Norwich Crag at Hill Farm pits, Wangford

NORWICH CRAG AT HILL FARM PITS, WANGFORD H. B.

Mottram

In common with much of the region the Norwich Crag strata at Hill Farm Pits are largely composed of sands. Here the exposed strata were mainly deposited as sand flats in a shallow North Sea. Fluctuations of the marine environment, possibly influenced by fluvial activity, allowed the formation of often thin but locally persistent shelly, gravelly and clayey beds. It was only at the end of this period that major channels developed and substantial thicknesses of large pebbles accumulated, implying that good deposits of gravel are much rarer than previously assumed.

Hill Farm Pits are located in the Lower Blyth Valley about 4km west of Southwold at Nat. Grid. Ref. TM 465 780. Several pits are referred to and these are identified in Fig. 1. Although there have been several suggested changes in nomenclature in recent years the term Norwich Crag is used throughout to mean all of the Plio-Pleistocene sediments of the area below or older than extensive deposits whose gravel content is significantly quartzose.

Interpolation of well data for Henham and Southwold suggests that the

HILL FARM PITS-LOCATION PLAN

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Fig. 1

N O R W I C H CRAG AT H I L L FARM PITS, W A N G F O R D

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Norwich Crag extends down to about - 3 5 m OD at Wangford. Unfortunately, boreholes at Wangford only penetrate to - 9 m OD. At this depth clean unimodal (well sorted) fine sands have been recorded with occasional clay laminae, bones of sea fish (R. A. D. Markham, pers. comm.) and winkles; this is consistent with a shallow marine environment, possibly of subtidal flats. Unimodal sands exposed by excavation at Hill Farm Pits are generally slightly coarser (fine to medium, Fig. 3a) suggesting shallower water. The sedimentary structures exhibited at these levels allow the recognition of sand and mud flats with associated channels and confirm upward shallowing from subtidal to intertidal conditions. However, there is some evidence to suggest that this progression may have been significantly disrupted since there is a truncation plane at +7.4m O D which may correlate with the erosion plane at Blyth River Gravel Pits (Mottram, 1987). Fig. 2 summarises much of the evidence for all of these interpretations.

When Norwich Crag sands are devoid of shells it is often thought that the sands could have been decalcified. In some instances this is undisputed but it should be remembered that the absence or presence of shells primarily depends upon the environments prevailing at the time of deposition. This is to some extent reflected in the coarser nature of sands with a high content of shell debris (Fig. 3d).

0-5km north of Hill Farm shells occurred between - 3 m and +4m OD. At this location they appeared to be mixed with sand and gravel which is comparable to the description given by Whitaker (1887) for Southwold's Kilcock Cliff (TM 5110 7645). At Hill Farm, in pit C the shelly sand was seen to be developed irregularly but separately, so that in part it underlay a localised accumulation of sand and gravel (Fig. 2). Adjacent to this the shelly sand extended up to a maximum level of +4.4m OD. In pit A1 it attains a maximum level of +5-7m O D at the present time, although Clarke and Auton (1984) infer peaking around +7m OD. In pit A1 it is horizontally and slightly trough bedded and appears to pass up into non-shelly sand. Distinctive erosive features and cross-bedding at Bulcamp (Prestwich, 1871, and Funnell, 1983) indicates strong current activity. At Hill Farm the initial appearance implies that mounding has occurred, but further consideration suggests that erosion is equally plausible. The poor sorting of the shelly sand and the associated sand and gravel (Figs. 3c & d) indicates rapid deposition from turbulent water that occurred during storms or in fast flowing channels.

The former would have promoted mounding and the latter erosion. It should be noted that very little of this sand and gravel is within the 16-32mm size range traditionally used when examining the composition of pebbly materials. However, the limited analysis made of this fraction shows it to be composed of rounded flints.

In the overlying clay laminae are fragmented tubes, up to 1mm in diameter, believed to be worm burrows. They were perhaps produced by a type of capitellid polychaete (M. A. Whyte, pers. comm.) such as are common in intertidal mud flats of the present day North Sea. Although the sand between the clay laminae may show root traces the presence of another form of burrow, of the Skolithos group, implies that conditions were not quiescent.

