Welsh Journals

FLOODPLAIN AND VALLEY FILL DEVELOPMENT IN SOUTH WALES: SOME INITIAL FINDINGS FROM THE ILSTON TRIBUTARY OF THE PENNARD PILL VALLEY, GOWER GORDON E. SAUNDERS, PAUL J. BURRIN SIMON J. WOOD CAMBRIA Saunders, Gordon E., Burrin, Paul J. and Wood, Simon J. 1986. Floodplain and valley fill development in South Wales: some initial findings from the Ilston tributary of the Pennard Pill Valley, Gower. Cambria Vol. 13 (2) pp 189-196 ISSN 0306-9796. Field evidence from the Ilston valley indicates that contemporary floodplain development is taking place by processes of lateral accretion with sedimentation being dominated by coarse-grained materials. Subsurface investigations into the valley fill deposits have revealed significant thickness of fine-grained sediments with no trace of the coarser bedload typical of the present river system. The geomorphological development of the valley floor is considered and a model presented as to its probable evolution. Gordon E. Saunders, Paul J. Burrin and Simon J. Wood, Department of Geography, Goldsmiths' College, University of London, New Cross, London, SE14 6NW. (Hevised October 1985). INTRODUCTION There have been few published studies concerning the nature and origin of valley floor deposits in South Wales, and, to the authors' knowledge, none from the Gower. Although there are accounts of coastal alluvial sequences from adjacent areas such as the Cardiff district (Anderson and Blundell 1965), the lower Usk (Williams 1968) and Swansea Bay (George and Griffiths 1938, Godwin 1940), little attention has been focussed on the more inland floodplain alluvial sequences. Typically, floodplains develop by the lateral migration of river channels across valley bottoms (Wolman and Leopold 1957). Progressive erosion occurs along the outer concave bank with the deposition of sands and gravels as an accretionary point bar on the inner, convex margins. As the channel continues to migrate, so the point bar grows with the result that a floodplain is gradually extended across the valley in the wake of the channel. These deposits of lateral accretion typically form 80-90% of floodplain alluvium with fine-grained silts and clays derived from overbank sedimentation accounting for the remainder of