153 GRAVELS AT RUSHFORD, SUFFOLK

Trans. Suffolk Nat. Soc. 48 (2012)

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 tide-dependent, 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].

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Suffolk Natural History, Vol. 48 154

Originating from ocean floor volcanoes as the North Atlantic opened up with the break-up of Greenland-Rockall plate, the ash was wind-blown into the North Sea area. The ash deposits can be correlated over the North Sea Basin and similar bands are found in Denmark & NW Germany. Over 30 layers are recognised in Suffolk and northern Essex and can be dated to about 53 million years. Most ash bands are 10–80 mm thick, weathered to bentonite, and can contain graded bedding and cross-lamination. The ash content comprises angular, brown glass shards, some of which are streaky and resemble pumice, crystal fragments (often plagioclase feldspar) and lithic fragments. These are best seen in the Harwich Stone Bed (HSB) concretions at Harkstead, where they have been protected from weathering (Plates 19 & 20). The whole sequence has reverse magnetic polarity.

Also of note are calcareous mudstone concretions, ‘septaria’, found in thin bands up to 30 cm thick forming distinct horizons of pale limestone, sometimes discontinuous, within the London Clay; fallen blocks litter the beach. The HSB is the most significant of these bands and has been used as a marker horizon in eastern Essex and south Suffolk. Arnott (1950) erroneously says that septaria are a form of fossilized jelly-fish, presumably on account of the frequently fractured surface texture; he does not give his source of reference, but the myth, still perpetuated occasionally today, is probably attributable to folklore.

Septaria were widely used as a building stone, indeed the streets of Harwich were once paved with them, seen today in east Suffolk’s Norman churches. Dredging of the Orwell and Stour Estuaries and West Rocks, off Harwich, was once widespread; there were 50 vessels and a stone dresser’s workshop at Pin Mill alone. Unfortunately, septaria fracture and weather easily, sometimes leading to collapse – repair and restoration have been constant themes! Septaria were also used for making ‘Roman’ cement, for example by William Lockwood at Lime Kiln Quay, Woodbridge; developed in the 1780s, it was made obsolete by the C19th introduction of Portland Cement. It has long been maintained that the extensive dredging has exacerbated coastal erosion.

Nacton Shore (Plate 18) Cars may be parked at the small Suffolk Coastal Council car park at National Grid Reference TM220392. From there walk the short distance to the Orwell Estuary and turn eastwards, along the edge of the parkland garden of Broke Hall, predominantly of Georgian brick but with earlier origins, landscaped by Humphrey Repton. Bricks did not become much used until the reigns of Henry VIII and Elizabeth I, when they became most fashionable – for example in Seckford Hall, built near Woodbridge in the 1540s, and the Tudor towers of Waldringfield and Hemley churches. Later, as their use became more widespread, many local brick pits were developed using the London Clay, as at Broke Hall.

Continue along the shore for about 500 m until the cliffs begin. It takes only seven or eight minutes to walk from the car park to the first London Clay exposure, low in the cliffs, and only two or three more minutes to good

155 GRAVELS AT RUSHFORD, SUFFOLK

Trans. Suffolk Nat. Soc. 48 (2012)

THE LONDON CLAY OF NACTON AND HARKSTEAD SHORES

exposure. The walk is across relatively firm sand, often muddy or with shingle. The cliffs, designated a County GeoSite, extend for approximately 1 km, below Home Wood and nearly to Levington Creek, reaching about 20 m in height (TM224388–TM233384). The section was briefly described by the author in a field meeting report (Dixon 2005).

The initial 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 London Clay.

Currently about 15 m of blue-grey silty London Clay is well exposed in the cliff and on the foreshore at Nacton. The bedding can be picked out quite clearly in the cliffs, showing that the London Clay is gently tilted to the west, contrary to the general regional dip eastwards, and there are at least four small faults with down-throws of up to 1 m and fault planes dipping at around 45˚ to the east; associated drag folds may be well displayed (Plate 20). Often exposure of these features is limited because of rain-wash and other superficial surface debris – repeat visits may be needed to see them.

Also of note is the ria-type Orwell Estuary itself, with views downstream as far as the cranes of Felixstowe docks, upstream to Woolverstone Park, and southwards across the river to Pin Mill and the plateau of the Shotley Peninsula. This ‘drowned’ river valley was formed by the post-glacial rise in sea level about 9,500 years ago as the ice caps melted. Inhabiting the extensive foreshore mud flats are Hydrobia ulvae, Littorina littorea, Mya arenaria, Cerastoderma edule, Scobicularia plana and Venerupis spp communities, with barnacle colonies abundant on the harder substrates provided by pebbles; feeding trails are much in evidence.

