Pleistocene history of the Vale of St. Albans

Abstract

The stratigraphy of the Pleistocene sequence in the area of the Vale of St Albans has been investigated and the results used to link the sequences in East Anglia and the Middle Thames region. Lithological units have been characterized using various methods and thirteen formal lithostratigraphical units are proposed. The bulk of the deposits are included in the Anglian glacial stage during which two major ice advances occurred. Both advances dammed the eastward-flowing river in the Vale causing, respectively, a minor and a more extensive proglacial lake. The river was finally diverted and after the ice retreat a westward-flowing river occupied the Vale. Organic remains fill kettle holes resting on the glacial deposits. A new site at London Colney shows a late Anglian to Hoxnian Ho1 pollen sequence. Wolstonian soliflucted gravels overlie the hollow fills. It is probable, since pre-Ipswichian and Ipswichian deposits occur near the present river level, that the modern river valleys were initiated during this stage. Overlying the Ipswichian deposits in river valleys and on interfluves is brown silt, containing a substantial loess component. This is considered to represent early Devensian river aggradation. A period of downcutting then occurred, before the accumulation of valley gravels which contain organic horizons radiocarbon-dated to the Late Devensian. The sequence is extended north to Stevenage and Hitchin and east into Essex. The Middle Thames Terrace sequence is equated with the Vale using relative heights and lithology, showing that the Thames flowed through the Vale until late Anglian times, when it was diverted southwards and took up a course close to that which it follows today.