed at Sugworth, SW of Oxford, a dissected terrace feature, the Sugworth Bench, covered by a pebbly clay resembling the ‘Plateau Drift’ or ‘Northern Drift’ of the Cotswolds. This rested on Kimmeridge Clay or upon a number of sand and gravel-filled channels cut into the Kimmeridge Clay. These channels are considered to be ancient meanders of an old River Thames with a flow probably seven times that of the present river. One of these channels provided a fauna and flora that indicate a Cromerian Ill b age. The essential facts leading to this conclusion are given here, but detailed discussion of the vertebrates, beetles, ostracods, molluscs and palaeobotany are given in a series of separate papers. Mainly to the NW of Sugworth, extending as far as the Oolite scarp of the Cotswolds, occur isolated patches of what has been called Plateau Drift, with its coarser constituents dominantly of pebbles from the Triassic Bunter Pebble Beds of the west Midlands and with a much smaller content of flint. These sediments have been examined from the aspects of topographical position, pebble content and heavy mineralogy. The following conclusions are drawn: 1. Much of the Plateau Drift is water-deposited, but where it was originally gravel, decalcification by solution of limestone clasts, together with cryoturbation, may give it a spurious resemblance to till. This interpretation does not preclude the original occurrence of some true till. 2. The lowest occurrences of ‘Plateau Drift’ along the Evenlode Valley lie on a surface that is a continuation upstream of the Sugworth Bench. 3. The high level occurrence of Plateau Drift represents the original incursion of west Midland erratic material on to the Cotswold Scarp and its dip plane to the southeast and must be ascribed to glacial action, though it is probable that the more easterly outcrops represent fluvioglacial outwash rather than till. 4. Intermediate levels of Plateau Drift are suggested to be the result of redistribution of original material at various times between the early glaciation and the stage of the Sugworth Bench (i.e. Terrace). Correlation is attempted with the pre-Boyn Hill terraces and summit deposits of the Thames below the Goring Gap. The Sugworth Terrace, initiated in Cromerian Ill b time and probably completed in the Anglian, is correlated with the Winter Hill Terrace, possibly only with the Lower Winter Hill. The highest Plateau Drift is equated with the high level Westland Gravels of Hertfordshire, which show the first introduction of Bunter pebbles to the Middle Thames. Only the broadest of correlations can as yet be made between those deposits of intermediate level in the Middle Thames and the Evenlode. Both Hey and Turner, from evidence in eastern England, have suggested a Baventian Glaciation. The high level Plateau Drift Glaciation may also be of this period, but there is as yet no better estimate of its age than that it preceded Cromerian III b by a considerable period.