Geological details of two pits near Kesgrave

GEOLOGICAL DETAILS OF TWO PITS NEAR KESGRAVE H. B. MOTTRAM Beneath Sink's Pit the Chalk is encountered at about -15m O . D . and below Foxhall Pits at about -12m O . D . There are insufficient data to accurately contour the upper surface of the Chalk other than to show that in this area it slopes down to the south-east (Notcutt, 1978, IGS, 1981). T h e Chalk is overlain by Lower Tertiary strata. The bottom half of this group of strata consists of fine sands and silts, the lowermost beds of which may represent a thin remnant of the Thanet Sands Formation but, in the main, they represent the Woolwich and Reading Formation. The top half of the Lower Tertiary group consists of silty clay, the London Clay Formation. The general slope of the upper surface of the London Clay here is down to the east-south-east.

There is more information available for the London Clay surface than for the Chalk. Where close groupings of boreholes have been made it can be shown that the surface of the London Clay is not entirely planar and that it has some distinct lows and highs. Below Foxhall Pits a 6m high ridge, truncated by the post-glacial Mill River Valley, and two shallow elongate depressions have been identified (Fig. 1). Possible explanations for these features come from several sources.

Boatman (1976) was of the opinion that the London Clay had been folded.

Although he didn't indicate an origin for the possible folding the most likely cause would have been the Alpine orogeny. The latter is known to have produced south-west to north-east trending low amplitude folds in the Wealden strata (Lower Cretaceous) of Sussex and Kent. Alpine earth movements could also have caused activation of faults of south-west to northeast alignment. Bristow (1983), conjectured that fault-bounded blocks of this nature were responsible for different thicknesses of Crag in central Suffolk.

Another alternative results from direct observations of the London Clay surface in the Chelmsford area where steep sides ridges and furrows of 3 to 20m height or depth occur. These have been attributed to ice-heaving by Bristow (1985). However, this mechanism can be discounted around Kesgrave because the lows and highs here have a gentle profile which cannot represent the eroded remnants of abrupt glacial or peri-glacial structures since they occur below pre-glacial Crag. Although Carr (1967) purported erosion by a large, northward-flowing river of pre-Coralline Crag age, the general slope and detailed topography of the London Clay surface do not accord with this. Funnell (1972) discussed the possible development of the south-west to north-east orientated central Suffolk depression around Stradbroke in terms of tidal scour of the Chalk surface. Dixon (1979) extended the concept to account for troughlike features in the London Clay surface. It was considered that erosion by south-westerly flowing tidal currents of Crag age produced the features. Mathers and Zalasiewicz (1988) also favoured this view.

Overall it seems most likely that erosive events rather than earth movements were responsible for sculpting the basic shape of the London Clay surface. It is quite likely that some erosion by rivers occurred in the Upper

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GEOLOGICAL DETAILS OF T W O PITS NEAR REDGRAVE

'24 NOTECONTOURS SHOWN IN METRES ABOVE O D. G E N E R A L SOUTH EAST WARD SLOPE LOCALLY DISCERNABLE (IT IS C L E A R E R OVER A LARGER A R E A THAN SHOWN) HIGH A N D LOW C O N T O U R S AT F O X H A L L AND THEIR ORIENTATION FEATURE AT NORTHERN END OF M A R T L E S H A M MAY BE INNACURATE DUE TO TYPE OF DRILLING USED HERE VERY DISTINCT TREND OF CONTOURS AROUND POST-GLACIAL VALLEYS

THE LONDON CLAY SURFACE AROUND KESGRAVE

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Fig.1

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Tertiary epochs. However, there is an absence of river sediments and of distinct fluviatile features from this period. Hence the indication is that any river activity would have been limited in effect and, in all probability, obliterated by the tidal erosion of this period, i.e. during the Coralline Crag and Red Crag marine phases.

At Foxhall, on the surface of the London Clay ridge at TM 23606 43883, an isolated patch of clean, fine to medium sand was observed forming the base of the Red Crag. This is probably a rarely preserved remnant of the relatively short-lived inter-tidal phase of deposition during the transgression of the sea.

Elsewhere, the basal 0.5m or so consists of the better known mix of shelly sand with derived phosphatic nodules ('coprolites') and occasional worn sharks teeth from the London Clay and also round black flints. These basal beds represent the lag deposits of the spreading North Sea during the Red Crag time interval. These deposits are therefore compatible with the possible tidal erosion of the London Clay surface discussed above.

In the main, the lower part of the Red Crag is composed of coarse redbrown sand and shells. These beds are now accepted as having formed as sand waves in the sub-tidal North Sea at water depths of about 20m (Boatman, 1976; Dixon, 1979; Mathers & Zalasiewicz, 1988). Sand waves are elongate ridges aligned perpendicular to the water flow (Fig 2). They are 1 to 5m high with a wedge-shaped cross-section of 50 to 100m width. They therefore resemble enormous ripples and are sometimes called megaripples.

