- Email: [email protected]
Field Meeting to the Bawdsey Peninsula, Suffolk, England, 22nd May 2010, to examine London Clay, Coralline Crag and Red Crag deposits
Proceedings of the Geologists’ Association 122 (2011) 514–523
Contents lists available at ScienceDirect
Proceedings of the Geologists’ Association journal homepage: www.elsevier.com/locate/pgeola
Field Meeting to the Bawdsey Peninsula, Suffolk, England, 22nd May 2010, to examine London Clay, Coralline Crag and Red Crag deposits Leaders: Roger Dixon and Bob Markham Roger Dixon * The White House, 7 Chapel Street, Woodbridge, Suffolk IP12 4NF, United Kingdom
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 July 2010 Received in revised form 8 February 2011 Accepted 11 February 2011 Available online 16 March 2011
A one-day Geologists’ Association field excursion to the Bawdsey Peninsula in coastal Suffolk is described, with a guided tour of ‘The Pliocene Island’ at Sutton Knoll and visits to examine coastal erosion, the London Clay and Red Crag at Bawdsey East Lane, and the Red Crag at Alderton and Ramsholt. The depositional environments indicated by the Crag sediments and faunas are discussed, and nonderived Red Crag mollusc faunas at Sutton and sand-wave mollusc assemblages from Ramsholt are described. ß 2011 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.
Keywords: Pliocene Coralline Crag Red Crag Rockhall Wood ‘Pliocene Forest’ Mollusc faunas
1. Introduction
2. Locality 1: Sutton Knoll [TM 305440]
With the barometer rising, temperatures soaring into the 20s and not a breath of wind, 26 members of the Geologists’ Association and the GeoSuffolk team met on a gloriously sunny day for an excursion to the Bawdsey Peninsula in coastal Suffolk. The Geologists’ Association last visited the area in April 2004 for a long week-end (Dixon, 2005) – this was a visit to see just some of the locations not seen then (Fig. 1). The stratigraphy seen during the day consisted of: Eocene London Clay underlies the area and is unconformably overlain by outliers of mid-Pliocene Coralline Crag. Late Pliocene Red Crag unconformably overlies both these deposits, and there is evidence of the later Early and early Middle Pleistocene proto-Thames Kesgrave Sands and Gravels. After a short briefing at Sutton Heath (National Grid Reference TM 306475), the group drove the short distance to Sutton Knoll. Here was held a ‘mass opening’ of the new enclosure, funded by the Curry Fund of the Geologists’ Association, for an expanded ‘Pliocene Forest’. The group held on to a length of red ribbon, which was snipped by Curry Fund committee member David Bone, who spoke about the work of the Curry Fund (Fig. 2).
This famous site, which includes Rockhall Wood Site of Special Scientific Interest (SSSI), has been studied for over 170 years, long known as an island of Coralline Crag during Red Crag sea times, with its Coralline Crag cliffs and boulders ‘buried’ in Red Crag. The classic description was by Prestwich (1871a,b), some of the figures of which were reproduced by Boswell (1928), but Balson (1999) gives an excellent recent description and discussion. Other recent descriptions include Balson and Long (1988) and Balson et al. (1990, 1993) describe the stratigraphy of the Coralline Crag as a whole. Dixon (2006, 2007, 2009a) describes recent developments. The locality has frequently been visited by field groups and has often been used for educational purposes, including undergraduate project work. English Nature (now Natural England) cleared faces in 2006 as part of their ‘Facelift’ programme and members of the local GeoSuffolk group have been actively maintaining and developing sections. Furthermore, new research by GeoSuffolk members is currently underway to help interpret the site from a more modern perspective (Fig. 3).
* Corresponding author. E-mail address: [email protected].
2.1. Coralline Crag Suffolk’s Pliocene Coralline Crag, deposited approximately 3.75 million years ago, outcrops as a 12 km inlier ridge running from the north of Aldeburgh south-westwards to the Butley River, with further small outliers, including Sutton, to the southwest, and rests
0016-7878/$ – see front matter ß 2011 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.pgeola.2011.02.002
[()TD$FIG]
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
Fig. 1. Location map of the sites visited during the Geologists’ Association field excursion on May 22nd, 2010. 1, Sutton Knoll; 2, Bawdsey, East Lane; 3, Alderton House Pit; 4, Tarrant’s Pit.
unconformably on the London Clay (see Fig. 4). The bioclastic sands, about 20 m thick, can be broadly divided into three sedimentary and faunal units, two of which outcrop at Sutton: a lower Ramsholt Member and an overlying Sudbourne Member
515
(seen also at Richmond Farm (TM 413493) and Crag Farm (TM 428523) SSSIs and elsewhere). The beds are possibly best known for their bryozoan fossils (‘Corallines’) and the common occurrence of one of Britain’s largest fossil brachiopods, ‘Terebratula grandis’. Over 300 species of mollusc have been recorded from these beds by Searles Wood and others, many extant forms having ‘southern’ or Lusitanic distributions, and suggest summer water temperatures approaching 17–20 8C and winter temperatures of 6–9 8C. However, ‘mixed signals’ are given by different faunal elements (Williams et al., 2009) and there is no consensus of opinion. A useful overview and discussion of Neogene correlation and environmental interpretation, with particular reference to the role of ostracods is given by Wood (2009). The group first examined the lower beds of the Sudbourne Member at the North Face. This 50 m long, 3 m high section on the northern edge of the Knoll was cleared by GeoSuffolk members in 2008/2009. The beds contain small and medium scale sand-waves with mud drapes and laminations, possibly of tidal origin, and abundant infaunal burrow types, including those of worms, crustaceans and spatangoid echinoids, and a good bryozoan fauna. The beds yield a predominance of smaller, more fragile shells (e.g. Lucina, Venus, Aloidis, Pteromeris and Anomia) with abundant foraminifera, which possibly have been deposited selectively by currents. A well-preserved 3 m solution hollow was clearly observed (Suffolk’s own karstic landscape); infill is mostly of more recent structureless and unfossiliferous regolith, making dating of it impossible, but the hollow itself may pre-date the Red Crag, when subaerial processes prevailed. A bed of finer grained silty Crag at the base of the face marks the boundary between the Sudbourne and underlying Ramsholt Members. Following the work by English Nature, the lower part of the Bullockyard Pit, seen next by the party on the eastern side of the Knoll, currently exposes about 5 m of silty/muddy sands of the Ramsholt Member, possibly dating to as old as c.4.2 Ma (Wood, 2009), making the section better than it has been since the days this when was a working pit. Exploratory excavations have extended below the water-table to the London Clay. Medium-scale sand-waves are evident, although often indistinct, with muddrapes, smaller ripples and extensive bioturbation, indicating weaker currents and slow depositional rates. Estimates of water depth vary between <20 m and >50 m, but the sedimentary
[()TD$FIG]
Fig. 2. David Bone cutting the ribbon to open the new extension to the ‘Pliocene Forest’ (Photo: R Gwyn-Thomas).
[()TD$FIG]
516
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
Fig. 3. Sutton Knoll site plan. The area of Site of Special Scientific Interest is stippled.
structures clearly indicate a depth shallower than storm base. An abundant benthic fauna of molluscs and bryozoans is present, and differences occur in the faunas from different beds. Well-defined accumulations of larger, more robust shells (e.g. the bivalves Arctica islandica, Cardita senilis, Pecten spp, Astarte spp, Venus spp, Glycimeris glycimeris and brachiopod Terebratula), often articulated, form shell-beds (currently exposed in a drainage sump). The corals Spenotrochus and Cryptangia and fish otoliths are not uncommon in the shell beds; other common mollusc species include Aloidis gibba, Pecten tigrinus, P. maximus, Lucina borealis and Cardita scalaris. The presence of a full size range for many species; articulated valves of juveniles of many species, including Terebratula; entire echinoid tests, including the full size range of Temnechinus, with fragments of Echinus sp. and Spatangid sea urchins; and a good fauna of smaller species indicate that these shell beds may represent non-derived shell-gravel faunas. Later in the morning, the group stopped to view the Coralline Crag ‘Rock Bed’, found on the north-western side of the Knoll in the Quarry Pit. About 5 m of shelly sands (Sudbourne Member) are exposed, cemented to form a relatively durable limestone, with evident bioturbation. The member exhibits well-developed medium- and large-scale trough cross-bedding, which is interpreted as storm and tidal sand-waves migrating in relatively shallow water (c.30–50 m) in a generally south to south-west direction. In this bed, post-depositional solution processes have dissolved aragonite shells, and re-precipitated carbonate in the form of calcite to form a
cement. The ‘Rock Bed’ processes occurred before the deposition of the later Red Crag, as pebbles and boulders of ‘Rock Bed’ are found in the Red Crag, and the term has no other stratigraphic implication. It was often used locally as a building stone in farm walls, including on the Sutton Estate, and in other structures such as the river walls. It can also be seen as major components in nearby Chillesford and Wantisden churches, and Orford Castle. Joint sets are well developed in the ‘Rock Bed’ and can be seen in the pit walls; they may be related to the flexuring of the North Sea Basin. Occasional infilled solution pipes can be seen extending down from the surface. 2.2. Red Crag The Red Crag is of late Pliocene age, around 2.5 Ma, unconformably overlying the Coralline Crag and London Clay (Fig. 5). It was formed in a high energy, shallowing sea dominated by strong tidal currents, with submarine sand waves piling up against the shoreline to the west; the bedforms are considered classic of their type. It is dominated by molluscan shell gravel assemblages, which indicate climatic conditions similar to those of Britain today: Glycimeris-Tapes assemblages occur at Walton-onthe-Naze and elsewhere in the south, with Spisula, Macoma, Mya and Mytilus assemblages in the Deben/Butley area (Dixon, 1979). Until recently surprisingly little was known about the Red Crag at Sutton, and no modern (post-war) quantitative or detailed
[()TD$FIG]
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
517
Fig. 4. Illustrative sketch section of the Coralline Crag ‘island’ at Sutton during the time of the Red Crag sea (first published in Dixon, 2007 and used on the interpretive panel at the site).
molluscan studies or sedimentological studies have been done. However, it is now clear that very different localised molluscan communities were able to develop in the different microenvironments that must have existed around the ‘island’ (Dixon, 2007, 2009a). The Red Crag abutting and unconformable on Coralline Crag is best seen on the east face of the ‘island’ at the southern end of the Bullockyard Pit, but is much overgrown. Prestwich (1871a) recorded over 2 m of Red Crag and his sketch sections show Red Crag banked up against a cliff of Coralline Crag. It contains much derived Coralline Crag fauna, and even pebbles of that Crag encrusted with Red Crag barnacles. The ‘Facelift’ clearance work
[()TD$FIG]
exposed over 13 m of the wave-cut platform cut by the Red Crag sea into the Coralline Crag. It dips southwards at over 108 and blocks of Coralline Crag can be seen resting on it. The precise boundary is not easy to see at first sight because of the amount of derived Coralline Crag material within the Red Crag, and it takes a well-practised eye to trace it laterally. When cleared in 2006 it was probably the first time it had been exposed for over 100 years! Colonies of Mytilus edulis and Neptunea contraria are found at the base of the Red Crag adjacent to the Red Crag/Coralline Crag boundary, and indeed were noted by Prestwich. In fact, they can help to identify the boundary itself when the two lithologies may at first sight appear so similar. The valves are articulated and
Fig. 5. The basal Red Crag pebble bed of phosphatic nodules and Boxstones overlies London Clay at Bawdsey, East Lane.
