RUNNING HEAD
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THE RED CRAG OF SUTTON KNOLL ROGER DIXON Introduction Sutton Knoll (Rockhall Wood Pit SSSI) has been studied for over 170 years and has long been known as a ‘fossil island’ of Coralline Crag during Red Crag sea times, with Coralline Crag wave-cut platform, cliffs and boulders ‘buried’ in the later Red Crag about 2 ½ million years ago. The classic description was by Joseph Prestwich in 1871, who gave illustrations of this old shoreline, and a great number of papers have been published since then. More recently, in 1999, Peter Balson described the Coralline Crag of the locality as part of the Geological Conservation Review, and the author gave an overview of the site in the Transactions of the Suffolk Naturalists’ Society, 2006, to which readers are referred for background information. Over 20 m of the junction between Coralline and Red Crags was exposed in two places as a direct result of English Nature’s (now Natural England) “Facelift” operations and on-going geo-conservation work by GeoSuffolk members. This boundary probably hasn’t been seen on this scale for 140 years or more, for Boswell in his 1928 Geological Survey Memoir indicates that it was then grown over. Certainly Sutton Knoll is the only known locality where the junction can now be seen. Although the Coralline Crag has been well described historically, surprisingly little is known about the Red Crag at this site, and no modern (post-war) quantitative or detailed molluscan or sedimentological studies have been done on the Red Crag here. This has been the focus of the author’s current research. It is clear that very different localised molluscan communities were able to develop in the different micro-environments that must have existed around the ‘island’. Non-derived Mytilus-dominated, Venerupis-dominated and Zirfaea assemblages were described by the author in the Transactions for 2007. These are characterised by articulated bivalves, probably in the life position and with adult and juvenile forms present. In addition, non-derived articulated Terebratula grandis, probably in life position, occur, and barnacle communities on shells and Coralline Crag pebbles are common. Consequently, very precise palaeo-environmental interpretations were able to be made. The purpose of this communication is to record the “Facelift” work and present some of the data collected away from the rocky Red Crag shore itself. In 2006, after consultation with GeoSuffolk members, English Nature embarked on a programme of site clearance to re-expose sections, not only for research workers, but to make the geology better accessible for field groups and other interested parties. At the same time, in order to interpret the Sutton Knoll site better, exploratory excavations were made, mainly to ascertain the nature of the Red Crag [these excavations were later filled in for safety reasons], and samples were collected by the author. The “Facelift” Excavations Bullock-yard Pit: The ‘fossil beach’ was cleared and three exploratory holes were dug out on 6 and 7 February, 2006. Further work was done clearing a permanent exposure of Coralline Crag in the main part of the pit.
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The ‘fossil beach’: The initial small section [Hole 2], to establish the boundary, was extended northwards to expose 13Â5 m of ‘fossil beach’. Clinometer measurement showed the average beach slope to be c. 10o to the south, but was highly irregular. The boundary between Red and Coralline Crags was not easy to identify at first because of the amount of re-worked Coralline Crag incorporated into the Red Crag – very close examination of the fauna was required. However, it was noted that the Red Crag contained mussels at its base and is ‘dirtier’ and fairly structureless, passing upwards into soil horizons. The basal Red Crag also contains silty mud bands. The Red Crag contained some occasional phosphate pebbles, but Coralline Crag boulders c. 0Â42 m across, although weathered, were easily recognisable. In general, the Red Crag shells were much