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

Holywell Coombe: is a valley cut into the scarp-face of the North Downs Cha lk cuesta, near Folkestone, Kent. Its geological importance stems from a hi ghly fossiliferous sequence of Lateglacial and Holocene deposits that line the valley floor. These have yielded a molluscan succession of particular i mportance, providing a record of environmental change throughout the past 1 3,000 radiocarbon years. Waterlogging of the basal deposits has prevented o xidation, leading to the preservation of a range of organic fossils such as plant and insect remains, that normally do not survive in calcareous envir onments. This enables linkage between faunal and vegetational records, allo wing the differential rates of response of particular groups to be critical ly 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 multidisciplin ary investigations, the results of which are reviewed here. A three-dimensi onal picture of the valley infill was established from a network of 180 bor eholes. Critical parts of the sequence were investigated in specially excav ated trenches and sections exposed during construction of the tunnel. Syste matic sampling at a number of locations within the valley provided a palaeo ntological record from the full stratigraphical succession. A number of Lat eglacial and Holocene soils were found to be represented in the sequence, i ncluding that formed during the Allerod phase of the Lateglacial interstadi al. The molluscan zonation scheme previously defined at Holywell Coombe, an d applicable over large areas of southern Britain and possibly further afie ld, has been refined and dated with greater precision. The Lateglacial sequ ence has been extended back to the early part of the Lateglacial interstadi al 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 1 3,000 and 9000 yr BP. The earliest sediments, marsh deposits with thermophi lous 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 accumul ation of thick colluvial deposits. By 11,500 yr BP the climate had stabiliz ed and slope movement had ceased, allowing the formation of the 'Allerod so il'. There followed a major deterioration to the arctic climate of the Youn ger Dryas, during which renewed erosion from the valley sides brought furth er material onto its floor, burying and sealing the earlier sediments. The beginning of the Holocene saw the onset of tufa formation around two domina nt springs in the upper valley. There was progressive development of forest , hazel-dominated woodland being established by 9500 yr BP. There is some e vidence for thinning of the forest canopy during the late Mesolithic and Ne olithic, but the major clearance occurred during the Early Bronze Age, caus ing renewed instability on slopes and consequent hillwash accumulation. Thi s 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-m arks, and contains an extensive sequence of artefacts. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.