30 FIELD MEETING REPORT – NACTON SHORE LEADER: BOB MARKHAM ROGER DIXON Although the forecast had not been encouraging and there was a blustery wind blowing, the weather stayed dry and largely sunny, providing near-perfect conditions for the afternoon excursion. Nacton Shore fulfils many of the requirements for a RIGS site: there is easy access for the public, with more than adequate parking facilities; safety issues are minimal, assuming a modicum of common sense; the walking is easy; and the educational value crosses several disciplines, from present day invertebrate habitats to active geomorphological processes and geology. The purpose of the trip was to demonstrate these values. The site has been enhanced by Suffolk Coast & Heaths and partners, who have provided three public display panels describing the coastal Suffolk’s Area of Outstanding Natural Beauty, the “Nacton Shoreline & River Orwell”, and the “Stour & Orwell Estuaries”. Our walk started at the western end of the section (NGR TM219391), on the edge of the parkland garden landscaped by Humphrey Repton of 16th Century Broke Hall, and from where we saw superb views of the ria-type Orwell Estuary downstream as far as the cranes of Felixstowe docks, upstream to Woolverstone Park, and southwards across the river to Pinmill and the plateau of the Shotley Peninsula. This ‘drowned’ river valley was formed by the post-glacial rise in sea level about 11,500 years ago as the ice caps melted. Immediately noticeable on the foreshore were the extensive mud flats with Hydrobia ulvae, Littorina littorea, Mya arenaria, Cerastoderma edule, Scobicularia plana and Venerupis sp communities, with barnacle colonies abundant on the harder substrates provided by pebbles. Feeding trails were much in evdence. Walking eastwards, a low 2 m cliff exposes late Pleistocene river terrace gravels, mainly composed of flint but with a significant proportion of quartz and other components. These are banked up against the Harwich Formation of the London Clay. To the east the London Clay becomes well exposed, the cliffs reaching a height of nearly 20m. In fact, the site is one of the best London Clay outcrops in the County to which public access is not restricted. Pale bands of volcanic ash, relating to the rifting and opening of the North Atlantic some 52 million years ago, could clearly be seen. Beds of calcareous mudstone concretions - septaria - up to 30 cm thick formed distinct horizons and fallen blocks littered the beach. The septaria have been extensively used as building stones since late Saxon times, including for the controversial restoration work on Colchester and Orford castles. Some pieces of pyritised wood from the Clay were found on the foreshore. Fish vertebrae, sharks teeth and turtles have also been recorded from this location. Contrary to the expected general trend, the London Clay here dips gently to the west or lies almost horizontal. However, four normal faults were observed, with planes dipping at about 45° to the east and downthrows of up to 80 cm, and often exhibiting perfect drag folds.
GEOSUFFOLK RIGS
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Of particular interest to the geomorphologist were numerous land slides and rotational slips, initiated by very obvious undercutting of the cliff at about the high water tide mark. Bob outlined a field project carried out by Amberfield School pupils where the approximate age of a slide could be determined by measuring and comparing the girths of trees (and hence their age) which were growing before the slide took place with those that have grown afterwards. The cliff decreases further to the east as the inlet and salt marsh of Levington Creek are approached. At this point the excursion - and the day’s proceedings – formally ended. The trip was not only most informative but also thought-provoking, for many of the issues raised earlier in the day during the presentations could be discussed further and in a practical way.