The bedding at Sink's Pit and Foxhall Pits consists of both tabular and troughed forms. The bedding has well developed steep forests which indicate strong currents. Measurement of the foreset orientations shows that the currents flowed to the south-west, i.e., parallel to the assumed coastline.

Longshore currents are actually ebb-tidal currents and in the near-shore area of the sub-tidal zone they are generally deflected clockwise around the British coast by the Coriollis effect. The effect of the weaker flood-tidal direction is less often preserved in the sand waves.

The molluscan shells in the Red Crag are well known. It is believed that the organisms lived in water of temperatures similar to those off the present day coast of Northern Ireland (Dixon, 1977; Mathers et al, 1984). Among the shells found at Foxhall and Sink's Pits are those of Cerastoderma edule, C. parkinsoni, Glycimeris glycimeris and Neptunea contraria. Today, Cerastoderma edule lives in or just below the inter-tidal zone. This is also the case for many other Red Crag molluscs and gives rise to the view that many shells were swept into the less shallow water to be incorporated into the sand waves. It is, however, thought that some fauna, including Glycimeris, were able to cope with the constantly shifting crest areas of the sand waves where the vigorous current carried a good food supply. Some of the more mud loving fauna inhabited the less active 'Valleys' between adjacent sand waves (Dixon, 1977).

The shelly Red Crag at Foxhall typically has 45% by weight of shell material which approximates to lA of the volume of these beds. Where decalcification of formerly shelly Red Crag has occurred at Foxhall, the absence of distinct collapse or disruption of the overlying beds suggests that the shells dissolved either at a fairly uniform rate or before the overlying beds

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were deposited. At Sink's Pit the shell content is less than at Foxhall, only being 2 0 % by weight. Nonetheless, the loss of volume which may have occurred here is also considered to be significant.

In Foxhall Pits at T M 2416 4382 there is a horizon of mainly fine to medium sand of orange-brown colour. This sand exhibits symmetrical, sinuous crested ripples with mud drapes. Examination of the ripple orientations indicates that the current flow was to either the north-east or the south-west, or both. This sand probably accumulated under the influence of ebb-tidal currents and also, perhaps, flood-tidal or eddy effects in the more sheltered conditions of a 'valley' between adjacent sand waves. Also present are subvertical burrows of small size, rarely larger than 15mm x 2mm diameter.

Identical burrows have been reported in shelly Red Crag at Buckanaye Farm, Alderton (Zalasiewicz et al, 1988). At Foxhall a small Channel of mainly coarse sand was also noted. The ripples in the Channel indicated flow to the south-east. The Channel is interpreted as having been a short-lived feature which served storm generated flows returning from the shoreline along the seabed.

C e r a s t o d e r m a edule

Glycimeris glycimeris

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Norwich C r a g is not k n o w n at Foxhall Pits. H o w e v e r , it clearly occurs in Sink's Pint in t h e n o r t h e r n f a c e , b e t w e e n T M 2229 4652 and T M 2252 4660, where it overlies the R e d C r a g at + 2 2 m O . D . , but w h e t h e r it was present at the eastern e n d of this site is unclear. T h e Sediment is a clean, well-sorted fine to m e d i u m sand of pale grey to pale yellow colour. In part the sand passes up into a b r o a d , shallow Channel of grey m u d . T h e Norwich C r a g here was deposited in an inter-tidal flat e n v i r o n m e n t .

Overlying t h e Crags a r e gravelly sands which are i n t e r p r e t e d as the deposits of a large, b r a i d e d river believed to be an ancestoral River T h a m e s (Rose et cd, 1978; W h i t e m a n , 1992). This is the Kesgrave Sand and Gravel F o r m a t i o n . A t Foxhall Pits this c o m m e n c e s at a r o u n d + 2 2 m O . D . A t Sink's Pit it is currently visible in t h e n o r t h e r n face above + 2 8 m O . D . and f u r t h e r south-eastwards at a b o u t + 2 3 m O . D . T h e sand and gravel contains n o c h a l k particles. T h e gravel c o m p o n e n t is mainly c o m p o s e d of flint with q u a r t z o s e pebbles providing 13% of t h e 16 to 32mm size ränge (Allen, 1984). T h e deposit occurs in flat and cross-bedded units. T h e foresets at Sink's Pit indicate w a t e r flow to the n o r t h - n o r t h - e a s t , almost identical to t h e river's

Cerastoderma parkinsoni

Neptunea contraria

lern

1cm

Drawings produced by C. J. Hawes Specimens provided by R. A. D. Markham, Ipswich Museum

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generally north-easterly flow in this area (Rose & Allen, 1977). The river is thought to have been of Lower Pleistocene age and to have had virtually no tributaries fed by glacial melt water.

