The geology of Hall Heath, Lackford

48

T H E GEOLOGY OF HALL HEATH, LACKFORD H. B. MOTTRAM Hall Heath lies on the southern edge of the Suffolk Breckland where arable farming predominates. Tucked in behind Victorian deciduous woodlands is a shallow, former gravel working which is currently used as a landfill site.

Centered at National Grid Reference T L 799 692, the site reaches down to the Chalk. According to the British Geological Survey (B.G.S.) this is the Upper Chalk close to its junction with the underlying Middle Chalk. The top surface of the Chalk here occurs at between +28 and +32m O.D. It is fairly flat with occasional hummocks and hollows. Some Solution pipes are known and these are infilled with either a chocolate-brown clay with black flecks or a reddishbrown, cobbly gravel. Where the infilling consists of both materials the clay tends to line the base and sides, with the cobbly gravel Alling the centre and top of the pipes. An admixture of the two materials is uncommon. The gravel is composed of flint material. Flints of cobble and boulder size are not worn and are often unbroken - characteristics which indicate minimal transportation and thus local derivation from the Chalk. It is likely that these clay and flint deposits were derived from weathering of the Chalk and soils which formed upon the Chalk, during warm and moist climates in the Tertiary to Lower Pleistocene, (Thorez, et al, 1971; Matthews, 1977).

Previous accounts of the sand and gravel At Hall Heath the Chalk is generally overlain with sand and gravel which has traditionally been interpreted as a glacial outwash deposit. Following its remapping work in the region, the B.G.S. has reaffirmed its support for this inteipretation (Hawkins, 1981; Clayton, 1983; Bristow, 1990). More recently, an interesting alternative interpretation was put forward at a quickly rearranged field meeting of the Joint Association for Quaternary Research. Examination of the composition and sedimentary structures of the sand and gravel at Hall Heath suggested that this deposit was attributable to the Ingham Sand and Gravel (Bridgland & Lewis, 1991; Hunt, et al, 1991). It was therefore reinterpreted as having been laid down by the large. pre-glacial 'Ingham' river of Lower Pleistocene age which flowed approximately eastwards hereabouts.

New evidence Later excavation in the pit at Hall Heath has revealed better exposures than previous researchers had the opportunity to see. A summary description of these is given below. The details are part of a county wide R.I.G.S. survey but must not be taken to infer any such status on the site.

The 16 to 32mm fraction of the sand and gravel deposit has an average composition of 75% flinty, 23% quartzose and 2% other pebble types. The flinty component is essentially composed of angular flint while the quartzose component is mainly made up of Lower Palaeozoic quartzite. Jurassic fossils and pebbles of chalk are absent, although trace quantities of chalk pebbles are known in boreholes adjacent to the site (Hawkins, 1981). The sand fraction is devoid of chalk grains. In part, the sand and gravel passes into laminated muds and also tili (boulder clay), and these have been shown to directly overlie the

Trans. Suffolk Nat. Soc. 30 (1994)

THE GEOLOGY OF HALL HEATH

49

Chalk to the west (Clayton, 1983) and to the south of the excavated area of the pit. The surface of the Chalk does not show a Step up or down as the covering strata changes from sand and gravel to mud or tili. A localised development of black, relatively carbon-rich clay has also been recorded. The orientations of foresets in sand and ripples in alternations of sand and mud have indicated that water flow was in the ränge of south-westward to south-eastward.

Conclusions From the foregoing it is important to note that the sand and gravel at Hall Heath appears to be intimately associated with tili. Till is indisputedly of glacial origin and so the implication is that at this site the sand and gravel must also be glacial.

In addition, the approximately southward waterflow accords better with the general direction of glacial drainage than with that of the pre-glacial rivers which deposited either the Ingham or Kesgrave Sands and Gravels. Although chalk fragments and occasional Jurassic fossils are found in many glacial sands and gravels in East Anglia their absence does not preclude a glacial origin.

Therefore, it is preferable to retain the traditional interpretation that the sand and gravel at Hall Heath represents the glacial Sediment of fairly clean meltwater which formed outwash streams and rivers flowing approximately southwards. These waters could have reworked the Ingham Sand and Gravel and have thus brought the much recycled Lower Palaeozoic quartzite pebbles some 3km southwards to Hall Heath. The occurrence of tili in discontinuous sheets at variable levels is taken to indicate that tili was deposited at the site from the wasting of ice blocks caught up in the outwash. The laminated muds could have accumulated during quiet periods in either a shallow lake or the subsidiary Channels of a braided river. Stagnation in these environments would have resulted in the deposition of organic clay. The extremely restricted spread of the organic clay is more typical of a subsidiary Channel.

References Bridgland, D. R. & Lewis, S. G. (1991). Introduction to the Pleistocene geology and drainage history of the Lark Valley, in Lewis et al. (eds), 37.

Bristow, C. R. (1990). Geology of the country around Bury St. Edmunds. Mem. Brit. Geol. Surv., sheet 189 (England and Wales).

Clayton, A. R. (1983). The sand and gravel resources of the country between Mildenhall and Barrow, Suffolk: description of 1:25,000 sheets TL 76, TL 77 and part of TL 87. Miner. Assess. Rep. Inst. Geol. Sei. N° 123.

Hawkins, M. R (1981). The sand and gravel resources of the Bury St. Edmunds (Suffolk) area: description of 1:25,000 sheet TL 86. Miner. Assess. Rep. Inst.

Geol. Sei. N° 72.

Hunt, C. O., Lewis, S. G„ Rose, J. & Wymer, J. J. (1991). Lackford, Suffolk (TL 815 704), in Lewis et al. (eds), 85.

Lewis, S. G„ Whiteman, C. A. & Bridgland, D. R. (eds) (1991). Central East Anglia and the Fen Basin. Field Guide. Quaternary Research Association.

Matthews, B. (1977). Clay-with-flints on the Yorkshire and Lincolnshire Wolds. Proc. Yorks. Geol. Soc. 41, 231.

Trans. Suffolk Nat. Soc. 30 (1994)

50

Suffolk Natural History, Vol. 30

Thorez, J. P„ Bullock, P , Catt, J. A. & Weir, A. H . (1971). The petrography and origin of deposits Alling Solution pipes in the Chalk near South Mimms, Hertfordshire. Geol. Mag. 108, 413.

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

Trans. Suffolk Nat. Soc. 30 (1994)