Late Pleistocene deposits at Wretton, Norfolk. I I. Devensian deposits

Abstract

The low terrace at Wretton, Norfolk, is shown to consist of up to 6 m of mainly fluviatile sands and gravels of Devensian age. It contains many organic horizons rich in plant remains, molluscs and bones, and shows well-developed periglacial structures, including involutions and ice-wedge casts. Organic deposits lie in channels beneath, within and above the fluviatile sediments, and also within small depressions, a few metres across, which are interpreted as features formed by the melting of ground-ice mounds. The succession within the terrace is complex, but the detailed sections observed, with petrographical, palaeobotanical and molluscan studies, allow a synthesis of the environmental changes which accompanied terrace formation. The petrography of the sediments in the depressions indicates that certain of the ground-ice mounds formed in lenses of sandy clay derived from weathered interglacial sediments. The sediments redeposited on the melting of the mounds have a characteristic particle-size distribution. Times of coversand formation within the terrace sequence are identified. The palaeobotanical study, by analysis of pollen and macroscopic plant remains, reveals a sequence of pollen assemblage biozones. There are three periods with biozones characterized by herb pollen spectra. These are separated by periods with biozones characterized by pollen spectra indicating the presence of woodland. The earlier sequence of the woodland biozones, named the Wretton interstadial, shows birch-pine woodland and heath. The later sequence shows pine-birch-spruce woodland and heath, and is correlated with the Chelford interstadial. A detailed consideration of the flora and vegetation of the herb biozones is given, with a comparison of the pollen spectra with recent pollen spectra from the Arctic. It is concluded that in these biozones vegetation physiognomically akin to grassland prevailed in the region. Molluscan faunas, found at several horizons, are typical of Early Devensian deposits in other parts of the Fenland drainage basin. The number of species is restricted, but southern forms do survive in small numbers from the Ipswichian interglacial even into the third grassland biozone of the Devensian. On the basis of the periglacial features and plant and animal remains, it seems that the climate was generally continental, with mean annual temperatures at or below 0$^\circ$C, probably as low as -8$^\circ$C at times of ice-wedge formation, with mean July temperatures as high as 15 to 20$^\circ$C. In the pollen-based interstadials the climate was considerably more oceanic. The methods of subdivision of the Devensian are considered, and the sequence then correlated with other Early Devensian sites in England and on the Continent. The Wretton and Chelford pollen-interstadials may be of the same age as the Amersfoort and Brorup interstadials respectively of the Netherlands. The associated herb biozones represent periods before, between and after the pollen-interstadials. The transition from the Ipswichian interglacial to the Devensian in the region of the eastern Fenland margin is considered. Grassland was already present in the late Ipswichian, and the transition to the Devensian was accompanied by a loss of some taxa and a gain of others, with many taxa present in both stages. The coleopteran and moss assemblages from the deposits are described in appendices. There is an unresolved discrepancy between the environmental evidence of the Coleoptera and the pollen in the later woodland biozone, the Coleoptera indicating a barren sandy landscape.