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THE WESTLETON BEDS – A CASE FOR RIGS H. B. MOTTRAM Introduction In most exposures, Norwich Crag is a well sorted sand. This is typical of nearshore environments that include both beaches and tidal flats. Genuine “crag” (shell deposits), muds and gravels do occur but are the exception rather than the rule. About 1·9 million years ago the Norwich Crag was coming to an end and some gravely deposits were laid down in ENE Suffolk and in parts of Norfolk. These deposits are often referred to as the “Westleton Beds” but strictly speaking they not a discrete set of beds, they are a local variant. The gravelly deposits occur in several ways. Descriptions and accounts of these are given in the references listed. The different gravelly occurrences are summarised in Table 1. It is not necessarily an exhaustive list but it sets out some of the most significant types of occurrence. Discussion Although we have a good understanding of gravels deposited by rivers our understanding of gravels deposited in the sea is much poorer. Also, most gravels that occur within the geological record were deposited by ancient rivers eg the Kesgrave Formation. To find examples of rip-channels in the geological record is difficult enough but to find gravelly examples of them is, as far as I know it, impossible. Therefore it is important to preserve some of our “Westletonian” ones. As for gravelly tidal inlets, well they are even more unique. The sites therefore have educational value. There are several issues to unravel and as knowledge advances we will want to re-evaluate earlier work so the sites are clearly important to ensure continuing research. The sites also have historical value. Prestwich used the term Westleton Beds in 1871 and 1890 while the Geological Survey (noteably Whitaker, Dalton and Blake) mapped and described Pebbly Series (1884–1890). Both sets of workers considered that they were separate from the Norwich Crag. Although Solomon added a little more information in 1935 (he actually used the term Westleton Series) it was really Richard Hey’s thoughtful work in 1967 that kick-started current interest. There are currently two RIGS, the cliffs at Dunwich and at Minsmere. They are accessible from public car parks that have toilets and refreshment facilities. They fit comfortably alongside other publicly accessible countryside amenities. In conclusion, sites exhibiting the so called “Westleton Beds” in ENE Suffolk have great RIGS potential.
References Hey, R. W. (1967). The Westleton Beds reconsidered. Proc. Geol. Assoc. 78: 427. Mathers, S. & Zalasiewicz, J.(1996). A gravel beach-rip channel system: the Westleton Beds (Pleistocene) of Suffolk, England. Proc. Geol. Assoc. 107: 57.
flint but can have 5% to 10% quartzose pebbles not analysed
well sorted (bi-modal)
up to 4 m thick
Interpreted as
Inter-tidal Creek (Meander Lag Deposits) Storm Deposits
Covehithe, Easton Rip Channel Lags & Infills Bavents & Minsmere (In Beach Shorefaces) Covehihe & Thorington Sub-tidal Channels & Sheets (In Beach Shorefaces or Lagoons?) Wenhaston Cliff-Base Lag Deposit
Dunwich, Minsmere & Tidal inlet infills Wangford (Between Barrier Islands)
Examples at
flint shape is semi- Wangford discoidal poorly sorted (wide spread lint fabric is matrix Wangford of grain sizes) suppported
very well sorted
not analysed
flint
well sorted (bi-modal)
up to 0·5 m thick, ribbon like development very thin (max 0·05 m)
flint fabric is often clast supported
very well sorted (strongly bi-modal)
channels up to 15 m deep × 600 m wide, ribbon like development, generally orientated approx WNW to ESE channels up to 4 m deep × 30 m wide, gravel layers max 0·5 m thick up to 5 m thick
Clasts
Grading
Geometry of Occurrences
Table 1: SUMMARY OF GRAVELLY OCCURRENCES
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GEOSUFFOLK RIGS
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Mottram, H. B. (1987). Norwich Crag at a site near Halesworth. Trans. Suffolk. Nat. Soc. 23: 64. Mottram, H. B. (1988). Norwich Crag at Hill Farm Pits, Wangford. Trans. Suffolk. Nat. Soc. 24: 88. Mottram, H. B. (1989). The Upper Caenozoic sequences around the Long Row, Covehithe. Trans. Suffolk. Nat. Soc. 25: 86. Mottram, H. B. (1994). Some critical locations in the gravelly Norwich Crag of Suffolk. Quaternary Newsletter 73: 29. Mottram, H. B. (1997). The channel gravels of the Dunwich and Minsmere cliff. Trans. Suffolk. Nat. Soc. 33: 77. Prestwich, J. (1871). On the structure of the crag-beds of Suffolk and Norfolk with some observations on their organic remains, Part 3. The Norwich Crag and Westleton Beds. Q. J. Geol. Soc. London. 27: 452. Prestwich, J. (1890). On the relations of the Westleton Beds, or Pebbly Sands of Suffolk, to those of Norfolk, and on their extension inland. Q. J. Geol. Soc. London. 46: 389. Richards, A. E., Gibbard, P. L. & Pettit, M. E. (1999). The sedimentology and palaeoecology of the Westleton Member of the Norwich Crag Formation (Early Pleistocene) at Thorington, Suffolk, England. Geol. Mag., 136: 453. Sinclair, J. M. Plio-Pleistecene relative sea-level changes in East Anglia: evidence from the Westleton Member and related deposits. Proc. Geol. Assoc. 110: 149. Solomon, J. D. (1935). The Westleton Series of East Anglia: its age, distribution and relations. Q. J. Geol. Soc. London. 91: 216. Whitaker, W. (1887). The geology of Southwold and the Suffolk coast from Dunwich to Covehithe (explanation of quarter-sheet 49, N.). Mem. Geol. Surv. Whitaker, W. & Dalton W. H. (1887). The geology of the country around Halesworth and Harleston. Mem. Geol. Surv. H. B. Mottram Environment and Transport Dept Suffolk County Council Endeavour House 8 Russell Road
Ipswich IP1 2BX.