comminuted and there was insufficient material for analysis. However, c. 0Â5 m long, 0Â10 m high lenses of mussels, many articulated open and closed, were observed at or near the base and adjacent to the Coralline Crag boulders, evidently in the life position. Hole 1: Dug at the southern end of pit, adjacent to perimeter track about 1Â5 m below track level. Coralline Crag was struck at 2Â5 m depth, c. 1Â6 m below that 14 m away in the ‘beach’. There were abundant phosphate nodules forming a pebble bed at the base of the Red Crag. The Red Crag was noticeably Mytilus and Neptunea contraria rich with Glycimeris and barnacles on pebbles and shells. Sedimentary structures were not apparent. A sample was taken from Red Crag 0Â5 m above the boundary. Hole 3: An exploratory hole dug in lowest part of pit. No obvious in situ Red Crag was found, it being much disturbed and weathered. The Coralline Crag surface was struck at 0Â75 m depth. Articulated mussels in iron pan and barnacle-encrusted whelks were found in the spoil. Chicken Pit: Sections were cleared on 20 September, 2006. 1. A face at the north end of the pit, which exposed Red Crag: An upper layer comprising a shelly sand-wave c. 1 m high, mussel-rich with small phosphate pebbles at base, and probably avalanche material. An articulated Mya arenaria was found. The palaeo-current direction was measured at 154o. Sampled at base (2). Below is finer comminuted shell sand, possibly the top of another sand wave. Sampled (1). 2. A face between the trees showing Red Crag with many phosphatic pebbles and Coralline Crag cobbles on in situ Coralline Crag. Pockets of fine and medium sand, with good iron-stain patterns, occupy hollows between blocks, containing only occasional shells. Abundant mussels of all sizes from large adults to small juveniles, often articulated, dominate the fauna [see list in Dixon 2007]. The surface of a huge cliff-fall Coralline Crag block was exposed at the base of the section.
Trans. Suffolk Nat. Soc. 45 (2009)
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3. A small section was cleared adjacent to the pond exposing comminuted Red Crag. In addition to the “Facelift” work, GeoSuffolk members have, since 2004, cleared and maintained two other exposures in the Chicken Pit: Barry’s Hole: 1 m ×1 m× c 75c m deep excavation – several Coralline Crag boulders and pebbles up to 0Â30 m across set in Red Crag sand matrix. Interstitial pockets contain a Tapes dominated fauna, including articulated valves, juveniles to adult in size, many being small and delicate. Elsewhere mussel-rich assemblages occur, including articulated colonies. The unusual fauna and its context meant that statistical sampling was not possible; instead a species list was drawn up (in Dixon 2007). Pond: a c. 0Â5 m face of shelly Red Crag sand-wave, rich in phosphatic nodules and other cobbles and pebbles rests on London Clay, the surface of which is bored by abundant Zirfaea burrows. The palaeo-current direction is c. 2450. Sampled at base. The Field: Six exploratory excavations were dug on 20 October 2006, four in the field adjacent to and northwest of the SSSI, facing the Quarry, and two within the SSSI near the quarry. Their locations are marked on the site plan. Hole 1: 1Â46 m soil and disturbed decalcified Crag 0Â37 m sparsely shelly sand 0Â30 m comminuted sand 0Â26 m shelly sand 0Â10 m green/grey unoxidised sand 0Â40 m shelly sand with abundant phosphatic pebbles, concentrated towards top Water table at 2Â89 m below surface London Clay not reached but not far below Hole 2: 1Â45 m soil & disturbed Crag 0Â88 m undifferentiated comminuted sand 0Â10 m shell layer with pebbles 0Â42 m comminuted shelly sand Water table at 2Â80 m, shelly sand below with many phosphatic nodules London Clay not reached but not far below Hole 3: 2Â2 m of topsoil and disturbed material 0Â80 m shelly sand Water table at 3 m
sampled sampled
sampled sampled
sampled
Holes 4 & 5: Both were located within the SSSI near the old quarry. Both were dug to a depth of 3 m, revealing much disturbed and decalcified Crag. The water table was not reached.