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Cutting into the sand sequence, as indicated by Fig. 2, is an exceptionally gravelly deposit (Fig. 3b). It is characterised by the dominance of medium to coarse rounded flints (Clarke & Auton, 1984) and is readily distinguished from other flinty deposits on this basis. The gravel fraction's dominant size infers strong currents, while its roundness and degree of sorting tend to suggest a well-worked source. Although the southern limit of the deposit has only ever been exposed in pit A2, where it is at present obscured, the available evidence indicates that it occupies a major channel of 0.4km width within Hill Farm Pits. The channel base seems to fall from about +3m O D at Hill Farm to - 7 m O D at Mardle Road 0-7km further east. A review of the borehole and pit data (Fig. 4) indicates that this channel can be traced from Henham to Southwold and that other major channel gravels probably occur in the area. Baden-Powell and West (1960) and Hey (1967) reported a generally south-easterly dip to the bedding of these gravels. Observations at Hill Farm Pits are in accord with this statement: in pit B3 and the western half of C, thin sand and ferruginious bands have been recorded dipping down at angles of 7° to 40° to the south-south-west: in pit A2 (Hey, 1967) and in the eastern half of C, thicker sand bands and lenses have been seen dipping down in a somewhat south-easterly direction. If deposited in the late Caenozoic North Sea as a nearshore sediment then the influence of southward longshore drift would seem to be quite normal for this region as shown by many Norwich Crag sands, the Red Crag (Dixon, 1979) and present day sediments.

The longshore current could have forced the line of the channel to continually drift southwards so that the currents flowing along the channel deposited their load on the northern bank. Some sand may have accumulated in this way, but overall it seems more likely that the gravel was spread down onto the northern bank by the longshore currents. The deep nature of the major channel suggests that it is a marine extension of a river. It is also possible that there was substantially increased fluvial discharge in response to a change of climate or sea level at this time.

Also shown cutting into the sand sequence in Fig. 2 is a slightly darker sand. It is not quite so well sorted as the unimodal sands previously described, contains clay fragments and has a gravel fraction often composed of flint chippings. The base of the deposit is extremely irregular and banding occurs sub-parallel to the base immediately beneath which ice wedges have been recorded. Upwards it appears to pass without break into sand that is greatly disturbed by cryoturbation, slumping and folding, and which may contain lenses of till. This sand must therefore be interpreted as being younger than the Norwich Crag and to be a product of glacial melt water which reworked the more easily eroded unimodal sands in preference to the gravels.

References Allen, P. (1984) (ed.) Field guide to the Gipping and Waveney Valleys, Suffolk. May 1982 Quaternary Research Association. Cambridge.

Baden-Powell D. F. W. & West R. G. (1960). Summer field meeting in East Anglia, 14-28 August 1958. Proc. Geol. Ass. 71, 61.

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Cambridge, P. G. (1971). Report on field meetings to Wangford and East Suffolk. Bull. Geol. Soc. Norfolk. 20, 34.

Clarke, M. R. & Auton, C. A. (1984). The early Pleistocene deposits of the lower Waveney Valley, in Allen ed. 108. (see above).

Dixon, R. G. (1979). Sedimentary facies of the Red Crag. Proc. Geol. Ass. 90, 117.

Funnell, B. M. (1983). The Crag at Bulcamp, Suffolk. Bull. Geol. Soc. Norfolk. 33, 35.

Hey, R. W. (1967). The Westleton Beds reconsidered. Proc. Geol. Ass. 78, All. Mottram, H. B. (1987). Norwich Crag at a site near Halesworth. Trans.

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Prestwich, J. (1871). On the structure of the crag-beds of Suffolk and Norfolk with some observations on their organic remains. Part 3. The Norwich Crag and Westleton Beds. Q. J. Geol. Soc. London. 27, 452.

Spencer, H. E. P. (1971). A contribution to the geology of Suffolk. Part 5.

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Whitaker, W. (1887). The geology of Southwold and the Suffolk coast from Dunwich to Covehithe (explanation of quarter-sheet 49, N.). Mem. Geol.

Survey.

H. B. Mottram, 66 Glastonbury Close, Ipswich IP2 9EE

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