Holywell Coombe, Folkestone: A 13,000 year history of an English Chalkland Valley

Abstract

Holywell Coombe is a valley cut into the scarp-face of the North Downs Chalk cuesta, near Folkestone, Kent. Its geological importance stems from a highly fossiliferous sequence of Lateglacial and Holocene deposits that line the valley floor. These have yielded a molluscan succession of particular importance, providing a record of environmental change throughout the past 13,000 radiocarbon years. Waterlogging of the basal deposits has prevented oxidation, leading to the preservation of a range of organic fossils, such as plant and insect remains, that normally do not survive in calcareous environments. This enables linkage between faunal and vegetational records, allowing the differential rates of response of particular groups to be critically compared. The importance of the site was revealed in 1968 in trial pits connected with an aborted Channel Tunnel project. Resurrection of plans to build a tunnel led in 1987 to major ‘rescue’ excavations and multidisciplinary investigations, the results of which are reviewed here. A three-dimensional picture of the valley infill was established from a network of 180 boreholes. Critical parts of the sequence were investigated in specially excavated trenches and sections exposed during construction of the tunnel. Systematic sampling at a number of locations within the valley provided a palaeontological record from the full stratigraphical succession. A number of Lateglacial and Holocene soils were found to be represented in the sequence, including that formed during the Allerød phase of the Lateglacial interstadial. The molluscan zonation scheme previously defined at Holywell Coombe, and applicable over large areas of southern Britain and possibly further afield, has been refined and dated with greater precision. The Lateglacial sequence has been extended back to the early part of the Lateglacial interstadial by this study and the site chronology is now underpinned by over 35 new radiocarbon dates. Quantitative palaeoclimatic reconstructions from beetle remains, using the Mutual Climatic Range method, cover the period between 13,000 and 9000 yr BP. The earliest sediments, marsh deposits with thermophilous insect taxa and a species-poor molluscan assemblage, date from around 13,000–12,000 yr BP. Just before the end of this period, changes in beetle faunas record climatic cooling, heralding slope instability and the accumulation of thick colluvial deposits. By 11,500 yr BP the climate had stabilized and slope movement had ceased, allowing the formation of the ‘Allerød soil’. There followed a major deterioration to the arctic climate of the Younger Dryas, during which renewed erosion from the valley sides brought further material onto its floor, burying and sealing the earlier sediments. The beginning of the Holocene saw the onset of tufa formation around two dominant springs in the upper valley. There was progressive development of forest, hazel-dominated woodland being established by 9500 yr BP. There is some evidence for thinning of the forest canopy during the late Mesolithic and Neolithic, but the major clearance occurred during the Early Bronze Age, causing renewed instability on slopes and consequent hillwash accumulation. This final depositional phase continued, with pauses marked by soil formation in the Early Bronze Age and the Iron Age, to the present day. The hillwash seals structures relating to prehistoric human activity, including plough-marks, and contains an extensive sequence of artefacts.