Of particular interest to the geomorphologist are numerous land-slides and rotational slips, initiated by undercutting of the cliff at about the high water tide mark.

Harkstead Shore (Plate 21) The section, cliffs up to 5 m high, extends along the north shore of the Stour Estuary for some 500 m, from TM184343–TM189335. It lies within Stour Estuary SSSI, but the geodiversity interest is not mentioned in the citation. Parking in the village may be difficult (it is a narrow street), but it may be possible to use the Village Hall car park. Access to the shore is along a well signed 500 m public footpath, which leaves the main street next to the Bakers Arms at TM187347, down Walnut Tree Lane for c.20 m, and right opposite Harksleigh Cottage. You walk past gardens and through a field; go straight across a driveway, down a track and past a house sign, “Copperas Reach”, a reference to the name of this part of the river. Copperas (iron sulphide from pyritised wood) was collected from the C17th to the mid-C19th from the beach or dredged by fishing smacks, providing employment during hard times. It was used locally in the dyeing and tanning industries and for making sulphuric acid in works on Ipswich quay. The name still survives on the opposite bank of the Estuary in Copperas Bay, Copperas Creek and Copperas Wood. When the beach is reached, turn eastwards (left). It takes under 15 minutes to reach the beach, and just over five minutes more to

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Suffolk Natural History, Vol. 48 156

reach good cliff exposure. On the way, there is a small, often polluted stream to cross. Note the present-day shell fauna, similar to the typical estuarine fauna at Nacton, and the lovely view westwards of the Royal Hospital School at Holbrook.

The HSB, 6∙5 m above base of Harwich Formation and c.30 cm thick, outcrops at the top of the beach and low in the main cliff about 1 m above current beach level. Note the fallen blocks (septaria) littering the beach. The HSB contains a substantial ash content, and ash bands in the clay are clearly visible (Plate 22). The HSB at the start of the section dips westwards, while at the far end of the section it dips eastwards; thus, as you walk the section you will be traversing a large open fold.

George (2012) lists and figures the Harwich Formation fossils he has found at this locality; the best place to search is on the low foreshore at the eastern end of the section. Pyritised wood is common and sharks’ teeth can be found with a diligent search.

Pleistocene deposits include channel infill sands and gravels with silt bands. They are strongly cryoturbated (Plate 23) – well displayed at the eastern end of the section, evidence of a former post-Anglian peri-glacial climate. A brickearth at the eastern end of the section has yielded a mammoth fauna dated at c.210 Ka. Archaeological finds have also been recorded.

Endpiece The Suffolk GeoCoast project was launched by GeoSuffolk in 2005 to promote the geology within the Suffolk Coast and Heaths AONB. To date a series of field excursions and leaflets have resulted, and the first of a series of guided walks and drives was published in Transactions Volume 47 on the sections at Bawdsey (Dixon, 2012). This is the second of the series, the ultimate aim being to produce a printed geological guide to the Suffolk coast.

References Arnott, W. G. 1950. Suffolk Estuary. Norman Adlard & Co, Ipswich. Daley, B. 1999. In: Daley, B. & Balson, P. British Tertiary Stratigraphy.

Geological Conservation Review Series No. 15, Joint Nature Conservation Committee, Peterborough.

Dixon, R. G. 2005. Field Meeting Report – Nacton Shore. Trans. Suffolk Nat. Soc. 41, 29–30.

Dixon, R. G. 2012. The Suffolk GeoCoast – Bawdsey East Lane to the Manor. Trans. Suffolk Nat. Soc. 47, 98–103.

George, W. H. 2012. Field Guide to the Harwich Formation and Pleistocene Deposits of Harkstead Shore. In: Dixon, R.G. (Ed.) A Celebration of Suffolk Geology – GeoSuffolk 10th Anniversary Volume, 133–148.

Roger Dixon The White House 7 Chapel Street Woodbridge Suffolk IP12 4NF

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Plate 18: General view of Nacton Shore, looking westwards (p. 154).

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Plate 19: Nacton: this section displays a 30cm thick bed of calcareous mudstone (with hand on) and thin pale ash bands (p. 154).

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Plate 20: One of the small faults (F – F) at Nacton, with a vertical displacement of 0.63m. Note the drag folds, picked out well by the bed of calcareous mudstone (CM), which has been ‘bent down’ by movement along the fault. Pale ash bands (A) show up clearly. The mudstone is well fractured (p. 155).

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Plate 21: General view of the section at Harkstead, looking westwards (p. 155).

Plate 22: The HSB at Harkstead (with hand on) and ash bands clearly displayed. (p. 156).

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Plate 23: The darker London Clay at Harkstead contrasts with paler Pleistocene deposits to show the high degree of contortion that occurred during Tundra-like periglacial conditions after the Anglian glaciation (p. 156).

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