[()TD$FIG]
518
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
Fig. 6. The Bone Tooth – a rare Red Crag rhino molar found in the Red Crag at Bawdsey, East Lane (Photo: A Bone).
closed, and the full size range is present, juveniles as well as adults. Mytilus is typically an intertidal rocky shore species, attaching itself to firm substrates by means of its byssal thread, and the communities discovered at Sutton are undoubtedly in their life positions (Fig. 6). Group members then walked over the knoll, pausing at the summit to find quartz and quartzite pebbles. These may suggest the former presence of Kesgrave Sands and Gravels, deposited between 0.6 and 1 Ma before the Anglian glaciation. The gravels, found in much of eastern Suffolk and Essex, mark the position of the former course of the proto-Thames. A further stop was made at the southern end of the SSSI to view from a distance Ramsholt Rocks, a further small outlier of Coralline Crag on the left bank of the River Deben, and to discuss the Crag Basement Pebble Bed (seen later at Bawdsey). At the base of both Crags is often found a discontinuous basal lag or remanie´ pebble bed containing cetacean remains, shark teeth and other fish remains, flints, ‘boxstones’ (rounded pebbles of late Miocene/early Pliocene sandstone, c.6 Ma old), crustaceans and, importantly, phosphatic nodules (mistakenly called ‘coprolites’) derived from the London Clay. These ‘coprolites’ were excavated on a large scale from Sutton and elsewhere in the local area during the midnineteenth century for making phosphate fertiliser. The waste material of flints and ‘boxstones’ was used as a building material in the nineteenth century restoration of Sutton and Shottisham churches and in some other local buildings, including barns on the Sutton Estate. Of particular interest was the recording by Prestwich (1871a) in this layer of a ton porphyry block from a trial pit in 1860 on the western flank of the Knoll, a site just visible from our view-point. It was the only known coprolite pit dug in the Coralline Crag (rather than Red Crag), but did not prove profitable and was filled in by the mid-1860s. It is not known what happened to the porphyry
boulder – there is apparently no record of its whereabouts and it does not appear to be on the Estate. It may well still be buried. The location of the pit has been discovered using old map records and by ‘walking the ground’, and it is hoped that geophysical methods may be able to detect the boulder at depth – the pit was 22 feet (6.7 m) deep – and that it can subsequently be retrieved. In 2006 the Red Crag/London Clay boundary was temporarily exposed in the Chicken Pit ‘pond’, actually an exploratory excavation originally dug by Bob Markham in September 1989. The entire exposed London Clay outcrop surface, about 2 m 2 m, is marked by abundant Zirfaea burrows, c.50 mm deep and c.40 mm apart. Occurrence in these numbers is unique for the Red Crag – only isolated examples of Zirfaea burrows have been found before at other localities. Zirfaea crispata is an infralittoral bivalve, commonly found from extreme low water spring tide to a depth of c.10 m. It mechanically bores into soft rock, firm clays, shales, silts, peat and wood. It is evident that the monospecific community thrived in this habitat before being overwhelmed by sand-waves. Prestwich also comments that Pholas (Zirfaea) was found boring into Coralline Crag in the Bullock-yard pit. Careful clearing in and around Barry’s pit, at the western end of the Chicken Pit, has revealed another unique fauna. The section shows blocks of Coralline Crag, which evidently littered the Red Crag beach/cliff zone, dominated by articulated Mytilus-rich assemblages. Many shells and cobbles of Coralline Crag have Red Crag barnacles attached. The blocks are interspersed by pockets of relatively clean sand, which reveal many perfectly preserved shells, including articulated Tapes and the brachiopod Terebratula apparently in the life position and with adult and juvenile forms present. Articulated Ensis and Macoma have also been found. Anomia, Buccinum, Capulus, Colus, Neptunea and the Naticidae family are also particularly well represented. It is evident that the fallen cliff blocks provided an ideal sheltered microenvironment for this unusual assemblage. Tapes typically occurs in shallow (extreme low water spring tide and below) sublittoral shell gravels and coarse sand. This important localised non-derived fauna is the subject of current research by GeoSuffolk members. Away from the ‘island’, exploratory excavations by English Nature for interpretive research, described by Dixon (2009a), revealed shelly sand-wave facies containing many phosphatic nodules. Because of the nature of the excavations, bedforms were indistinct at best. It is clear, however, that a sand-wave facies occurs away from the ‘island’, and sand-waves can be seen in the more permanently exposed faces of the Chicken Pit. From these, it can be observed that although variable, current directions trend generally towards the southwest, as elsewhere. Shell beds and other shell-rich layers probably represent shell avalanche material accumulating at the base of sand-wave foresets, again seen better in nearby sections. Pebble beds are lag and remanie´ deposits, left behind after currents have swept finer material away; the lag deposits indicate that current velocities must frequently been well in excess of c.0.5 m/s, scouring the sea bed surface and winnowing sediment. Offshore shell gravel assemblages dominate the mollusc fauna. Glycimeris-Venerupis assemblages tend to be found lower down the sequence, those two species making nearly 25% of some samples. Glycimeris is well-adapted to the physically rigorous conditions of a current-swept sand wave; associated Aloidis gibba, specialised for life in the muddy bottomset sands between sand-waves, is also common. Spisula shell gravel assemblages occur higher up the sequence, Spisula itself making over 20% of some samples. Extinct Spisula ovalis is possibly an eco- or phenotype of the extant S. solida (Long, 2000), which occurs in coarse sand from low beach to shallow depths, frequently in shell beds. Extinct Macoma species also become more frequent. Gastropod frequencies tend to be somewhat variable – they are easily rolled and winnowed by
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
currents. Indeed, although the assemblages are good indicators of the original mollusc communities, it is clear that they have been significantly modified by sedimentary processes. The vertical change in faunal assemblages can be identified elsewhere in the Red Crag basin, with Glycimeris-Venerupis assemblages being associated with older Red Crag outcropping to the south of the Deben and at depth, and younger Spisula assemblages outcropping to the north. This sequence can be taken to indicate a general shallowing during Red Crag times. Changes in the nature of sedimentary structures seen in better exposures in nearby pits also lead to this conclusion. It is also evident that the influence of the island rocky shore becomes more important towards the top of the sequence with a greater influx of Mytilus and other shoreline species, presumably being swept offshore by currents. A general picture builds up, then, of an initial Red Crag sea transgression with a period of intense scouring of the sea bed by strong currents, and erosion of the London Clay and Coralline Crag surfaces. Coarse pebble beds were left behind as remanie´ deposits. There then followed a period of sand-wave migration, the sedimentary structures and shell gravel faunas indicating a shallowing, regressive sequence. As the Crag basin became shallower the ‘island’ became a more positive feature and possibly emergent, with the development of non-derived shoreline and infralittoral faunas where suitable conditions allowed, but with much material being washed out to sea. It was at this stage that the ‘fossil’ cliffs, wave-cut platform and ‘beach’ were formed. It should be noted that this interpretation was the subject of some discussion by group members! After visiting the Quarry Pit, and ignoring demands for a lunch break (it was only 12.30 after all), the leader handed over the group to Barry Hall, who was celebrating his 25th wedding anniversary that day, for a guided tour of ‘The Pliocene Forest’. 2.3. ‘The Pliocene Forest’ In March 2009 GeoSuffolk applied for, and was awarded, a £372 grant from the Curry Fund of the Geologists’ Association to create a ‘Pliocene Forest’ – a plantation of representative plant genera that had existed during Coralline Crag times. Inspired by a pollen analysis from a borehole site near Orford by Andrew and West (1977) and, later, by Gibbard and Pegler (1988) from Sutton itself, the concept arose as part of a continuing programme of site development and enhancement. Pollen analyses are uncommon for the Crags – preservation is very poor (if at all) in the coarse, oxidised sediments, and British Pliocene Crag floras are still relatively poorly known. However, pollen analyses from the Coralline Crag have shown Pinus-dominated assemblages with Sequoia, Picea, Sciadopitys, Abies, Tsuga and others, including Liquidambar and Juniper, with heathers and grasses. They indicate a nearby Pliocene landscape dominated by forests containing a significant number of exotic genera now native to Eastern Asia and North America. GeoSuffolk member Barry Hall, a horticulturalist by profession, guided the work on the project. Deer and rabbits are a major threat to the plants in their early years; this meant the erection of protective fencing, creating a 10 m 4 m safe enclosure. A special manure-based compost and weathered bonfire ash was used at each planting site to improve water retention and the structure of the free-draining sandy soil. Continued nurturing and maintenance was needed until the plants become truly established. After discussion with Professor Richard West, GeoSuffolk planted four extant ‘Coralline Crag trees’ near the explanatory panel and close to the public footpath, using 2 m ‘standards’ to provide immediate visual impact: Sciadopitys verticillata (Japanese Umbrella Pine), having a fossil record that goes back some 230 Ma
519