The top of the Kesgrave Sand and Gravel Formation was modified by two soil processes. Initially reddening with slight clay enrichment took place under temperate conditions. This was followed by freeze-thaw disturbance under peri-glacial conditions. The combined effect at Sink's Pit has been to produce reddish convolutions in the top 0.5m or so of the Kesgrave Sand and Gravel Formation. Where the effect has not been cut-out by subsequent geological activity it forms a useful marker horizon for defining the junction with any overlying sand and gravel.

At Sink's Pit, above the forementioned marker horizon, there is a thin gravelly sand known as the Barham Sands and Gravels (Rose & Allen, 1977).

Trace of chalk have been recorded elsewhere but they are absent here. Flint is again the main component of the gravel fraction but the quartzose component of the 16 to 32mm fraction has risen to 32% (Allen, 1984). The deposit occurs as thin, flat and cross-bedded units. Foresets indicate flow to the east-north-east although the general direction of flow in the area was to the east-south-east. This Stratum has been accounted for in terms of deposition from a shallow, braided river fed by melt water from the encroaching Anglian ice sheet during the early part of the Middle Pleistocene. The Barham Sands and Gravels have never been recognised in the Foxhall Pits. It is possible that some sand with chalky clay near ground level may be a scanty development here.

No fully glacial deposits are known at the two sites. Post glacial river erosion has cut down into the London Clay immediately to the south of Foxhall Pits and to the north and south of Sink's Pit. In these Valleys the London Clay is often overlain by thin post-glacial alluvium, typically sand followed by peat and then mud. Additional sand, due to hill wash and recent river activity, may also occur.

References Allen, P. (ed). (1984). Field guide (revised edition, October 1984) to the Gipping and Waveney Valleys, Suffolk. May 1982. Quat. Res. Ass. Cambridge.

Allender, R. E. & Hollyer, S. E. (1972). The sand and gravel resourcesof the area south and west of Woodbridge, Suffolk: description of 1:25,000 resource sheet TM 24. Rep. Inst. Geol. Sei. 72/9.

Boatman, A. R. C. (1976). Sedimentary characteristics of the Red Crag.

Bull. Ips. Geol. Grp. 18, 12.

Bristow, C. R. (1983). The stratigraphy and struetures of the Crag of midSuffolk, England. Proc. Geol. 4 m . 94, 1.

Bristow, C. R. (1985). The geology ofthe country around Chelmsford. Mem. Brit. Geol. Surv. Keyworth.

Carr, A. P. (1967). The London Clay Surface in part of Suffolk. Geol. Mag. 104, 514.

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Dixon, R. G . (1977). A note on the palaeoecology of the Red Crag (Lower Pleistocene). Quat. Newsl. 23, 1.

Dixon, R. G . (1979). Sedimentary facies in the Red Crag (Lower Pleistocene, East Anglia). Proc. Geol. -4m. 90, 117.

Funneil, B. M. (1972). T h e history of the North Sea. Bull. Geol. Soc. Norfolk. 22, 2.

Gibbard, P. L. & Zalasiewicz, J. A . (eds). (1988). T h e Pliocene - Middle Pleistocene of East Anglia. Field Guide. Quat. Res. Am. Cambridge.

IGS, (1981). Hydrogeological map of southern East Anglia Sheet 2. Chalk, Crag and Lower Greensand: geological structure. Brit. Geol. Surv. Keyworth.

Long, P. E. & Cambridge, P. G . in Gibbard, P. L. & Zalasiewicz, J. A .

(1988). Crag mollusca: an overview. 53.

Mathers, S. J., Zalasiewicz, J. A. & Balson, P. S. (1984). A guide to the geology of south-east Suffolk. Bull. Geol. Soc. Norfolk, 34, 65.

Mathers, S. J. & Zalasiewicz, J. A. (1988). The Red Crag and Norwich Crag Formations of southern East Anglia. Proc. Geol. Ass. 99, 261.

Notcutt, G . J., (1978). T h e concealed Chalk surface of mid-Suffolk. Trans.

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Rose, J. & Allen, P. (1977). Middle Pleistocene stratigraphy in south-east Suffolk. J. Geol. Soc. 133, 83.

Rose, J., Allen, P. & Wymer, J. J. (1978). Weekend field meeting in southeast Suffolk. 15-17 October 1976. Proc. Geol. Ass. 89, 81.

Whiteman, C. A . (1992). The palaeogeography and correlation of preAnglian glaciation terraces of the River Thames in Essex and the London Basin. Proc. Geol. Am. 103, 37.

Zalasiewicz, J. A . & Mathers, S. J. (1985). Lithostratigraphy of the Red and Norwich Crags of the A l d e b u r g h - O r f o r d area, south-east Suffolk. Geol.

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Zalasiewicz, J. A . , Mathers, S. J. & Cambridge, P., in Gibbard, P. L. & Zalasiewicz, J. A. (1988). Buckanaye Farm. 73.

H. B. M o t t r a m , 66, Glastonbury Close, Ipswich. IP2 9 E E .

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