Norwich Crag at a site near Halesworth

NORWICH CRAG AT A SITE NEAR HALESWORTH H. B.

MOTTRAM

Blyth River Gravel Pit, Wenhaston This little known site located at Nat. Grid. Ref. T M 410 766 was investigated in 1984. T h e following is a record of the features revealed during this investigation.

From borehole evidence it is known that at this location the Lower Tertiary strata (London Clay etc.) have been eroded down to between - 3 m and - 5 m O D , usually terminating at claystones. The immediately succeeding strata have only been described from these boreholes. Assumed to be part of the Norwich Crag Formation, they are grey silty fine sands containing some laterally discontinuous silts and clays. Fragmented marine shells also occur. These beds probably represent shallow marine conditions made locally more muddy due to reworking of Lower Tertiary strata. Above and seen in working faces are clean fine to medium well sorted (unimodal) sands.

Distinction between a lower sequence containing occasional clay laminae (mud drapes?) and a higher one without is not only difficult throughout most of the pit but initially appears to be of no significance. However, in the south-western corner of the pit the sequences are clearly separated. This is illustrated.

Here the lower sand sequence is composed of three beach facies. The first facies is that of unimodal sand which frequently has foresets steeply inclined down to the south-west (Fig. R). This represents a wave dominated foreshore (intertidal) environment with low sand ridges. The ridges would have been driven in a south-westerly direction by a combination of westward (landward) wave surge and southward longshore drift. As the water shallowed, reverse (antidune) ripples formed. Above this is a bioturbated facies of otherwise flat bedded sand with some clay laminae formed in the foreshore swash zone (Fig. B). H e r e there are vertical tubulär burrows approx. 100mm X 10mm dia. which have been identified as Skolithos (G. E. Farrow, pers. comm.). This trace fossil is typical of foreshore and other intertidal sands. No other faunal evidence has been observed at this or adjacent levels but it is feit that the burrows were produced by a bivalve mollusc such as Macoma.

The highest of the three facies consists of horizontally laminated clay with some sand (Fig. C). It represents mud flats formed under quieter conditions around high water mark. The clayey facies is suggested to extend from the disused brickworks ('Haiesworth Kiln') 2km further west (TM 393 762) and beyond; Whitaker and Dalton (1887) record clay of similar appearance and level at Huntingfield (TM 33 72) and several surrounding villages. As shallowing continued the facies would have followed the sea's eastward withdrawal. Eventually the sea returned and eroded previous sediments during this westward transgression. As the cliff line retreated it was 'drowned' at one point (Fig. RCL) due to a relative rise of sea level and it became buried by the upper sand sequence.

Beneath the poorly permeable clayey facies iron oxide is scattered

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NORWICH CRAG AT A SITE NEAR HALESWORTH

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throughout the bioturbated sand and it helps to define foresets and other planes some 2m beneath. At the foot of the cliff feature a 0-5m thick gravel lag is particularly ferruginious having been cemented into a conglomerate (Fig. G). An iron pan arises here and forms a useful reference plane for separating the two sand sequences in the rest of the pit where the bioturbated and clayey facies have been cut out.

The uppermost deposits at the site have been ill exposed for a number of years. They are gravelly but appear to be an irregulär development about 2m thick and subparallel to the ground surface. It is unlikely that they are part of the Norwich Crag Formation and so it is possible that the south-easterly dipping flint gravels recognised by Hey (1967) have been fully excavated.

It should be noted that the most interesting features at this site are now unfortunately obscured.

References Hey, R. W. (1967). The Westleton Beds reconsidered. Proc. Geol. /Imoc. , 78. 427.

Whitaker, W., & Dalton W. H. (1887). The geology of the country around Haiesworth and Harleston. Mem. Geol. Surv.

H. B. Mottram, 66, Glastonbury Close, Ipswich

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