and already in decline by the mid-Cretaceous; Liquidambar styraciflua (Sweetgum), with a similar form found in the Miocene of Europe; Tsuga heterophylla (Western Hemlock); and Tsuga canadensis (Eastern Hemlock). By the end of the year, additional donations had provided the two grasses, Stipa gigantea and S. tenuissima, and the shrub Atriplex halimus (Salt-bush, which perished in the harsh winter of 2009/2010) with the contrasting annual A. hortensis ‘rubra’ (Red Orach). The project was described by the author in the Magazine of the Geologists’ Association (Dixon, 2009b). The innovative original forest has aroused considerable interest among visitors and field groups that have visited the site. So much so that GeoSuffolk decided to seek to expand the ‘Pliocene Forest’ on a much larger scale and longer timescale. In August 2009 the landowner of Sutton Knoll, during a guided visit with his family to the ‘Pliocene Island’ by GeoSuffolk members, gave approval for GeoSuffolk’s ongoing Site Management Plan in general and the expansion of the ‘Pliocene Forest’ in particular. As Sutton Knoll [Rockhall Wood] is a SSSI, GeoSuffolk, through the landowner, also applied to Natural England for consent to make these changes, justified on scientific, educational, aesthetic and general interest grounds. In March 2010 a further grant of £1692 was awarded by the Curry Fund to fence an expanded planting area of some 800– 900 m2, centred more or less on the original enclosure, the work being carried out by Jeff Redgrave, a local geologist. Planting is arranged to have multiples of the same plant at different growth stages with a sub-theme of strong contrast between species of the same genera (e.g. Atriplex hortensis ‘Rubra’ and A. halimus). Additional representative species include Pinus coulteri (largest cones) and Cryptomeria japonicum ‘Sekkan-sugi’ (foliage and colour). Bush and shrub forms of some of the tree genera are also being planted as understorey. Helped by further fundraising, the spring 2010 season saw the planting of several donated and sponsored trees: Tsuga canadensis (Eastern Hemlock, tree form); Abies nordmanniana (Caucasian Fir – after a spell of duty as a Christmas tree); Cryptomeria japonica ‘Lobii nana’ (dwarf form); C. japonica ‘Sekkan-sugi’ (foliage); Platycarya strobilacea (Caucasian Walnut); Sciadopitys verticillata; Microbiota decussata (dwarf Cypress native to Siberia); Cunninghamia lanceolata (China Fir); Picea breweriana; P. omorika; P. pungens ‘Hoopsii’; Quercus suber; Betula utilis var. ‘Jacquemontii’; Metasequoia glyptostroboides (Dawn Redwood); Ostrya carpinifolia (Hop Hornbeam). Barry Hall is maintaining a ‘Forest’ database, which includes: name, type of plant, height & spread at planting, date of planting, location, source and cost, and donor/sponsor. Many other species of tree and shrub are under consideration. Monitoring and recording of rainfall has been initiated and the height of the water table being measured. 3. Locality 2: Bawdsey East Lane [TM 356397] The group then drove to East Lane car park, where lunch was taken amid remains of concrete bunkers from World War Two, prior to examining the foreshore south of the Martello Tower. 3.1. The London Clay A brief description of the London Clay was given by David Bone. The London Clay comprises brown and grey predominantly silty clays that were deposited in a shallow sea that covered much of south-east England during the Eocene Epoch about 53 million years ago. 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
520
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
[()TD$FIG]
compacted on the sea floor. Plant debris, logs and branches, seeds and fruits, are now preserved as pyritised fossils. Walton-on-theNaze, only 16 km away, is a key locality for the study of Palaeogene plants in Britain and has considerable potential for future research. A useful summary of the sections at Walton, Harwich and Wrabness is given by Daley (1999, pp. 61–72). Indeed, the London Clay exposed at Bawdsey is very similar to that exposed on the foreshore at Walton. Many species of sharks and other fish swam in the seas, and other vertebrates including turtles, have been found. Walton is also an internationally important site for finds of tiny bird bones – also found at Bawdsey. A small invertebrate (mostly mollusc) fauna also occurs. Well records show the London Clay at Bawdsey Manor to be 87 feet (26.5 m) thick (Whitaker, 1906) and Davis and Elliot (1951) refer to between 11 and 20 feet (3.4–6.6 m) of blue London Clay, overlain by Red Crag, to be seen in the cliff one mile north-east of Bawdsey Ferry (i.e. East Lane). Currently about 5 m of blue-grey silty London Clay is well exposed in the cliff and on the foreshore at East Lane, just to the south of the Martello tower, and is actively being eroded by the sea. When viewing the wave-cut platform from the beach or cliff top, the bedding can be picked out quite clearly, showing that the London Clay has been gently folded with some small faults. Calcareous mudstone concretions, ‘septaria’, once used locally for making ‘Roman’ cement, occur in thin bands within the London Clay. No evidence was found for the thin layers of pale creamy yellowish rust-coloured sediment of volcanic ash; it was suggested during discussion that the sequence at East Lane overlies the ash horizons, seen at Bawdsey Manor Cliffs and further south. The ash originated from ocean floor volcanoes as the North Atlantic opened up and with the break-up of the Greenland-Rockall plate, and was wind-blown from these volcanoes 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 correlate with Greenland volcanic events at 53 Ma (Daley, 1999, pp 14–21). Much pyritized debris was found washed out on the beach; there are also pockets of drifted woody material which contain fruits, diatoms and radiolaria. The foraminifera Astrorhiza is abundant. The teeth of the sharks Carcharias hopei, C. teretidens,
Striatolamia macrota and Otodus obliquus, and fish vertebrae can be found. In the cliff, the London Clay/Red Crag unconformity was well exposed. It is highly irregular, partly due to later cryoturbation, but in many ways resembles the eroded London Clay surface of the present wave-cut platform. There is c.2.5 m of Glycimeris-rich Crag with a well-developed but discontinuous basal pebble bed of phosphatic nodules and containing Boxstones, large flints and material derived from the London Clay. Several shark teeth, including that of the shark Isurus hastalis, fragments of ray palate and cetacean bone fragments were found, but the find of the day was undoubtedly that by David Bone of an extremely rare molar from the lower jaw of a rhinoceros, Dicerorhinus megarhinus (Shreve pers. commun.). 3.2. Coastal defences The Martello Tower was built in 1809 with 750,000 bricks at a cost of £9784 to help protect the shore from Napoleon’s forces. It is one of only 17 left on the East coast. Twenty years ago it was protected by a large expanse of shingle beach (c.20 m) and 25 m of land, and wooden groynes restricted long-shore drift. It was bought by architect John Fell-Clark, restored and converted into a 2-floor dwelling, now let as a holiday home. In 1996 a new erosional cycle started; a big storm in 1997 removed all the beach and some of the cliff. By 2005 the tower stood only 10 m from the cliff top. Following the storm, Suffolk Coastal District Council approved emergency funding for limited boulder-style defence work in front of the tower, but this served to exacerbate erosion immediately to the south. Furthermore, East Lane Point is considered of key strategic importance in controlling the movement of shingle between Aldeburgh and Felixstowe. Bawdsey scored low on Department of the Environment and Environment Agency priority lists for coastal defences. Consequently, the East Lane Trust was formed to raise privately the estimated £2.2m needed for sea defence work to give greater protection to farm land, homes and the tower. Locally owned farmland in surrounding villages (Hollesley, Bawdsey, Alderton) was donated to the Trust, which then sold it off for housing development. Special governmental consent was given in 2007
Fig. 7. At the time of the emplacement of the ‘rock armour’ to protect the Martello Topwer in 2009 a shingle beach and talus afforded some protection to the cliffs, which were slowly degrading. The shingle is now entirely removed; there is rapid basal cliff erosion with cliff falls assisted by water seepage at the London Clay/Red Crag junction.
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
allowing residential development on some of the plots that lay outside the ‘Local Plan’. In 2008 this privately funded project was finally given the goahead – it is believed to be unique in England, and work began in September. 22,000 tonnes of ‘granite’ (according to the press) were brought by barge from Norway, each boulder weighing c.6 tonnes, to make ‘rock armour’ for c.350 m of cliff. In fact, much of the material is Larvikite and Gabbro, with a large proportion of French Carboniferous Limestone, complete with crinoids, corals and brachiopods. Work was completed in June 2009 at a cost of c. £2.4m. Although some of the excellent exposure of London Clay and phosphatic basal Red Crag pebble bed had been lost as a result of the work and subsequent slope degradation, the focus of erosion has merely shifted slightly, c.150 m southwards, for within 2 months of completion, waves still reached the base of the cliffs and falls were still taking place. In February 2010 a further 12,000 tonnes of rock was brought in from Northern France to reinforce coastal protection north of the tower to prevent flooding of low-lying farmland at a projected cost, using ‘emergency funding’, of £1.5 million. Although the ‘rock armour’ was ‘‘holding up well and performing as intended’’ (Fell-Clark, letter to the Editor, East Anglian Daily Times 6.2.10), at the present time the entire shingle beach has been removed by storms and the cliff section is actively being rapidly eroded – it has never looked better! (Fig. 7). 4. Locality 3: Alderton House Pit [TM 328412] Members then drove the short distance to Locality 3, an old Red Crag section, now much overgrown, comprising an ‘old’ face and a ‘new’ 3 m face lying some 50 m to the west, first exposed in the 1950s. The older eastern section shows c.1.3 m thickness of largescale sand-wave foresets [Bed 2], indicating a current direction of 230–2408, the regional trend. It is stratigraphically overlain by about 60 cm of trough cross-bedded shelly Crag [Bed 3]. The ‘new’ face shows medium-scale cross-bedding, demonstrating subordinate 2608 regional current directions, but dominant cross-cutting tidal flow directions towards 0808, with channelling, mud drapes and other shallow water sedimentary structures [Bed 2]. Some small, thin endogenetic tubes/burrows (provisionally attributed to an amphipod) occur and bioturbation is common in silty partings. It would appear that Bed 2 of the ‘new’ face represents an accumulation of inter-ripple trough-zone sediments formed at the same time as the larger scale foresets of the ‘old’ face. A pebble lag deposit occurs at the base of the shellrich uppermost horizon [Bed 3], rich in phosphatic nodules and flints (including a flint belemnite and flint Terebratula). About 35 cm is currently exposed. Large fragments of Echinocardium cordatum test have been found in this horizon (Dixon, 2001a). Bed 2 in both faces is underlain by a further 1.8 m of sands [Bed 1] with comminuted shells interbedded with bioturbated silts containing Arenicola-type tubes, now completely obscured by talus but exposed in the 1970s. The Crag is rich in shells and this was a good opportunity for members to sieve material and collect. The mollusc fauna is dominated by Spisula shell gravel assemblages in all horizons – the frequencies of dominant species from selected spot samples are given in Table 1. The method used was given by Dixon (1977) but all species found were divided into their ‘ecological groups’ – the total frequency in each group is also given. It can be seen that there is a significant increase in intertidal Mytilus and Mya frequencies, and a corresponding decrease in sublittoral Glycimeris and Venerupis in the upper beds, reflecting the regional trend of shallowing water depths. There is also a notable abundance of the Pea Urchin, Echinocyamus pusillus (Dixon, 2001b), and the solitary
521
Table 1 Percentage frequencies of dominant Red Crag molluscan species and ecological groupings of all species from selected sample points at Alderton House Pit.