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Table 1. % frequencies of the dominant molluscs and their ecological groups from the Sutton Knoll site excavations Bullockyard: Field Hole 1: Field Hole 1: Field Hole 2: Field Hole 2: hole 1 shelly sand PO4 layer shell band PO4 layer Intertidal rocky shore Mytilus edulis 13Â6 total 14Â6 intertidal sandy shore Cerastoderma edule 0Â8 Mya arenaria 0Â3 total 1Â1 Infralittoral Zirfaea crispata 1Â6 total 2Â1 Sublittoral shell gravel Glycimeris glycimeris 7Â0 Venerupis rhomboides 5Â6 total 13Â6 Sublittoral muddy sand Aloidis gibba 5Â4 total 5Â7 Sublittoral clean sand Callista chione 0Â9 Dosinia exoleta 1Â2 total 2Â1 Sublittoral silt/mud tolerant Turritella triplicata 7Â8 total 8Â1 Sublittoral epifauna 3Â0 Miscellaneous 1Â9 Extinct Nassa granulata 2Â6 Natica hemiclausa 2Â2 N. catenoides 1Â3 N. multipunctata 0Â4 Astarte obliquata 2Â5 A. omalii 0Â7 Macoma obliqua 1Â9 M. praetenuis 3Â1 Spisula ovalis 18Â4 total 43Â7 Unidentifiable 4Â1 number of individuals % of one individual number of species
1099 0Â09 64
16Â0 16Â7
10Â8 11Â3
10Â4 10Â4
9Â4 9Â4
0Â0 1Â8 1Â8
2Â7 0Â9 3Â6
3Â0 3Â0 6Â0
4Â7 1Â2 5Â9
6Â2 6Â2
4Â5 4Â5
6Â0 6Â0
4Â7 5Â9
9Â9 5Â6 17Â9
7Â2 10Â4 19Â8
3Â0 8Â9 13Â4
9Â4 9Â4 18Â8
4Â9 5Â6
6Â3 6Â8
3Â0 3Â0
4Â7 4Â7
1Â2 3Â1 4Â3
0Â9 1Â8 2Â7
1Â5 3Â0 4Â5
1Â2 4Â7 5Â9
10Â5 12Â3 1Â2 1Â8
12Â2 13Â1 0Â0 1Â8
1Â5 3Â0 1Â5 3Â0
12Â9 14Â1 0Â0 3Â5
1Â2 0Â0 0Â0 0Â0 1Â2 3Â1 2Â5 3Â7 8Â6 29Â0 3Â2
2Â2 0Â9 0Â9 0Â9 3Â1 1Â8 1Â8 3Â1 7Â7 32Â4 4Â0
0Â0 1Â5 1Â5 1Â5 6Â0 6Â0 3Â0 11Â9 7Â5 44Â8 4Â4
0Â0 1Â2 0Â0 1Â2 3Â5 2Â4 1Â2 5Â9 5Â9 31Â7 0Â1
162 0Â62 37
Trans. Suffolk Nat. Soc. 45 (2009)
222 0Â45 38
67 1Â49 26
85 1Â18 29
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Table 1. Continued Field Hole 3 Field Hole 6: Field Hole 6: Field Hole 6: Chicken Pit: Chicken Pit: top middle bottom north end 1 north end 2
Pond
6Â7 6Â7
9Â7 10Â0
7Â8 7Â8
1Â1 3Â1
7Â4 8Â0
13Â7 14Â3
6Â6 7Â8
2Â7 1Â3 4Â0
2Â6 1Â3 3Â9
3Â9 2Â0 5Â9
3Â4 0Â8 4Â2
2Â9 0Â4 3Â3
2Â1 0Â2 2Â3
1Â8 0Â5 2Â3
1Â3 1Â3
3Â9 3Â9
3Â9 4Â4
2Â5 2Â5
4Â1 4Â1
2Â6 3Â6
2Â6 3Â0
10Â7 10Â7 26Â7
5Â7 14Â9 20Â6
11Â8 12Â2 24Â5
10Â4 9Â8 22Â7
6Â2 9Â1 16Â1
5Â4 8Â7 15Â2
12Â0 10Â9 24Â0
6Â7 8Â0
5Â7 5Â7
7Â4 7Â4
12Â9 12Â9
6Â8 7Â2
3Â6 3Â8
9Â3 10Â3
2Â7 1Â3 4Â0
0Â9 1Â3 2Â2
2Â0 0Â0 2Â0
1Â1 1Â1 2Â2
2Â5 2Â7 5Â2
1Â3 1Â6 3Â0
1Â2 4Â0 5Â4
2Â7 4Â0 1Â3 2Â7
13Â6 14Â0 0Â4 0Â4
10Â3 11Â3 1Â0 2Â5
22Â1 22Â1 0Â9 1Â4
12Â4 12Â8 1Â0 1Â0
9Â9 9.9 0Â7 2Â5
6Â2 6Â5 0Â8 1Â9
1Â3 0Â0 1Â3 0Â0 2Â7 4Â0 2Â7 5Â3 12Â0 38Â7 2Â6
1Â8 1Â8 0Â0 1Â3 0Â4 4Â4 2Â2 3Â9 11Â0 38Â6 0Â4
1Â5 1Â5 0Â0 0Â5 1Â0 2Â5 2Â5 2Â9 9Â3 29Â4 3Â8
3Â4 0Â0 0Â0 0Â0 2Â0 0Â8 0Â3 0Â3 4Â8 20Â2 7Â8
0Â6 1Â2 0Â4 0Â0 3Â3 1Â4 1.0 2Â7 13Â6 28Â5 12Â8
1Â2 0Â3 0Â5 0Â5 1Â5 1Â0 0.8 2Â6 20Â1 36Â8 7Â9
2Â4 0Â5 0Â3 0Â3 2Â9 1Â3 0.4 0Â2 9Â9 26Â1 11Â9
75 1Â33 28
229 0Â44 30
204 0Â49 30
357 0Â28 38
485 0Â21 45
607 0Â16 46
1739 0Â06 71
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Hole 6: 1Â05 m topsoil 1Â09 m coarse shelly sand 0Â48 m dark oxidised Crag, with fine sand at top 0Â25 m pale greenish unoxidised shelly sand Water table at 2Â87 m
sampled from top and base sampled
The field excavations clearly revealed the basal ‘Red Crag Pebble Bed’ – the concentration of phosphatic pebbles at the base of the Crag, but the London Clay surface on which it rests was not actually exposed in any of the holes. It evidently determined the presence of the water table, which limited digging, although the capacity of the digger was in any case approaching its limit.
