River terrace systems in north-west Europe: an archive of environmental change, uplift and early human occupation

Abstract

Staircases of large-scale aggradational river terraces are a notable feature of many valleys in the temperate lattitudes, particularly in areas beyond the reach of the erosive activities of Pleistocene ice sheets. It is now recognized that the cyclic fluctuations of climate during the Quaternary have driven the generation of terraces, through the direct and indirect influence of both temperature and precipitation on fluviatile activity. Where fossiliferous deposits are preserved within terrace sequences it is often possible to date these and to correlate them with the oceanic record, thus providing an important framework for the evidence of environmental change on land. Middle and Late Pleistocene terraces in different areas can commonly be seen to have formed in synchrony with glacial–interglacial cycles or with longer-periodicity megacycles. Climatic forcing alone is insufficient to cause terraces to form, however; uplift is also necessary, so that terrace sequences can provide a useful record of crustal movement. In northwest Europe, where some of the best known studies of river terrace sequences have been carried out, the fluviatile deposits are also an important repository for Palaeolithic artefacts, from which a record of early human occupation can be reconstructed.

Introduction

River terrace sediments have been a source of scientific interest for many years. These are the deposits that give rise to large-scale aggradational terraces, which are distinct from (a) minor aggradational terraces, formed in much the same way but during shorter time-scales and largely ephermeral beyond a single climatic cycle and (b) erosional or strath terraces, which lack any sedimentary or palaeontological evidence and are consequently more difficult to date and interpret (although they can be buried by or interspersed between datable aggradational terraces). In Britain and on the European continent the major aggradational terraces have provided the bulk of the artifactual evidence for the presence of Lower Palaeolithic hunter-gatherers (Wymer, 1968, Wymer, 1988; Roebroeks and van Kolfschoten, 1995). They also account for a high proportion of the interglacial sites in the unglaciated parts of this same area, since sporadic lenses of warm-climate sediment have survived within primarily periglacial fluviatile sequences. These localized survivals are often rich in vertebrate remains, sometimes even of hominid origin, such as at Swanscombe, UK (Conway et al., 1996), and Steinheim, Germany (Adam et al., 1995). Another fossil group sometimes represented is the Mollusca, which supply important biostratigraphical and palaeoenvironmental information (Keen, 1990; Preece, 1995a, Preece, 1999) and, in recent years, provide the raw material for the valuable geochronological method based on amino acid epimerization (Miller et al., 1979; Bowen et al., 1986; Bates, 1994). Recent work, however, suggests that mammalian fossils offer the most powerful tool for correlation of fluvial sequences with the global marine record of glacials and interglacials (Schreve, 1997; Bridgland and Schreve, in press).

Pollen, recognized as the most widespread source of environmental and stratigraphical evidence since the study of interglacials began (Pike and Godwin, 1953; West, 1956), is often present in organic fluviatile sediments, such as those representing infilled oxbow lakes. However, these seldom represent a large proportion of any particular interglacial, in contrast to sequences filling kettle hole lakes in glaciated areas, which have provided the most complete palynological records of such episodes (Mangerud, 1991). It is unfortunate, then, that some interglacial type localities, such as that for the British Ipswichian (West, 1957), are fluviatile sites that record a mere fragment of the climatostratigraphic stage represented.