Intertidal rocky shore Mytilus edulis Total Intertidal sandy shore Mya arenaria Total Infralittoral Sublittoral shell gravel Glycimeris glycimeris Venerupis spp Total Sublittoral muddy sand Aloidis gibba Total Sublittoral clean sand Dosinia exoleta Total Sublittoral mud tolerant Turritella triplicata Total Sublittoral epifauna Buccinum undatum Neptunea contraria Total Miscellaneous Chlamys opercularis Venus ovata Total Extinct Nassa granulata Natica multipunctata Polinices hemiclausa Nucella lapillus Astarte obliquata Cardita corbis C. scalaris Cardium interuptum Macoma obliqua M. praetenuis Spisula ovalis Venus imbricata Total Unidentifiable Total species % of 1 individual
Bed 1
Bed 2
Bed 3
9.0 9.0
8.9 9.3
13.5 14.3
0.0 0.0 0.0
0.0 1.0 1.2
3.9 4.4 1.0
5.0 7.4 13.0
4.1 9.5 14.9
4.3 0.0 6.1
9.3 9.6
8.1 8.5
1.7 2.0
2.0 3.1
2.8 3.3
1.2 1.4
6.9 7.2
13.5 14.0
8.9 9.2
0.0 0.0 1.0
0.0 0.0 0.0
1.1 1.1 2.8
0.0 1.0 2.5
0.0 1.1 2.2
3.6 0.0 4.5
0.0 0.0 0.0 0.0 1.9 2.2 2.9 1.3 2.2 2.9 27.1 1.7 46.2
0.0 0.0 0.0 0.0 1.7 1.0 2.2 1.4 1.6 4.6 14.6 1.4 36.6
3.3 1.1 1.1 4.0 0.0 0.0 0.0 1.0 2.2 4.9 16.3 0.0 44.1
6.0 57 0.22
9.3 73 0.16
9.3 81 0.14
corals Sphenotrochus and Balanophyllia are common here. Rolled bryozoans, derived from the Coralline Crag, are much in evidence, and shark and ray teeth are not uncommon. Of particular note is the finding of a Pika tooth, Prolagus, by Bob Markham from Bed 3 of the ‘new’ face (Markham pers. com.) and the presence of Scrobicularia plana in the upper beds (Dixon, 1976). In many respects, the sequence and mollusc assemblages resemble those at Vale Farm and Buckanay Farm, both seen during the Geologists’ Association excursion in 2004.
5. Locality 4: Tarrants Pit, Ramsholt [TM 303422] It was only a short drive to the final stop of the day, an 8 m deep working Red Crag pit. There can be seen medium- and large-scale sand-waves with beds of shelly material, small-scale sand-waves, mud drapes, pebbly lag deposits – at least four, with phosphatic nodules, flint pebbles and convex-up Glycimeris valves (best seen in the west and east faces). The section thus shows a series of superimposed sand-waves: pebble beds demonstrating episodes of inter-sand-wave scouring, followed by small-scale sand-waves with associated mud drapes forming as bottomsets in trough
[()TD$FIG]
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
522
Fig. 8. One of the pebbly lag deposits at Tarrant’s Pit; it indicates scour prior to bottomset deposition. Note the convex-up position of the Glycimeris valves.
zones, and finally larger scale foresets with shell avalanche material at their base. Reactivation surfaces can also be seen – a well-defined such surface can be seen in the east face, with a 2 m sand-wave above it. Current directions are predominantly towards the south-west and west, as is the regional trend, but subordinate cross-cutting directions towards the north-east are significant (Fig. 8). Although not at first sight rich in shells, even a cursory search yields Spisula/Macoma/Mytilus-rich assemblages – a shallow water fauna typical of the upper part of the Red Crag in the Ramsholt/ Alderton area. An abundance of small, thin, wiry tube-like trace fossils can be seen in some horizons, usually cleaner sands, as seen at Alderton House; they possibly belong to amphipods but their origin is uncertain. The top-most 1–1.5 m of Crag is decalcified. Two small reverse faults can be seen in the centre of the south face, with a combined throw of c.40 cm. Several members sieved material and collected an interesting range of fossils – of note were many Nucella and some phosphatic nodules encrusted with barnacles found in the pebbly lag horizons. One member found a much-rolled belemnite. Other mollusc species found included Glycimeris glycimeris, Arctica islandica, Chlamys opercularis, Pygocardia rustica, Neptunea contraria, N. ‘despecta’, Nucella lapillus, N. tetragona, Nassa reticosa and Scaphella lamberti. The pit was last visited by Geologists’ Association on Sunday 20th July, 1947, the final stop on a long week-end excursion based at the Crown and Anchor Hotel, Ipswich. The pit was not named, but stated as ‘‘. . .a quarry north-west of Ramsholt Church. . .’’. The Crag was described as intermediate between Newbournian and Butleyan and many well-preserved fossils were collected (Ovey and Pitcher, 1948). The meeting closed at about 17.30, after a good day enjoyed by all. Acknowledgements Sutton Knoll is privately owned; GeoSuffolk would like to thank Sutton Hall Farms for innumerable access permissions and their continuing kind support. Mr. James Adeane kindly granted access permission for both Alderton House Pit and Tarrants Pit, both also privately owned. Thanks go to Jim Rose and the two peer
reviewers, Jan Zalasiewicz and Peter Riches – the manuscript has greatly benefited from their critical appraisal and is now much improved. Thanks also go to David Bone, who commented on earlier drafts of the London Clay at Bawdsey. Barry Hall is to be commended for guiding the party around the ‘Pliocene Forest’. Very special thanks go to Bob Markham, whose input as co-leader was greatly appreciated and who made many useful suggestions on the first draft of this report.
References Andrew, R., West, R.G., 1977. Pollen spectra from Pliocene Crag at Orford, Suffolk. New Phytologist 78, 709–714. Balson, P.S., 1999. The Coralline Crag. In: Daley, B., Balson, P. (Eds.), British Tertiary Stratigraphy. Geological Conservation Review Series No.15. Joint Conservation Committee, Peterborough, pp. 253–288. Balson, P.S., Long, P.E., 1988. Rockhall wood, Sutton. In: Gibbard, P.L., Zalasiewicz, J.A. (Eds.), Pliocene–Middle Pleistocene of East Anglia Field Guide. Quaternary Research Association, Cambridge, pp. 66–72. Balson, P.S., Humphreys, B., Zalasiewicz, J.A., 1990. Field guide No. 3: Coralline and Red Crags of East Anglia.In: 13th International Sedimentological Congress, UK. Balson, P.S., Mathers, S.J., Zalasiewicz, J.A., 1993. The lithostratigraphy of the Coralline Crag (Pliocene) of Suffolk. Proceedings of the Geologists’ Association 104, 59–70. Boswell, P.G.H., 1928. The Geology of the country around Woodbridge, Felixstowe and Orford. Memoir of the Geological Survey of England and Wales. Daley, B., 1999. In: Daley, B., Balson, P. (Eds.), British Tertiary Stratigraphy. Geological Conservation Review Series No. 15, Joint Nature Conservation Committee, Peterborough. Davis, A.G., Elliot, G.F., 1951. The London clay of Coastal Suffolk and Essex. Geological Magazine 88, 329–337. Dixon, R.G., 1976. The occurrence of Scrobicularia plana in the Red Crag, Lower Pleistocene. Proceedings of the Geologists’ Association 87, 409–412. Dixon, R.G., 1977. A note on the palaeoecology of the Red Crag (Lower Pleistocene). Quaternary Newsletter 23, 1–5. Dixon, R.G., 1979. Sedimentary facies in the Red Crag (Lower Pleistocene, East Anglia). Proceedings of the Geologists’ Association 90, 117–132. Dixon, R.G., 2001a. A note on the occurrence of Echinocardium cordatum from the Red Crag, Pliocene, East Anglia. Bulletin of the Geological Society of Norfolk 51, 105–112. Dixon, R.G., 2001b. A short note on the distribution of Echinocyamus pusillus in the Red Crag. Bulletin of the Geological Society of Norfolk 51, 113–116. Dixon, R.G., 2005. Field meeting: Coastal Suffolk Crag week-end, 23–25 April, 2004. Proceedings of the Geologists’ Association 116, 149–160. Dixon, R.G., 2006. Sutton Knoll. Transactions of the Suffolk Naturalists’ Society 42, 97–101. Dixon, R.G., 2007. Autochthonous Mollusc Faunas from the Red Crag. In: Dixon, R.G. (Ed.), Proceedings of GeoSuffolk RIGS Meeting, Transactions of the Suffolk Naturalists’ Society, 43, pp. 12–16.
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523 Dixon, R.G., 2009a. The Red Crag at Sutton Knoll. Transactions of the Suffolk Naturalists’ Society 45, 1–9. Dixon, R.G., 2009b. The Pliocene forest. Magazine of the Geologists’ Association 8 (3), 22. Gibbard, P.L., Pegler, S.M., 1988. Pollen analysis from Rockhall Wood, Sutton. In: Gibbard, P.L., Zalasiewicz, (Eds.), Pliocene–Middle Pleistocene of East Anglia. Field Guide, Quaternary Research Association, Cambridge. Long, P.E., 2000. Norwich Crag Puzzles: two questions raised by molluscs. In: Dixon, R.G. (Ed.), The Geological Society of Norfolk Jubilee Volume. pp. 59–65. Ovey, C.D., Pitcher, W.S., 1948. Observations on the geology of East Suffolk. Proceedings of the Geologists’ Association 59, 23–34. Prestwich, J., 1871a. On the structure of the Crag-beds of Suffolk and Norfolk with some Observations on the organic remains. I. The Coralline Crag of Suffolk. Quarterly Journal of the Geological Society of London 27, 115–146.
523
Prestwich, J., 1871b. On the structure of the Crag-beds of Suffolk and Norfolk with some Observations on the organic remains. II. The Red Crag of Essex and Suffolk. Quarterly Journal of the Geological Society of London 27, 325–356. Whitaker, W. 1906. The Water Supply of Suffolk, from Underground Sources. Memoir of the Geological Survey of Great Britain. Williams, M., Haywood, A.M., Harper, E.M., Johnson, A.L.A., Knowles, T., Leng, M.J., Lint, D.J., Okamura, B., Taylor, P.D., Zalasiewiz, J., 2009. Pliocene climate and seasonality in North Atlantic shelf seas. Philosophical Transactions of the Royal Society A 367, 85–108. Wood, A.M., 2009. Neogene. In: Whitaker, J.E., Hart, M.B. (Eds.), Ostracods in British Stratigraphy. The Micropalaeontological Society, Special Publication, pp. 411–446.
