Plant macrofossil analysis of Holocene alluvium, with special reference to the Lower Thames Basin
Alluvium is an important archaeological and palaeoenvironmental resource in lowland Britain. The research presented here develops plant macrofossil analysis of alluvial facies, with special emphasis on the depositional and natural environments of the Lower Thames Basin. Plant macrofossil analysis is a poorly developed area of alluvial research, usually limited to superficial description of the fossils seen in section, or detailed analysis of a narrow suite of remains. A comprehensive, quantitative method of macrofossil analysis using counts and cover abundance scores is developed. Identification criteria for several groups of macrofossils are presented, including leaves, rootlets and epidermis. Potential macrofossil incorporation was investigated at eight wetland and alluvial sites, including saltmarsh, wet woodland and herb fen environments. Macrofossil collections were compared to extant vegetation and subject to multivariate analysis. The results showed that macrofossil assemblages produce spatially and temporally precise data of plant presences, although spatial and temporal fidelity varies in different depositional environments and between plant taxa. Vegetation dominants were favoured in the assemblages of all classes of macrofossils, with bulky Monocotyledons and Therophytes favourably preserved and sparsely distributed taxa, such as rosette plants, less well favoured. The depositional environment and position in relation to environmental gradients were also found to affect macrofossil composition. Multiple approaches to macrofossil analysis using a wide range of macrofossils were found to produce improved interpretations. The value of different macrofossil classes and occurrences of the major observed taxa in alluvial sediments are discussed. The method was applied to samples from the Medway River at Chatham. Vegetation history, hydrology and traces of human disturbance are discussed from 7000BP to 2000BP. Analysis showed a gradual increase in human disturbance over time, development of a distinctive human-influenced upper salt marsh flora from 3000BP and supports the trend across southern Britain for a change in hydrology by the same period.