GRAVELS AT RUSHFORD, SUFFOLK
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THE SUFFOLK GEOCOAST THE LONDON CLAY OF NACTON AND HARKSTEAD SHORES ROGER DIXON Introduction This paper describes two coastal walks along the Orwell and Stour estuaries along which the London Clay may be studied. The walks are ‘easy’ – along level sand for most of the way, but can be quite muddy. The walks are tidedependent, so time your walk to coincide with low tide and to within two hours either side of it. • Do follow the Suffolk Naturalists’ Society field excursion guidelines. • Parts of the coastline are rapidly eroding and cliff falls occur at frequent intervals. The cliff is therefore unstable – do not go near the cliff, top or bottom, where there are overhangs. • The London Clay surface is extremely slippery – tread with great care. • Stout footwear is essential; dress appropriately for the weather; take plenty of water. Neither section is described in the Geological Conservation Review (GCR) (Daley, 1999), but they are nevertheless extremely important local sections and are the best two publicly accessible London Clay outcrops in the county. The GCR is an essential source of further information on the London Clay, and the sections at Harwich, Wrabness and Walton-on-the-Naze are well described. Dixon (2012) describes the London Clay at nearby Bawdsey, while George (2012), in an excellent recent paper, describes the section at Harkstead and includes several useful figures, including many fossils found at the site. The London Clay The London Clay comprises brown and grey predominantly silty clays that were deposited in a shallow sea, up to 100 m deep, that covered much of south-east England during the Eocene Period. England had a warm subtropical climate, with luxuriant rain forest and coastal mangrove swamps. Large sluggish rivers carried sediment and plant debris out to sea, where it settled and compacted on the sea floor. Plant debris, logs and branches, now preserved as pyritised fossil wood, seeds and fruits, are. Many species of sharks and other fish swam in the seas – the teeth of the sharks Carcharias hopei, C. teretidens, Striatolamia macrota and Otodus obliquus, and fish vertebrae can often be found washed out on the foreshore at both sites. Other vertebrates, including turtles, have also been found. A microfauna of diatoms, radiolarian and foraminifera, and a small, poor invertebrate (mostly mollusc) fauna have been recorded. Of particular note at both Nacton and Harkstead are thin layers of pale creamy yellowish rust coloured sediment within the London Clay – these are layers of volcanic ash. The presence of ash bands indicate that this part of the London Clay sequence should more formally be referred to as the Harwich Formation [compare with the overlying, slightly younger beds at Bawdsey, which do not contain ash and are probably London Clay Formation sensu stricto].
Trans. Suffolk Nat. Soc. 48 (2012)