Lithospheric flexure, uplift, and landscape evolution in south-central England

The high level river gravels in Oxfordshire (the Northern Drift Group), which range in age from Early Pleistocene to around 450 ka, contain pebbles that were derived from a source area to the north near Birmingham. The pebbles could not have been transported across the Upper Triassic and Lower Jurassic clay outcrops of the Midlands unless the region was at or close to sea level. The present day Cotswold escarpment ranges from 70 to 215 m: this uplift must therefore have occurred after deposition of the Northern Drift Group, when the drainage of the Midlands appears to have shifted from towards the Thames basin to the Bristol Channel area. It is concluded that lithospheric flexure due to the removal of >500 km(3) of soft Late Triassic and Early Jurassic clays and marls from large areas of the Midlands and their re-deposition in the Celtic Deep might account for this Late Pleistocene uplift. Uplift in the Midlands is the likely explanation for both the tilted (approximately 0.3 degrees) plateau surface in north Oxfordshire and the change in strike of the Jurassic beds SW of Northamptonshire. Although some uplift occurred in SE England during the Early Cenozoic and Miocene, we demonstrate here that there has been significant uplift of the region in the Late Pleistocene. The tilted plateau surface in north Oxfordshire may therefore be a relatively young (i.e. post-450 ka) feature related to tectonic uplift in the Midlands.