Figure 1. Sutton Location Map. Sedimentology Because of the nature of the excavations, bed forms were indistinct at best or not identifiable at all in the exploratory pits. Often, distinct laminations only become clear after a period of weathering, which was not the case here, with only fresh, temporary faces exposed. 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 water current directions trend generally towards the southwest, a regional trend.
Trans. Suffolk Nat. Soc. 45 (2009)
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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 remanié deposits, left behind after currents have swept finer material away. Granulometric data suggests that current velocities of c. 0Â5 m/sec are needed to form the Red Crag sand-waves; the lag deposits indicate that currents must have frequently been well in excess of that, scouring the sea bed surface and winnowing sediment. Red Crag sand-waves can also tell us about water depths, their height being directly proportional. However, their true height here cannot be gauged because post-depositional currents have clearly modified them. The Mollusc Fauna The method Any method used to analyse Red Crag fossils must take into account the postdeath modifications that have taken place to animal communities, particularly where currents may have considerably altered them. The method thus has to take into account winnowing, transportation, and derivation. The counting method used here was that described by the author (Dixon, 1977) and subsequently used successfully in a number of publications. It was based on and modified from that used by Norton (1967) for Norwich Crag molluscs, which in turn was modified from Sorgenfrei working on Miocene molluscs of Jutland. Samples were washed and sieved down to 1 mm mesh. All gastropod apices were identified and counted, each apex representing one individual. The central part of all bivalve hinges containing the umbo were identified and counted, two hinges representing one individual. Frequency percentages were then calculated. Further, all species found were divided into ‘ecological groups’ in order to interpret the palaeo-environment. Results and Discussion Table 1 shows the dominant species forming the assemblages, and the relative importance of the different ecological groupings. At first sight it may not be easy to interpret the figures; however, when considered in a regional context the trends are evident. Off-shore shell gravel assemblages dominate. Glycimeris-Venerupis assemblages tend to be found lower down the sequence. Glycimeris is welladapted to the physically rigorous conditions of a current-swept sand wave. Aloidis, specialised for life in the muddy sands of inter-sand wave bottomsets, is also common. Spisula shell gravel assemblages occur higher up the sequence, and extinct Macoma species also become more frequent. It has been suggested that the extinct Spisula ovalis is 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. Gastropod frequencies tend to be somewhat variable – they are easily rolled and winnowed by 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.
