Investigations Bearing on the Age and Development of Chesil Beach, Dorset, and the Associated Area

Recent site investigations and research on Chesil Beach, Dorset, have provided new data which help to shed light on the origin of both the Beach and the lagoonal Fleet which is situated to landward. A channel of the order of 26 m is indicated between the Isle of Portland and the mainland. Under much of the Fleet there is a marked break of slope at about I5 m O.D. This is associated with pebbles, cobbles and boulders and takes the form of a storm beach with planation surface to seaward. Overlying this surface are various deposits dating from Pollen Zone VI onward. In the light of these data the surface must, as in the case of those at Orford, Suffolk, be entirely preFlandrian. A tentative chronology is given. THE purpose of this paper is to discuss aspects of the evolution of Chesil Beach in relation to the Fleet and the Portland Raised Beach. The paper describes certain facets of recent work carried out in the area by the former Physiographical Section of the Nature Conservancy and incorporates new data from elsewhere. Chesil Beach is one of the three major shingle structures on the coast of Great Britain and the only one without a cuspate form of development. The beach extends at least 8 km from Chesilton in the east, where it terminates against the cliffs of the so-called Isle of Portland, to an essentially arbitrary limit in the west. (For location of most places referred to in the text, see Figure ia.) At both ends the shingle structure is joined to the mainland but over the remaining 13 km it is backed by the shallow, tidal, Fleet. The latter varies in width from under ioo m to some 900 m, and in depth from less than -0 3 m to nearly -3-0 m O.D. (Ordnance Datum, Newlyn, is approximately mean sea level at this site.) Opposite the Fleet, Chesil Beach is between I50 and 200 m wide but it is narrower both adjacent to the cliffs in the west and at the extreme eastern end. The crest is broken intermittently at the western end but extends continuously from just east of Section 12 in Figure ia to Chesilton. While minor irregularities of the order of 0-5 m, and occasionally more, occur throughout this distance, the broad picture is of a progressively increasing ridge height from west to east, the maximum of I4'7 m O.D. being found near Chesilton. Pebble size above low water mark also increases in the same direction. Pebble composition is overwhelmingly of flint and chert (A. P. Carr and M. W. Blackley, I969). Offshore, the beach drops at a gradient broadly similar to that of the seaward face above low-water