Contents lists available at ScienceDirect
Proceedings of the Geologists’ Association journal homepage: www.elsevier.com/locate/pgeola
Field Meeting to the Bawdsey Peninsula, Suffolk, England, 22nd May 2010, to examine London Clay, Coralline Crag and Red Crag deposits Leaders: Roger Dixon and Bob Markham Roger Dixon * The White House, 7 Chapel Street, Woodbridge, Suffolk IP12 4NF, United Kingdom
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 July 2010 Received in revised form 8 February 2011 Accepted 11 February 2011 Available online 16 March 2011
A one-day Geologists’ Association field excursion to the Bawdsey Peninsula in coastal Suffolk is described, with a guided tour of ‘The Pliocene Island’ at Sutton Knoll and visits to examine coastal erosion, the London Clay and Red Crag at Bawdsey East Lane, and the Red Crag at Alderton and Ramsholt. The depositional environments indicated by the Crag sediments and faunas are discussed, and nonderived Red Crag mollusc faunas at Sutton and sand-wave mollusc assemblages from Ramsholt are described. ß 2011 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.
Keywords: Pliocene Coralline Crag Red Crag Rockhall Wood ‘Pliocene Forest’ Mollusc faunas
1. Introduction
2. Locality 1: Sutton Knoll [TM 305440]
With the barometer rising, temperatures soaring into the 20s and not a breath of wind, 26 members of the Geologists’ Association and the GeoSuffolk team met on a gloriously sunny day for an excursion to the Bawdsey Peninsula in coastal Suffolk. The Geologists’ Association last visited the area in April 2004 for a long week-end (Dixon, 2005) – this was a visit to see just some of the locations not seen then (Fig. 1). The stratigraphy seen during the day consisted of: Eocene London Clay underlies the area and is unconformably overlain by outliers of mid-Pliocene Coralline Crag. Late Pliocene Red Crag unconformably overlies both these deposits, and there is evidence of the later Early and early Middle Pleistocene proto-Thames Kesgrave Sands and Gravels. After a short briefing at Sutton Heath (National Grid Reference TM 306475), the group drove the short distance to Sutton Knoll. Here was held a ‘mass opening’ of the new enclosure, funded by the Curry Fund of the Geologists’ Association, for an expanded ‘Pliocene Forest’. The group held on to a length of red ribbon, which was snipped by Curry Fund committee member David Bone, who spoke about the work of the Curry Fund (Fig. 2).
This famous site, which includes Rockhall Wood Site of Special Scientific Interest (SSSI), has been studied for over 170 years, long known as an island of Coralline Crag during Red Crag sea times, with its Coralline Crag cliffs and boulders ‘buried’ in Red Crag. The classic description was by Prestwich (1871a,b), some of the figures of which were reproduced by Boswell (1928), but Balson (1999) gives an excellent recent description and discussion. Other recent descriptions include Balson and Long (1988) and Balson et al. (1990, 1993) describe the stratigraphy of the Coralline Crag as a whole. Dixon (2006, 2007, 2009a) describes recent developments. The locality has frequently been visited by field groups and has often been used for educational purposes, including undergraduate project work. English Nature (now Natural England) cleared faces in 2006 as part of their ‘Facelift’ programme and members of the local GeoSuffolk group have been actively maintaining and developing sections. Furthermore, new research by GeoSuffolk members is currently underway to help interpret the site from a more modern perspective (Fig. 3).
* Corresponding author. E-mail address: [email protected].
2.1. Coralline Crag Suffolk’s Pliocene Coralline Crag, deposited approximately 3.75 million years ago, outcrops as a 12 km inlier ridge running from the north of Aldeburgh south-westwards to the Butley River, with further small outliers, including Sutton, to the southwest, and rests
0016-7878/$ – see front matter ß 2011 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.pgeola.2011.02.002
[()TD$FIG]
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
Fig. 1. Location map of the sites visited during the Geologists’ Association field excursion on May 22nd, 2010. 1, Sutton Knoll; 2, Bawdsey, East Lane; 3, Alderton House Pit; 4, Tarrant’s Pit.
unconformably on the London Clay (see Fig. 4). The bioclastic sands, about 20 m thick, can be broadly divided into three sedimentary and faunal units, two of which outcrop at Sutton: a lower Ramsholt Member and an overlying Sudbourne Member
515
(seen also at Richmond Farm (TM 413493) and Crag Farm (TM 428523) SSSIs and elsewhere). The beds are possibly best known for their bryozoan fossils (‘Corallines’) and the common occurrence of one of Britain’s largest fossil brachiopods, ‘Terebratula grandis’. Over 300 species of mollusc have been recorded from these beds by Searles Wood and others, many extant forms having ‘southern’ or Lusitanic distributions, and suggest summer water temperatures approaching 17–20 8C and winter temperatures of 6–9 8C. However, ‘mixed signals’ are given by different faunal elements (Williams et al., 2009) and there is no consensus of opinion. A useful overview and discussion of Neogene correlation and environmental interpretation, with particular reference to the role of ostracods is given by Wood (2009). The group first examined the lower beds of the Sudbourne Member at the North Face. This 50 m long, 3 m high section on the northern edge of the Knoll was cleared by GeoSuffolk members in 2008/2009. The beds contain small and medium scale sand-waves with mud drapes and laminations, possibly of tidal origin, and abundant infaunal burrow types, including those of worms, crustaceans and spatangoid echinoids, and a good bryozoan fauna. The beds yield a predominance of smaller, more fragile shells (e.g. Lucina, Venus, Aloidis, Pteromeris and Anomia) with abundant foraminifera, which possibly have been deposited selectively by currents. A well-preserved 3 m solution hollow was clearly observed (Suffolk’s own karstic landscape); infill is mostly of more recent structureless and unfossiliferous regolith, making dating of it impossible, but the hollow itself may pre-date the Red Crag, when subaerial processes prevailed. A bed of finer grained silty Crag at the base of the face marks the boundary between the Sudbourne and underlying Ramsholt Members. Following the work by English Nature, the lower part of the Bullockyard Pit, seen next by the party on the eastern side of the Knoll, currently exposes about 5 m of silty/muddy sands of the Ramsholt Member, possibly dating to as old as c.4.2 Ma (Wood, 2009), making the section better than it has been since the days this when was a working pit. Exploratory excavations have extended below the water-table to the London Clay. Medium-scale sand-waves are evident, although often indistinct, with muddrapes, smaller ripples and extensive bioturbation, indicating weaker currents and slow depositional rates. Estimates of water depth vary between <20 m and >50 m, but the sedimentary
[()TD$FIG]
Fig. 2. David Bone cutting the ribbon to open the new extension to the ‘Pliocene Forest’ (Photo: R Gwyn-Thomas).
[()TD$FIG]
516
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
Fig. 3. Sutton Knoll site plan. The area of Site of Special Scientific Interest is stippled.
structures clearly indicate a depth shallower than storm base. An abundant benthic fauna of molluscs and bryozoans is present, and differences occur in the faunas from different beds. Well-defined accumulations of larger, more robust shells (e.g. the bivalves Arctica islandica, Cardita senilis, Pecten spp, Astarte spp, Venus spp, Glycimeris glycimeris and brachiopod Terebratula), often articulated, form shell-beds (currently exposed in a drainage sump). The corals Spenotrochus and Cryptangia and fish otoliths are not uncommon in the shell beds; other common mollusc species include Aloidis gibba, Pecten tigrinus, P. maximus, Lucina borealis and Cardita scalaris. The presence of a full size range for many species; articulated valves of juveniles of many species, including Terebratula; entire echinoid tests, including the full size range of Temnechinus, with fragments of Echinus sp. and Spatangid sea urchins; and a good fauna of smaller species indicate that these shell beds may represent non-derived shell-gravel faunas. Later in the morning, the group stopped to view the Coralline Crag ‘Rock Bed’, found on the north-western side of the Knoll in the Quarry Pit. About 5 m of shelly sands (Sudbourne Member) are exposed, cemented to form a relatively durable limestone, with evident bioturbation. The member exhibits well-developed medium- and large-scale trough cross-bedding, which is interpreted as storm and tidal sand-waves migrating in relatively shallow water (c.30–50 m) in a generally south to south-west direction. In this bed, post-depositional solution processes have dissolved aragonite shells, and re-precipitated carbonate in the form of calcite to form a
cement. The ‘Rock Bed’ processes occurred before the deposition of the later Red Crag, as pebbles and boulders of ‘Rock Bed’ are found in the Red Crag, and the term has no other stratigraphic implication. It was often used locally as a building stone in farm walls, including on the Sutton Estate, and in other structures such as the river walls. It can also be seen as major components in nearby Chillesford and Wantisden churches, and Orford Castle. Joint sets are well developed in the ‘Rock Bed’ and can be seen in the pit walls; they may be related to the flexuring of the North Sea Basin. Occasional infilled solution pipes can be seen extending down from the surface. 2.2. Red Crag The Red Crag is of late Pliocene age, around 2.5 Ma, unconformably overlying the Coralline Crag and London Clay (Fig. 5). It was formed in a high energy, shallowing sea dominated by strong tidal currents, with submarine sand waves piling up against the shoreline to the west; the bedforms are considered classic of their type. It is dominated by molluscan shell gravel assemblages, which indicate climatic conditions similar to those of Britain today: Glycimeris-Tapes assemblages occur at Walton-onthe-Naze and elsewhere in the south, with Spisula, Macoma, Mya and Mytilus assemblages in the Deben/Butley area (Dixon, 1979). Until recently surprisingly little was known about the Red Crag at Sutton, and no modern (post-war) quantitative or detailed
[()TD$FIG]
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
517
Fig. 4. Illustrative sketch section of the Coralline Crag ‘island’ at Sutton during the time of the Red Crag sea (first published in Dixon, 2007 and used on the interpretive panel at the site).
molluscan studies or sedimentological studies have been done. However, it is now clear that very different localised molluscan communities were able to develop in the different microenvironments that must have existed around the ‘island’ (Dixon, 2007, 2009a). The Red Crag abutting and unconformable on Coralline Crag is best seen on the east face of the ‘island’ at the southern end of the Bullockyard Pit, but is much overgrown. Prestwich (1871a) recorded over 2 m of Red Crag and his sketch sections show Red Crag banked up against a cliff of Coralline Crag. It contains much derived Coralline Crag fauna, and even pebbles of that Crag encrusted with Red Crag barnacles. The ‘Facelift’ clearance work
[()TD$FIG]
exposed over 13 m of the wave-cut platform cut by the Red Crag sea into the Coralline Crag. It dips southwards at over 108 and blocks of Coralline Crag can be seen resting on it. The precise boundary is not easy to see at first sight because of the amount of derived Coralline Crag material within the Red Crag, and it takes a well-practised eye to trace it laterally. When cleared in 2006 it was probably the first time it had been exposed for over 100 years! Colonies of Mytilus edulis and Neptunea contraria are found at the base of the Red Crag adjacent to the Red Crag/Coralline Crag boundary, and indeed were noted by Prestwich. In fact, they can help to identify the boundary itself when the two lithologies may at first sight appear so similar. The valves are articulated and
Fig. 5. The basal Red Crag pebble bed of phosphatic nodules and Boxstones overlies London Clay at Bawdsey, East Lane.
