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

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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

Trans. Suffolk Nat. Soc. 28 (1992)


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A note on the geology of Bear's pit, Acton by Suffolk Naturalists' Society - Issuu