Welsh Journals

RHYL AND COASTAL EVOLUTION By JOHN MANLEY The aim of this note is to bring to the reader's attention the existence of a new and important source of data directly relevant to post-glacial coastal evolution in the Rhyl area. An understanding of the changing land and sea-levels is a necessary prerequisite for any evaluation of the likely human settlement pattern in the later prehistoric period. An earlier article (Manley 1981) considered much the same area; an outline of its conclusions is presented here as a prelude to a discussion of the new data. The most obvious indicator that changes in the relationship between land and sea have occurred, lies in the periodic observation of peat beds underlying areas of blown and marine sand on the shores of the present coast. Thomas Pennant (1784, 349) recorded that near Abergele (Fig. 1), at low water and remote from the shore, numbers of oak trees could occasionally be seen. The anticipated geological sequence in the Rhyl area should be as follows: the solid geology comprises red and buff, fine to coarse-grained sandstone of the Kinnerton Sand- stone, Chester Pebble Beds and Wilmslow Sandstone of Triassic age. This is overlain by extensive deposits of glacial till (boulder clay) resulting from the north- south advances and retreats of the Irish Sea ice mass. In turn, post-glacial oscillatory land and sea-level changes will have led to the development of inter- bedded sequences of peats, estuarine, marine and alluvial muds, silts and clays. Isostatic uplift, tectonic warping, and regional eustasy (Tooley 1985a, 114) which took place as a result of the return of water to the sea on the melting of the Pleistocene ice sheets, will have produced a complex sequence of vertical move- ments of both land and sea-levels, with correlated horizontal displacements of the position of the coastline. Work on sea-level change had been undertaken on the Lancashire coast (Tooley 1978), an area geologically very similar to the mouth of the Vale of Clwyd. Sea- level did not rise at a constant rate and its interaction with the land had produced a series of ten major marine transgressions from the Mesolithic to the Medieval periods, an interval of approximately 10,000 years. Applying the result of Tooley's work to some borehole data from the mouth of the Vale of Clwyd the following observations were made. The first two major transgressions (I & II) at the beginning of the Mesolithic will have penetrated perhaps as far as present-day Rhuddlan, although an area of elevated boulder clay at Rhyl may not have been