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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. The faunal sequence has been taken to indicate a general shallowing of water depth 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 picture builds up, then, of a regressive depositional sedimentary and faunal sequence. Following the initial Red Crag sea transgression was 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 remanié deposits. Then followed a period of sand wave migration with a sequence of shell gravel faunas. 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. A simple reconstruction of the environmental interpretations was given by the author in the Transactions for 2007 (p. 13, see below).
Figure 2. Sketch showing the Coralline Crag island at Sutton during the time of the Red Crag sea and its environments.
Trans. Suffolk Nat. Soc. 45 (2009)
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Endpiece Sutton Knoll is among Suffolk’s premier geological sites. One of the principal aims of the excavations was to help interpret the site better and more fully. One result of the work that has been done is the placing by GeoSuffolk of an explanatory information panel adjacent to the public footpath at the northern end of the Knoll, officially ‘unveiled’ on 14 May 2009. The work would not have been possible without the encouragement and practical support of Natural England, frequent access consent given by Sutton Hall Farms Ltd, and regular site maintenance by Barry Hall and others from the GeoSuffolk team. The author would like to thank Jonathan Larwood and Bob Markham for their helpful comments on an earlier draft of this article. References Balson, P. S. (1999). The Coralline Crag. In: British Tertiary Stratigraphy. Geological Conservation Review Series No.15, (B. Daley & P. Balson). Joint Conservation Committee, Peterborough, pp.253–288. Boswell, P. G. H. (1928). The Geology of the country around Woodbridge, Felixstowe and Orford. Memoir of the Geological Survey of England and Wales. Dixon, R. G. (1977). A note on the palaeoecology of the Red Crag (Lower Pleistocene). Quaternary Newsletter. 23: 1–5. Dixon, R. G. (2006). Sutton Knoll. Trans. Suffolk Nat. Soc. 42: 97–101. Dixon, R. G. (2007). Autochthonous Mollusc Faunas from the Red Crag. In: Dixon, R. G. (ed): Proceedings of GeoSuffolk RIGS Meeting. Trans. Suffolk Nat. Soc. 43: 12–16. Long, P. (2000). Norwich Crag puzzles: two questions raised by molluscs. In: Dixon, R. G. (ed): The Geological Society of Norfolk Jubilee Volume, 59– 65. Norton, P. E. P. (1967). Marine Mollusca in the Early Pleistocene of Sidestrand, Bramerton and the Royal Society borehole at Ludham, Norfolk. Phil. Trans. R. Soc. B. 253: 161–200. Prestwich, J. (1871). On the structure of the Crag-beds of Suffolk and Norfolk with some observations on the organic remains. Part II: The Red Crag of Essex and Suffolk. Quarterly Journal of the Geological Society of London 27: 325–356. Roger Dixon The White House 7 Chapel Street Woodbridge Suffolk IP12 4NF
Trans. Suffolk Nat. Soc. 45 (2009)