[()TD$FIG]
518
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
Fig. 6. The Bone Tooth – a rare Red Crag rhino molar found in the Red Crag at Bawdsey, East Lane (Photo: A Bone).
closed, and the full size range is present, juveniles as well as adults. Mytilus is typically an intertidal rocky shore species, attaching itself to firm substrates by means of its byssal thread, and the communities discovered at Sutton are undoubtedly in their life positions (Fig. 6). Group members then walked over the knoll, pausing at the summit to find quartz and quartzite pebbles. These may suggest the former presence of Kesgrave Sands and Gravels, deposited between 0.6 and 1 Ma before the Anglian glaciation. The gravels, found in much of eastern Suffolk and Essex, mark the position of the former course of the proto-Thames. A further stop was made at the southern end of the SSSI to view from a distance Ramsholt Rocks, a further small outlier of Coralline Crag on the left bank of the River Deben, and to discuss the Crag Basement Pebble Bed (seen later at Bawdsey). At the base of both Crags is often found a discontinuous basal lag or remanie´ pebble bed containing cetacean remains, shark teeth and other fish remains, flints, ‘boxstones’ (rounded pebbles of late Miocene/early Pliocene sandstone, c.6 Ma old), crustaceans and, importantly, phosphatic nodules (mistakenly called ‘coprolites’) derived from the London Clay. These ‘coprolites’ were excavated on a large scale from Sutton and elsewhere in the local area during the midnineteenth century for making phosphate fertiliser. The waste material of flints and ‘boxstones’ was used as a building material in the nineteenth century restoration of Sutton and Shottisham churches and in some other local buildings, including barns on the Sutton Estate. Of particular interest was the recording by Prestwich (1871a) in this layer of a ton porphyry block from a trial pit in 1860 on the western flank of the Knoll, a site just visible from our view-point. It was the only known coprolite pit dug in the Coralline Crag (rather than Red Crag), but did not prove profitable and was filled in by the mid-1860s. It is not known what happened to the porphyry
boulder – there is apparently no record of its whereabouts and it does not appear to be on the Estate. It may well still be buried. The location of the pit has been discovered using old map records and by ‘walking the ground’, and it is hoped that geophysical methods may be able to detect the boulder at depth – the pit was 22 feet (6.7 m) deep – and that it can subsequently be retrieved. In 2006 the Red Crag/London Clay boundary was temporarily exposed in the Chicken Pit ‘pond’, actually an exploratory excavation originally dug by Bob Markham in September 1989. The entire exposed London Clay outcrop surface, about 2 m 2 m, is marked by abundant Zirfaea burrows, c.50 mm deep and c.40 mm apart. Occurrence in these numbers is unique for the Red Crag – only isolated examples of Zirfaea burrows have been found before at other localities. Zirfaea crispata is an infralittoral bivalve, commonly found from extreme low water spring tide to a depth of c.10 m. It mechanically bores into soft rock, firm clays, shales, silts, peat and wood. It is evident that the monospecific community thrived in this habitat before being overwhelmed by sand-waves. Prestwich also comments that Pholas (Zirfaea) was found boring into Coralline Crag in the Bullock-yard pit. Careful clearing in and around Barry’s pit, at the western end of the Chicken Pit, has revealed another unique fauna. The section shows blocks of Coralline Crag, which evidently littered the Red Crag beach/cliff zone, dominated by articulated Mytilus-rich assemblages. Many shells and cobbles of Coralline Crag have Red Crag barnacles attached. The blocks are interspersed by pockets of relatively clean sand, which reveal many perfectly preserved shells, including articulated Tapes and the brachiopod Terebratula apparently in the life position and with adult and juvenile forms present. Articulated Ensis and Macoma have also been found. Anomia, Buccinum, Capulus, Colus, Neptunea and the Naticidae family are also particularly well represented. It is evident that the fallen cliff blocks provided an ideal sheltered microenvironment for this unusual assemblage. Tapes typically occurs in shallow (extreme low water spring tide and below) sublittoral shell gravels and coarse sand. This important localised non-derived fauna is the subject of current research by GeoSuffolk members. Away from the ‘island’, exploratory excavations by English Nature for interpretive research, described by Dixon (2009a), revealed shelly sand-wave facies containing many phosphatic nodules. Because of the nature of the excavations, bedforms were indistinct at best. It is clear, however, that a sand-wave facies occurs away from the ‘island’, and sand-waves can be seen in the more permanently exposed faces of the Chicken Pit. From these, it can be observed that although variable, current directions trend generally towards the southwest, as elsewhere. Shell beds and other shell-rich layers probably represent shell avalanche material accumulating at the base of sand-wave foresets, again seen better in nearby sections. Pebble beds are lag and remanie´ deposits, left behind after currents have swept finer material away; the lag deposits indicate that current velocities must frequently been well in excess of c.0.5 m/s, scouring the sea bed surface and winnowing sediment. Offshore shell gravel assemblages dominate the mollusc fauna. Glycimeris-Venerupis assemblages tend to be found lower down the sequence, those two species making nearly 25% of some samples. Glycimeris is well-adapted to the physically rigorous conditions of a current-swept sand wave; associated Aloidis gibba, specialised for life in the muddy bottomset sands between sand-waves, is also common. Spisula shell gravel assemblages occur higher up the sequence, Spisula itself making over 20% of some samples. Extinct Spisula ovalis is possibly an eco- or phenotype of the extant S. solida (Long, 2000), which occurs in coarse sand from low beach to shallow depths, frequently in shell beds. Extinct Macoma species also become more frequent. Gastropod frequencies tend to be somewhat variable – they are easily rolled and winnowed by
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
currents. Indeed, although the assemblages are good indicators of the original mollusc communities, it is clear that they have been significantly modified by sedimentary processes. The vertical change in faunal assemblages can be identified elsewhere in the Red Crag basin, with Glycimeris-Venerupis assemblages being associated with older Red Crag outcropping to the south of the Deben and at depth, and younger Spisula assemblages outcropping to the north. This sequence can be taken to indicate a general shallowing during Red Crag times. Changes in the nature of sedimentary structures seen in better exposures in nearby pits also lead to this conclusion. It is also evident that the influence of the island rocky shore becomes more important towards the top of the sequence with a greater influx of Mytilus and other shoreline species, presumably being swept offshore by currents. A general picture builds up, then, of an initial Red Crag sea transgression with a period of intense scouring of the sea bed by strong currents, and erosion of the London Clay and Coralline Crag surfaces. Coarse pebble beds were left behind as remanie´ deposits. There then followed a period of sand-wave migration, the sedimentary structures and shell gravel faunas indicating a shallowing, regressive sequence. As the Crag basin became shallower the ‘island’ became a more positive feature and possibly emergent, with the development of non-derived shoreline and infralittoral faunas where suitable conditions allowed, but with much material being washed out to sea. It was at this stage that the ‘fossil’ cliffs, wave-cut platform and ‘beach’ were formed. It should be noted that this interpretation was the subject of some discussion by group members! After visiting the Quarry Pit, and ignoring demands for a lunch break (it was only 12.30 after all), the leader handed over the group to Barry Hall, who was celebrating his 25th wedding anniversary that day, for a guided tour of ‘The Pliocene Forest’. 2.3. ‘The Pliocene Forest’ In March 2009 GeoSuffolk applied for, and was awarded, a £372 grant from the Curry Fund of the Geologists’ Association to create a ‘Pliocene Forest’ – a plantation of representative plant genera that had existed during Coralline Crag times. Inspired by a pollen analysis from a borehole site near Orford by Andrew and West (1977) and, later, by Gibbard and Pegler (1988) from Sutton itself, the concept arose as part of a continuing programme of site development and enhancement. Pollen analyses are uncommon for the Crags – preservation is very poor (if at all) in the coarse, oxidised sediments, and British Pliocene Crag floras are still relatively poorly known. However, pollen analyses from the Coralline Crag have shown Pinus-dominated assemblages with Sequoia, Picea, Sciadopitys, Abies, Tsuga and others, including Liquidambar and Juniper, with heathers and grasses. They indicate a nearby Pliocene landscape dominated by forests containing a significant number of exotic genera now native to Eastern Asia and North America. GeoSuffolk member Barry Hall, a horticulturalist by profession, guided the work on the project. Deer and rabbits are a major threat to the plants in their early years; this meant the erection of protective fencing, creating a 10 m 4 m safe enclosure. A special manure-based compost and weathered bonfire ash was used at each planting site to improve water retention and the structure of the free-draining sandy soil. Continued nurturing and maintenance was needed until the plants become truly established. After discussion with Professor Richard West, GeoSuffolk planted four extant ‘Coralline Crag trees’ near the explanatory panel and close to the public footpath, using 2 m ‘standards’ to provide immediate visual impact: Sciadopitys verticillata (Japanese Umbrella Pine), having a fossil record that goes back some 230 Ma
519
