A Generic Geomorphological Approach to Archaeological Interpretation and Prospection in British River Valleys: A Guide for Archaeologists Investigating Holocene Landscapes
Howard, A. J., Macklin, M. G., Antiquity
Alluvial landscapes offer some of the most attractive environments for human activity and settlement. Utilized since early prehistoric times, they have been the subject of intense worldwide archaeological and geoarchaeological research (e.g. Berendsen 1993; Bettis 1995; Brown 1997; Jing et al. 1997; Joyce & Mueller 1997; Martin-Consuegra et al. 1998; Needham & Macklin 1992). Their continued attraction for transportation networks, settlement and exploitation of resources such as minerals (Allen et al. 1997) and groundwater (Parker-Pearson & Sydes 1997), however, has put intense pressure on both cultural and environmental archaeological remains (Darvill & Fulton 1998). Destruction of archaeological sites in river valleys most commonly occurs because of construction and development and through drainage of valley floors resulting in lower water-tables and oxidation of alluvium.
Research on Holocene environmental change and archaeology within British river valleys over the last two decades has concentrated on a number of key themes. These include assessments of the potential attractiveness of river valley landscapes to human communities (Evans 1991) and analysis of the utilization of particular river zones (Robinson 1978), to excavation and recording of individual floodplain sites (Nayling & Caseldine 1997), through to identifying causal mechanisms of changing catchment hydrology and sedimentation styles (Robinson & Lambrick 1984; Macklin & Lewin 1993). Whilst a growing number of alluvial archaeological studies have demonstrated the wealth of the resource (Needham 1991), particularly where water-tables are high (Bell & Neumann 1997; Pryor et al. 1986; Parker-Pearson & Sydes 1997), geomorphological investigation has shown the complex evolution of these natural sedimentary systems and how river processes and channel dynamics influence archaeological site location and the quality of preservation (Lewin 1992; Passmore & Macklin 1997; Macklin 1999). In Britain, the Holocene fluvial stratigraphic record indicates that, prior to large-scale land drainage and channelization since the industrial revolution, there was a greater diversity of channel types and floodplain sedimentation styles than found today (Macklin & Needham 1992). These contrasting river patterns, included braided (laterally mobile multi-channelled rivers with wide and shallow, rapidly shifting channels, that divide and rejoin around sand and gravel bars, and vegetated islands) and anastomosed channel systems (inter-connected networks of low-gradient, relatively deep and narrow channels of variable sinuosity, characterized by stable, vegetated banks composed of fine-grained silt and clay; Smith & Smith 1983) which produce different alluvial sedimentary sequences (Brown & Keough 1992a; Passmore et al. 1993). Although many archaeologists and [TABULAR DATA FOR TABLE 1 OMITTED] geomorphologists in Britain are increasingly aware of this, few have considered how temporal and spatial variability in river dynamics have affected the preservation and visibility of the cultural record in alluvial environments (for examples of those who have, see Brown & Keough 1992b; Lewin 1992; Macklin et al. 1992c; Macklin 1999). This paper considers the current state of research in this important area and presents an empirically-based model of Holocene river-valley evolution in Britain which highlights the impact that fluvial processes have had on the archaeological record.
River channel and floodplain classification
More than a century of study by fluvial geomorphologists has resulted in the classification of rivers and their floodplains on the basis of channel planform and sedimentation processes. Four styles of fluvial channel are commonly recognized; braided, meandering, anastomosing and straight (Leopold & Wolman 1957; Leopold et al. 1964). All of these channel types have been identified in British Holocene fluvial sedimentary sequences, though divided multi-channel river systems (braided or anastomosing) are relatively rare in Britain today. …