Late Flandrian shoreline oscillations in the Severn Estuary: the Rumney formation at its typesite (Cardiff area)

Abstract

The shore of the former tidal wetland northeast of Cardiff, called the Wentlooge Level and reclaimed during the Roman period, exposes a series of late Flandrian lithostratigraphic units dominated by estuarine clays. Chief among these are the (upper) Wentlooge Formation (up to 2.5 m thick) and the Rumney Formation (up to 2.7 m thick). The (upper) Wentlooge Formation consists of greenish grey slightly silty clays and is capped by an immature palaeosol yielding signs of clay illuviation. Extending down from a level within the palaeosol is a reticulate system of deep drainage ditches, some of which are locally filled with Romano-British cultural debris, regarded as dating their excavation. Such fills are sealed by the soil. Marine erosion after the Roman period but before late mediaeval times destroyed much of the upper Wentlooge Formation together with a presumed Roman sea defence. The then unprotected drainage ditches were invaded during this episode of coastal retreat and widened out to form a series of large embayments that defined flat-topped headlands on which the palaeosol formed a capping. The overlying Rumney Formation is a series of pink estuarine silty clays with some sands and gravels. It infills the embayments of the coastline cut in the Wentlooge beds and smothers its headlands to a thickness of about 1.4 m. Deposition in the embayments began in the 15th century, but on the headlands was delayed for a further 100-200 years. The outward movement of the shore recorded by the mudflat and marsh deposits of the earlier part of the Rumney Formation was eventually reversed with the formation of a bold mud cliff that has continued to retreat inland to its present position. On the upper part of this cliff are exposed sand sheets which record sand bodies formed during the retreat, at the heads of inlets, as pocket beaches, and as landward-facing bars cast up on to the salt marsh during high tides and storms. Two younger lithostratigraphic units of recent date, the Awre Formation and the Northwick Formation, have stratigraphical relations similar to the Rumney Formation and record further movements of the strand. The coastal oscillations recorded by the lithostratigraphy are attributable to fluctuations in the role and strength of waves at the shore, as governed by either medium-term weather changes or the positions of offshore shoals, or by the morphology of the shore itself.