and already in decline by the mid-Cretaceous; Liquidambar styraciflua (Sweetgum), with a similar form found in the Miocene of Europe; Tsuga heterophylla (Western Hemlock); and Tsuga canadensis (Eastern Hemlock). By the end of the year, additional donations had provided the two grasses, Stipa gigantea and S. tenuissima, and the shrub Atriplex halimus (Salt-bush, which perished in the harsh winter of 2009/2010) with the contrasting annual A. hortensis ‘rubra’ (Red Orach). The project was described by the author in the Magazine of the Geologists’ Association (Dixon, 2009b). The innovative original forest has aroused considerable interest among visitors and field groups that have visited the site. So much so that GeoSuffolk decided to seek to expand the ‘Pliocene Forest’ on a much larger scale and longer timescale. In August 2009 the landowner of Sutton Knoll, during a guided visit with his family to the ‘Pliocene Island’ by GeoSuffolk members, gave approval for GeoSuffolk’s ongoing Site Management Plan in general and the expansion of the ‘Pliocene Forest’ in particular. As Sutton Knoll [Rockhall Wood] is a SSSI, GeoSuffolk, through the landowner, also applied to Natural England for consent to make these changes, justified on scientific, educational, aesthetic and general interest grounds. In March 2010 a further grant of £1692 was awarded by the Curry Fund to fence an expanded planting area of some 800– 900 m2, centred more or less on the original enclosure, the work being carried out by Jeff Redgrave, a local geologist. Planting is arranged to have multiples of the same plant at different growth stages with a sub-theme of strong contrast between species of the same genera (e.g. Atriplex hortensis ‘Rubra’ and A. halimus). Additional representative species include Pinus coulteri (largest cones) and Cryptomeria japonicum ‘Sekkan-sugi’ (foliage and colour). Bush and shrub forms of some of the tree genera are also being planted as understorey. Helped by further fundraising, the spring 2010 season saw the planting of several donated and sponsored trees: Tsuga canadensis (Eastern Hemlock, tree form); Abies nordmanniana (Caucasian Fir – after a spell of duty as a Christmas tree); Cryptomeria japonica ‘Lobii nana’ (dwarf form); C. japonica ‘Sekkan-sugi’ (foliage); Platycarya strobilacea (Caucasian Walnut); Sciadopitys verticillata; Microbiota decussata (dwarf Cypress native to Siberia); Cunninghamia lanceolata (China Fir); Picea breweriana; P. omorika; P. pungens ‘Hoopsii’; Quercus suber; Betula utilis var. ‘Jacquemontii’; Metasequoia glyptostroboides (Dawn Redwood); Ostrya carpinifolia (Hop Hornbeam). Barry Hall is maintaining a ‘Forest’ database, which includes: name, type of plant, height & spread at planting, date of planting, location, source and cost, and donor/sponsor. Many other species of tree and shrub are under consideration. Monitoring and recording of rainfall has been initiated and the height of the water table being measured. 3. Locality 2: Bawdsey East Lane [TM 356397] The group then drove to East Lane car park, where lunch was taken amid remains of concrete bunkers from World War Two, prior to examining the foreshore south of the Martello Tower. 3.1. The London Clay A brief description of the London Clay was given by David Bone. The London Clay comprises brown and grey predominantly silty clays that were deposited in a shallow sea that covered much of south-east England during the Eocene Epoch about 53 million years ago. 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
520
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
[()TD$FIG]
compacted on the sea floor. Plant debris, logs and branches, seeds and fruits, are now preserved as pyritised fossils. Walton-on-theNaze, only 16 km away, is a key locality for the study of Palaeogene plants in Britain and has considerable potential for future research. A useful summary of the sections at Walton, Harwich and Wrabness is given by Daley (1999, pp. 61–72). Indeed, the London Clay exposed at Bawdsey is very similar to that exposed on the foreshore at Walton. Many species of sharks and other fish swam in the seas, and other vertebrates including turtles, have been found. Walton is also an internationally important site for finds of tiny bird bones – also found at Bawdsey. A small invertebrate (mostly mollusc) fauna also occurs. Well records show the London Clay at Bawdsey Manor to be 87 feet (26.5 m) thick (Whitaker, 1906) and Davis and Elliot (1951) refer to between 11 and 20 feet (3.4–6.6 m) of blue London Clay, overlain by Red Crag, to be seen in the cliff one mile north-east of Bawdsey Ferry (i.e. East Lane). Currently about 5 m of blue-grey silty London Clay is well exposed in the cliff and on the foreshore at East Lane, just to the south of the Martello tower, and is actively being eroded by the sea. When viewing the wave-cut platform from the beach or cliff top, the bedding can be picked out quite clearly, showing that the London Clay has been gently folded with some small faults. Calcareous mudstone concretions, ‘septaria’, once used locally for making ‘Roman’ cement, occur in thin bands within the London Clay. No evidence was found for the thin layers of pale creamy yellowish rust-coloured sediment of volcanic ash; it was suggested during discussion that the sequence at East Lane overlies the ash horizons, seen at Bawdsey Manor Cliffs and further south. The ash originated from ocean floor volcanoes as the North Atlantic opened up and with the break-up of the Greenland-Rockall plate, and was wind-blown from these volcanoes 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 correlate with Greenland volcanic events at 53 Ma (Daley, 1999, pp 14–21). Much pyritized debris was found washed out on the beach; there are also pockets of drifted woody material which contain fruits, diatoms and radiolaria. The foraminifera Astrorhiza is abundant. The teeth of the sharks Carcharias hopei, C. teretidens,
Striatolamia macrota and Otodus obliquus, and fish vertebrae can be found. In the cliff, the London Clay/Red Crag unconformity was well exposed. It is highly irregular, partly due to later cryoturbation, but in many ways resembles the eroded London Clay surface of the present wave-cut platform. There is c.2.5 m of Glycimeris-rich Crag with a well-developed but discontinuous basal pebble bed of phosphatic nodules and containing Boxstones, large flints and material derived from the London Clay. Several shark teeth, including that of the shark Isurus hastalis, fragments of ray palate and cetacean bone fragments were found, but the find of the day was undoubtedly that by David Bone of an extremely rare molar from the lower jaw of a rhinoceros, Dicerorhinus megarhinus (Shreve pers. commun.). 3.2. Coastal defences The Martello Tower was built in 1809 with 750,000 bricks at a cost of £9784 to help protect the shore from Napoleon’s forces. It is one of only 17 left on the East coast. Twenty years ago it was protected by a large expanse of shingle beach (c.20 m) and 25 m of land, and wooden groynes restricted long-shore drift. It was bought by architect John Fell-Clark, restored and converted into a 2-floor dwelling, now let as a holiday home. In 1996 a new erosional cycle started; a big storm in 1997 removed all the beach and some of the cliff. By 2005 the tower stood only 10 m from the cliff top. Following the storm, Suffolk Coastal District Council approved emergency funding for limited boulder-style defence work in front of the tower, but this served to exacerbate erosion immediately to the south. Furthermore, East Lane Point is considered of key strategic importance in controlling the movement of shingle between Aldeburgh and Felixstowe. Bawdsey scored low on Department of the Environment and Environment Agency priority lists for coastal defences. Consequently, the East Lane Trust was formed to raise privately the estimated £2.2m needed for sea defence work to give greater protection to farm land, homes and the tower. Locally owned farmland in surrounding villages (Hollesley, Bawdsey, Alderton) was donated to the Trust, which then sold it off for housing development. Special governmental consent was given in 2007
Fig. 7. At the time of the emplacement of the ‘rock armour’ to protect the Martello Topwer in 2009 a shingle beach and talus afforded some protection to the cliffs, which were slowly degrading. The shingle is now entirely removed; there is rapid basal cliff erosion with cliff falls assisted by water seepage at the London Clay/Red Crag junction.
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
allowing residential development on some of the plots that lay outside the ‘Local Plan’. In 2008 this privately funded project was finally given the goahead – it is believed to be unique in England, and work began in September. 22,000 tonnes of ‘granite’ (according to the press) were brought by barge from Norway, each boulder weighing c.6 tonnes, to make ‘rock armour’ for c.350 m of cliff. In fact, much of the material is Larvikite and Gabbro, with a large proportion of French Carboniferous Limestone, complete with crinoids, corals and brachiopods. Work was completed in June 2009 at a cost of c. £2.4m. Although some of the excellent exposure of London Clay and phosphatic basal Red Crag pebble bed had been lost as a result of the work and subsequent slope degradation, the focus of erosion has merely shifted slightly, c.150 m southwards, for within 2 months of completion, waves still reached the base of the cliffs and falls were still taking place. In February 2010 a further 12,000 tonnes of rock was brought in from Northern France to reinforce coastal protection north of the tower to prevent flooding of low-lying farmland at a projected cost, using ‘emergency funding’, of £1.5 million. Although the ‘rock armour’ was ‘‘holding up well and performing as intended’’ (Fell-Clark, letter to the Editor, East Anglian Daily Times 6.2.10), at the present time the entire shingle beach has been removed by storms and the cliff section is actively being rapidly eroded – it has never looked better! (Fig. 7). 4. Locality 3: Alderton House Pit [TM 328412] Members then drove the short distance to Locality 3, an old Red Crag section, now much overgrown, comprising an ‘old’ face and a ‘new’ 3 m face lying some 50 m to the west, first exposed in the 1950s. The older eastern section shows c.1.3 m thickness of largescale sand-wave foresets [Bed 2], indicating a current direction of 230–2408, the regional trend. It is stratigraphically overlain by about 60 cm of trough cross-bedded shelly Crag [Bed 3]. The ‘new’ face shows medium-scale cross-bedding, demonstrating subordinate 2608 regional current directions, but dominant cross-cutting tidal flow directions towards 0808, with channelling, mud drapes and other shallow water sedimentary structures [Bed 2]. Some small, thin endogenetic tubes/burrows (provisionally attributed to an amphipod) occur and bioturbation is common in silty partings. It would appear that Bed 2 of the ‘new’ face represents an accumulation of inter-ripple trough-zone sediments formed at the same time as the larger scale foresets of the ‘old’ face. A pebble lag deposit occurs at the base of the shellrich uppermost horizon [Bed 3], rich in phosphatic nodules and flints (including a flint belemnite and flint Terebratula). About 35 cm is currently exposed. Large fragments of Echinocardium cordatum test have been found in this horizon (Dixon, 2001a). Bed 2 in both faces is underlain by a further 1.8 m of sands [Bed 1] with comminuted shells interbedded with bioturbated silts containing Arenicola-type tubes, now completely obscured by talus but exposed in the 1970s. The Crag is rich in shells and this was a good opportunity for members to sieve material and collect. The mollusc fauna is dominated by Spisula shell gravel assemblages in all horizons – the frequencies of dominant species from selected spot samples are given in Table 1. The method used was given by Dixon (1977) but all species found were divided into their ‘ecological groups’ – the total frequency in each group is also given. It can be seen that there is a significant increase in intertidal Mytilus and Mya frequencies, and a corresponding decrease in sublittoral Glycimeris and Venerupis in the upper beds, reflecting the regional trend of shallowing water depths. There is also a notable abundance of the Pea Urchin, Echinocyamus pusillus (Dixon, 2001b), and the solitary
521
Table 1 Percentage frequencies of dominant Red Crag molluscan species and ecological groupings of all species from selected sample points at Alderton House Pit.
