Interglacial marine deposits of the Somerset Levels, South West England

Abstract

Sands and gravels underlain by silts and clays, known as the Burtle Beds, from the Somerset Levels of South West England have been examined in a specially excavated pit. The Mollusca, Foraminifera and the Ostracoda, and the sparse macroscopic plant remains included in the deposit, all indicate a prgressive marine transgression in which water temperatures obtained approximating to those of the present sea in the adjacent Bristol Channel. The deposits are shown to be estuarine/marine near shore and intertidal sediments of interglacial age. Radiometric assays and studies of palaeomagnetism have failed to indicate clearly which interglacial marine transgressive phase is involved. Geomorphological considerations supported to some extent by the Ostracoda suggest that a last interglacial (Ipswichian) age is more likely than an earlier (Hoxnian?) date. Estimates of the height of sea levels involved are, of necessity, based on evidence which cannot be regarded as wholly reliable. Having regard to the environmental conditions indicated by the included fauna, and making allowance for post depositional erosion, it is suggested that mean sea level at the height of the ‘Burtle’ transgression was between 9 and 12 m above that of the present (O. D.). Assuming a similar tidal range to that of today, a MHWST (Mean High Water of Spring Tides) level of between 15 and 18 m above O. D. is indicated