A note on the geology of Bear's pit, Acton

A NOTE ON THE GEOLOGY OF BEAR'S PIT, ACTON H . B . MOTTRAM

For many years there was a gravel pit off Bull Lane in Acton, some 5km north of Sudbury. The pit was centered at Nat. Grid Ref. TL 884 461. It is uncertain as to when the pit was first worked but its absence from the 1" Ordnance Survey (O.S.) map of 1838 may indicate either that excavation had not begun at this time or that it was too restricted to be shown at the 1" scale. However, an excavation is shown on the 25" O.S. map of 1886 and, although this suggests that the pit was shallow, descriptions from this period (Whitaker et al., 1878) indicate that a face some l i m high was in existence. The 1926 edition of the 25" O.S. map clearly indicates a deep pit and Boswell (1929) described the face as being about 13m deep. From our present understanding of the water table it can be inferred that by this time excavation had reached underground water.

Sand and gravel continued to be extracted, at one time by W. F. Bear, after whom the pit takes its name, but the pit largely feil into disuse by 1970 and exposures became degraded. The site was reopened in 1978 as a landfill facility. Fortunately, far from further obscuring the isolated geological details, the relationships of which were poorly understood, the change in use required digging-out with consequent exposure of the strata, as well as the drilling of boreholes below the visible excavation level, all of which has allowed a significantly better understanding of the geology.

The Chalk underlies the area at variable levels. Although Tertiary and preglacial Pleistocene deposits are known to overlie the Chalk in the region they are absent in the locality of Bear's Pit. Resting on the Chalk here is a thick development of sand and gravel which also contains beds of boulders, cobbles, silt and more rarely clay. Typically the sand and gravel is poorly sorted. The main constituent of the gravel fraction is angular flint. Hopson (1982) recorded minor amounts of rounded flint, quartz and quartzite, subangular sandstone and more rarely, igneous and metamorphic material.

Generally chalk is absent from the gravel fraction but it is common in the sand fraction and it dominates the silt fraction; see for example photograph 16 of McKeowan and Samuel (1985). Sedimentary structures were not well developed in the sand and gravel but uneven beddingdid occur in it throughout the site. The best exposed and recorded structures were in the central area where beds of boulders and cobbles took the form of Channel infill. This suggests that deposition was from high velocity surges of melt water with silt sometimes accummulating when conditions abated. The poor sorting of grain sizes and the sedimentary structures recorded are untypical of sands and gravels from a glacial outwash piain (sandur) and Millward (1980) suggested that the melt water at this site was associated with the development of the nearby Stour sub-glacial tunnel Valley. Slabs of tili occasionally occur within the deposit; these would have developed from blocks of melting ice which implies that the edge of the melting ice sheet was not too distant. In common with other exposures of glacial sand and gravel in the area (Millward, 1980), slump structures have been noted: see B.G.S. photograph

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A NOTE ON THE GEOLOGY OF BEAR'S PIT, ACTON

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Suffolk Natural History, Vol. 28

A13249. The slumping may have resulted from subsidence of the sand and gravel into the voids left by the progressive melting of blocks of ice which had been caught up in the sand and gravel.

Jutting into the sand and gravel from the north was a horizontal tongue of tenaceous, structureless tili, 6 to 7m thick, which laterally interdigitated into the sand and gravel bordering its eastern and possibly western edges. The tili was essentially dark grey, only being brown at the top and sometimes at the base. It contained innumerable small fragments of chalk. Fragments of Kimmeridge Clay, Keuper Marl (Mercia Mudstone) and belemnite phragmacones were occasionally recognisable. The tili resembles lodgement tili but due to its limited extent and intimate relationship with the sand and gravel it is interpreted as a melt-out tili. It would have formed from ice that had broken away from the basal (debris rieh) part of the ice sheet.

The sand and gravel was only known to be buckled in the central area of the site. Spencer (1967) suggested that the disturbance could have resulted from the movement of the ice sheet. However, subsequent excavation also showed that the sand and gravel dipped underneath this area in a steep depression. It is therefore thought more likely that the sand and gravel accumulated around the core of a large block of melting ice so that spasmodic collapsing produced the buckling.

Overlying the sand and gravel at the centre of the site was a thin development of brown and grey laminated silt and clay, see B.G.S. photographs AI3246 to A13248. It is understood that it was from here that Wilson & Lake (1983) identified dropstones (pebbles which feil from floating ice blocks). On this basis Wilson & Lake proposed that these clays and silts were laid down under water, i.e. in a lake. These clays and silts passed up into brown banded sandy tili. It is thought that the latter is the 'boulder clay' described by Whitaker et al. (1878), Boswell (1929) and Donner and West (1956), but it is now considered that this was in fact a flow tili. This tili would have formed when wet, somewhat fluid, debris, released from the surface of the melting ice, flowed down an adjacent slope. In so doing the banding was produced and pebbles became aligned in the same direction.

Capping the banded tili was a thin development of dark grey tili identical to the tongue of melt-out tili described earlier. It is uncertain whether this uppermost tili was also of localised basal melt-out rather than lodgement origin.

References B.G.S. (unpub photos). Sheet album 206. British Geological Survey, Keyworth.

Boswell, P. G. H. (1929). The geology of the country around Sudbury, Suffolk. Mem. Geol. Surv.

Hopson, P. (1982). The sand and gravel resources of the country around Sudbury, Suffolk: description of 1:25,000 resource sheet TL 84. Miner.

Assess. Rep. Inst. Geol. Sei. No. 118.

McKeowan, M. C. & Samuel, M. D. A. (1985). Regional study of the sand and gravel resources of Essex and south Suffolk. British Geol. Surv.

Trans. Suffolk Nat. Soc. 28 (1992)

A NOTE ON THE GEOLOGY OF BEAR'S PIT, ACTON

49 Millward, D. (1980). Notes and local details for the area around Long Melford (TL 84 NE, SE); part of the 1:50,000 sheet 206 (Sudbury). Open File Rep. Inst. Geol. Sei. No. 1980/4.

Spencer, H. E. P. (1967). A contribution to the geology of Suffolk; Part 3.

The glacial epochs. Trans. Suff. Nat. Soc. 13, 366.

West, R. G. & Donner, J. J. (1956). The glaciations of East Anglia and the East Midlands: a differentiation based on stone-orientation measurements of the tili. Q. J. Geol. Soc. London. 112, 69.

Whitaker, W., Penning, W. H„ Dalton, W. D. & Bennett, F. S. (1878). The geology of part of north-west Essex with parts of Cambridgeshire and Suffolk and north-east parts of Herts. Mem. Geol. Survey.

Wilson, D. & Lake, R. (1983). Field meeting to North Essex and West Suffolk, (20-22 June 1980). Proe. Geol. Ass. 94, 75.

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

Trans. Suffolk Nat. Soc. 28 (1992)