Intertidal rocky shore Mytilus edulis Total Intertidal sandy shore Mya arenaria Total Infralittoral Sublittoral shell gravel Glycimeris glycimeris Venerupis spp Total Sublittoral muddy sand Aloidis gibba Total Sublittoral clean sand Dosinia exoleta Total Sublittoral mud tolerant Turritella triplicata Total Sublittoral epifauna Buccinum undatum Neptunea contraria Total Miscellaneous Chlamys opercularis Venus ovata Total Extinct Nassa granulata Natica multipunctata Polinices hemiclausa Nucella lapillus Astarte obliquata Cardita corbis C. scalaris Cardium interuptum Macoma obliqua M. praetenuis Spisula ovalis Venus imbricata Total Unidentifiable Total species % of 1 individual
Bed 1
Bed 2
Bed 3
9.0 9.0
8.9 9.3
13.5 14.3
0.0 0.0 0.0
0.0 1.0 1.2
3.9 4.4 1.0
5.0 7.4 13.0
4.1 9.5 14.9
4.3 0.0 6.1
9.3 9.6
8.1 8.5
1.7 2.0
2.0 3.1
2.8 3.3
1.2 1.4
6.9 7.2
13.5 14.0
8.9 9.2
0.0 0.0 1.0
0.0 0.0 0.0
1.1 1.1 2.8
0.0 1.0 2.5
0.0 1.1 2.2
3.6 0.0 4.5
0.0 0.0 0.0 0.0 1.9 2.2 2.9 1.3 2.2 2.9 27.1 1.7 46.2
0.0 0.0 0.0 0.0 1.7 1.0 2.2 1.4 1.6 4.6 14.6 1.4 36.6
3.3 1.1 1.1 4.0 0.0 0.0 0.0 1.0 2.2 4.9 16.3 0.0 44.1
6.0 57 0.22
9.3 73 0.16
9.3 81 0.14
corals Sphenotrochus and Balanophyllia are common here. Rolled bryozoans, derived from the Coralline Crag, are much in evidence, and shark and ray teeth are not uncommon. Of particular note is the finding of a Pika tooth, Prolagus, by Bob Markham from Bed 3 of the ‘new’ face (Markham pers. com.) and the presence of Scrobicularia plana in the upper beds (Dixon, 1976). In many respects, the sequence and mollusc assemblages resemble those at Vale Farm and Buckanay Farm, both seen during the Geologists’ Association excursion in 2004.
5. Locality 4: Tarrants Pit, Ramsholt [TM 303422] It was only a short drive to the final stop of the day, an 8 m deep working Red Crag pit. There can be seen medium- and large-scale sand-waves with beds of shelly material, small-scale sand-waves, mud drapes, pebbly lag deposits – at least four, with phosphatic nodules, flint pebbles and convex-up Glycimeris valves (best seen in the west and east faces). The section thus shows a series of superimposed sand-waves: pebble beds demonstrating episodes of inter-sand-wave scouring, followed by small-scale sand-waves with associated mud drapes forming as bottomsets in trough
[()TD$FIG]
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523
522
Fig. 8. One of the pebbly lag deposits at Tarrant’s Pit; it indicates scour prior to bottomset deposition. Note the convex-up position of the Glycimeris valves.
zones, and finally larger scale foresets with shell avalanche material at their base. Reactivation surfaces can also be seen – a well-defined such surface can be seen in the east face, with a 2 m sand-wave above it. Current directions are predominantly towards the south-west and west, as is the regional trend, but subordinate cross-cutting directions towards the north-east are significant (Fig. 8). Although not at first sight rich in shells, even a cursory search yields Spisula/Macoma/Mytilus-rich assemblages – a shallow water fauna typical of the upper part of the Red Crag in the Ramsholt/ Alderton area. An abundance of small, thin, wiry tube-like trace fossils can be seen in some horizons, usually cleaner sands, as seen at Alderton House; they possibly belong to amphipods but their origin is uncertain. The top-most 1–1.5 m of Crag is decalcified. Two small reverse faults can be seen in the centre of the south face, with a combined throw of c.40 cm. Several members sieved material and collected an interesting range of fossils – of note were many Nucella and some phosphatic nodules encrusted with barnacles found in the pebbly lag horizons. One member found a much-rolled belemnite. Other mollusc species found included Glycimeris glycimeris, Arctica islandica, Chlamys opercularis, Pygocardia rustica, Neptunea contraria, N. ‘despecta’, Nucella lapillus, N. tetragona, Nassa reticosa and Scaphella lamberti. The pit was last visited by Geologists’ Association on Sunday 20th July, 1947, the final stop on a long week-end excursion based at the Crown and Anchor Hotel, Ipswich. The pit was not named, but stated as ‘‘. . .a quarry north-west of Ramsholt Church. . .’’. The Crag was described as intermediate between Newbournian and Butleyan and many well-preserved fossils were collected (Ovey and Pitcher, 1948). The meeting closed at about 17.30, after a good day enjoyed by all. Acknowledgements Sutton Knoll is privately owned; GeoSuffolk would like to thank Sutton Hall Farms for innumerable access permissions and their continuing kind support. Mr. James Adeane kindly granted access permission for both Alderton House Pit and Tarrants Pit, both also privately owned. Thanks go to Jim Rose and the two peer
reviewers, Jan Zalasiewicz and Peter Riches – the manuscript has greatly benefited from their critical appraisal and is now much improved. Thanks also go to David Bone, who commented on earlier drafts of the London Clay at Bawdsey. Barry Hall is to be commended for guiding the party around the ‘Pliocene Forest’. Very special thanks go to Bob Markham, whose input as co-leader was greatly appreciated and who made many useful suggestions on the first draft of this report.
References Andrew, R., West, R.G., 1977. Pollen spectra from Pliocene Crag at Orford, Suffolk. New Phytologist 78, 709–714. Balson, P.S., 1999. The Coralline Crag. In: Daley, B., Balson, P. (Eds.), British Tertiary Stratigraphy. Geological Conservation Review Series No.15. Joint Conservation Committee, Peterborough, pp. 253–288. Balson, P.S., Long, P.E., 1988. Rockhall wood, Sutton. In: Gibbard, P.L., Zalasiewicz, J.A. (Eds.), Pliocene–Middle Pleistocene of East Anglia Field Guide. Quaternary Research Association, Cambridge, pp. 66–72. Balson, P.S., Humphreys, B., Zalasiewicz, J.A., 1990. Field guide No. 3: Coralline and Red Crags of East Anglia.In: 13th International Sedimentological Congress, UK. Balson, P.S., Mathers, S.J., Zalasiewicz, J.A., 1993. The lithostratigraphy of the Coralline Crag (Pliocene) of Suffolk. Proceedings of the Geologists’ Association 104, 59–70. Boswell, P.G.H., 1928. The Geology of the country around Woodbridge, Felixstowe and Orford. Memoir of the Geological Survey of England and Wales. Daley, B., 1999. In: Daley, B., Balson, P. (Eds.), British Tertiary Stratigraphy. Geological Conservation Review Series No. 15, Joint Nature Conservation Committee, Peterborough. Davis, A.G., Elliot, G.F., 1951. The London clay of Coastal Suffolk and Essex. Geological Magazine 88, 329–337. Dixon, R.G., 1976. The occurrence of Scrobicularia plana in the Red Crag, Lower Pleistocene. Proceedings of the Geologists’ Association 87, 409–412. Dixon, R.G., 1977. A note on the palaeoecology of the Red Crag (Lower Pleistocene). Quaternary Newsletter 23, 1–5. Dixon, R.G., 1979. Sedimentary facies in the Red Crag (Lower Pleistocene, East Anglia). Proceedings of the Geologists’ Association 90, 117–132. Dixon, R.G., 2001a. A note on the occurrence of Echinocardium cordatum from the Red Crag, Pliocene, East Anglia. Bulletin of the Geological Society of Norfolk 51, 105–112. Dixon, R.G., 2001b. A short note on the distribution of Echinocyamus pusillus in the Red Crag. Bulletin of the Geological Society of Norfolk 51, 113–116. Dixon, R.G., 2005. Field meeting: Coastal Suffolk Crag week-end, 23–25 April, 2004. Proceedings of the Geologists’ Association 116, 149–160. Dixon, R.G., 2006. Sutton Knoll. Transactions of the Suffolk Naturalists’ Society 42, 97–101. Dixon, R.G., 2007. Autochthonous Mollusc Faunas from the Red Crag. In: Dixon, R.G. (Ed.), Proceedings of GeoSuffolk RIGS Meeting, Transactions of the Suffolk Naturalists’ Society, 43, pp. 12–16.
R. Dixon / Proceedings of the Geologists’ Association 122 (2011) 514–523 Dixon, R.G., 2009a. The Red Crag at Sutton Knoll. Transactions of the Suffolk Naturalists’ Society 45, 1–9. Dixon, R.G., 2009b. The Pliocene forest. Magazine of the Geologists’ Association 8 (3), 22. Gibbard, P.L., Pegler, S.M., 1988. Pollen analysis from Rockhall Wood, Sutton. In: Gibbard, P.L., Zalasiewicz, (Eds.), Pliocene–Middle Pleistocene of East Anglia. Field Guide, Quaternary Research Association, Cambridge. Long, P.E., 2000. Norwich Crag Puzzles: two questions raised by molluscs. In: Dixon, R.G. (Ed.), The Geological Society of Norfolk Jubilee Volume. pp. 59–65. Ovey, C.D., Pitcher, W.S., 1948. Observations on the geology of East Suffolk. Proceedings of the Geologists’ Association 59, 23–34. Prestwich, J., 1871a. On the structure of the Crag-beds of Suffolk and Norfolk with some Observations on the organic remains. I. The Coralline Crag of Suffolk. Quarterly Journal of the Geological Society of London 27, 115–146.
523
Prestwich, J., 1871b. On the structure of the Crag-beds of Suffolk and Norfolk with some Observations on the organic remains. II. The Red Crag of Essex and Suffolk. Quarterly Journal of the Geological Society of London 27, 325–356. Whitaker, W. 1906. The Water Supply of Suffolk, from Underground Sources. Memoir of the Geological Survey of Great Britain. Williams, M., Haywood, A.M., Harper, E.M., Johnson, A.L.A., Knowles, T., Leng, M.J., Lint, D.J., Okamura, B., Taylor, P.D., Zalasiewiz, J., 2009. Pliocene climate and seasonality in North Atlantic shelf seas. Philosophical Transactions of the Royal Society A 367, 85–108. Wood, A.M., 2009. Neogene. In: Whitaker, J.E., Hart, M.B. (Eds.), Ostracods in British Stratigraphy. The Micropalaeontological Society, Special Publication, pp. 411–446.