THE CLIFF AGAINST DUNWICH HEATH – UNDERSTANDING THE NORWICH CRAG SANDS & GRAVELS

81 THE CLIFF AGAINST DUNWICH HEATH – ITS FURTHER IMPORTANCE IN OUR UNDERSTANDING OF THE NORWICH CRAG SANDS AND GRAVELS HOWARD MOTTRAM As we have previously noted in these Transactions (Mottram, 1989), there are some gravelly beds in the cliff at Covehithe and among those in the low to mid-levels of the cliff are a minority that have a recognisable proportion (say > 5%) of pebbles that are quartzose (quartz and quartzite). For some researchers this has led to an increasing desire to reclassify these more quartzose beds from the Norwich Crag to the younger Wroxham Crag and, by default, to also apply this to the immediately overlying beds (Hamblin, 2001; Hamblin & Rose, 2012). However, where the immediately overlying beds are gravelly they have virtually no quartzose pebbles at all; a lack of quartzose pebbles in gravel is one of the characteristics of typical Norwich Crag. These immediately overlying beds include rip-channels. The cliff against Dunwich Heath holds the key to this dilemma.

As we have also previously noted in these Transactions (Mottram, 1997), there is a large channel, some 600 m wide, in the cliff against Dunwich Heath (“Minsmere Cliff” on some maps). In the northern part of this channel, the pebbles are usually of large size and tightly packed so that the ground here is more resilient and can sustain the cliff top at virtually its original Dunwich Heath level of nearly +20 m OD. In the central and southern parts of the channel, gravel is much less common than sand and this results in the cliff being less stable and accordingly the cliff top is of reduced height.

In the cliff just south of the large channel there is a much smaller channel, some 33 m wide and 3 m deep (see Plates 20 & 21). This is a rip-channel that would have been produced by especially vigorous rip-currents moving down the submerged part of the Crag beach-face of that time. If we compare this rip-channel to the other Crag rip-channels in the cliff between here and Benacre Broad, we can see that this rip-channel is of the same sort of size and structure. Therefore, this rip-channel cannot have lost any significant height or width due to the cliff top having been eroded down. Rip currents form in shallow topographic lows and a gravelly beach-face would be far less likely to provide these lows than a sandy beach-face. So, if we reconstruct the cliff top back to its original Dunwich Heath level then the rip-channel would not need to be extended higher and wider and certainly not over immediately adjacent gravelly beds.

But what of the large channel? When it was formed its northern and southern flanks may have been a few hundred metres apart but they would have been cut down from the same plane. Allowing for variations in this plane, it is still extremely unlikely that the final position of the southern flank would have been cut down from a significantly different level to that of the northern flank. Today the preserved northern flank extends down from virtually the maximum surface of Dunwich Heath (nearly +20 m OD) and so if we reconstruct the central and southern parts of the channel they need to be extended up to the same sort of level for which the more conservative level of the adjacent land of just over +16 m OD has been used rather than the level of nearly +20 m OD mentioned for the northern flank. Nonetheless, in

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Figure 1. Sketch Section of Cliff against Dunwich Heath. so doing, the southern flank would still rise up and over the small rip-channel previously discussed. See Fig 1.

The rip-channels and the large channels of this area have previously been accounted for as having been contemporaries of one another and having been part of the same dynamic environment (Mottram 1997; 2012). The new evidence presented here does not change that interpretation but what it indicates is that the rip-channel here underlies the large channel and so the rip-channel cannot be younger than the large channel. As the large channel is undisputedly of Norwich Crag age, then it is reconfirmed that the ripchannels of this area of Suffolk are of Norwich Crag age and are not of Wroxham Crag age.

The possibility of the occasional presence of noticeable concentrations of quartzose pebbles in the Norwich Crag should perhaps be expected. If we go back in time by around 2 million years to the Norwich Crag period, the climate oscillated in response to 41,000 yr Milankovitch cycles and in consequence the position of the East Anglian mainland relative to the North Sea moved backwards (westwards) and forwards (eastwards) over each 41,000 yr cycle. It is not know precisely how long the Norwich Crag period lasted but it may have been 700,000 yrs. Even if it was only half this length of time there would have undoubtedly been periods when rivers, such as perhaps an early River Bytham, had the opportunity to more strongly influence the sedimentation by providing quartzose pebbles picked up from the English Midlands.

In summary, the cliff against Dunwich Heath continues to be an important geological site. It has already helped us understand the nature of the large bodies of gravel in the Norwich Crag and it is now helping us confirm that small quantities of quartzose pebbles can be found in the Norwich Crag at

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83 other locations i.e. some time before the younger more quartzose Wroxham Crag was deposited.

References Hamblin, R. J. O. 2001. Presidential Address – The later Crags and associated fluvial deposits of East Anglia. Mercian Geologist 15: 134–138.

Hamblin, R. J. O and Rose, J. 2012. The Red, Norwich and Wroxham Crags of Northern Suffolk. in Dixon, R. G. ed., A celebration of Suffolk geology.

Geosuffolk. 175–197.

Mottram, H. B. 1989. The Upper Caenozoic sequence around the Long Row, Covehithe. Trans. Suff. Nat. Soc. 25: 86–91.

Mottram, H. B. 1997. The channel gravels of the Dunwich and Minsmere Cliffs. Trans. Suff. Nat. Soc. 33: 77–80.

Mottram, H. B. 2012. The Norwich Crag geology of the country around Westleton. in Dixon, R. G. ed., A celebration of Suffolk geology.

Geosuffolk. 251–258.

H. B. Mottram, The Warren, Duckamere, Bramford, Ipswich IP8 4AH

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Plate 20: Cliff against Dunwich Heath; rip channel in 2009. The post labelled S. Stn. is a Suffolk Coastal DC survey station (p. 81).

Plate 21: Cliff against Dunwich Heath; rip channel in September 2014. Note that the present day beach shingle has replenished the beach and is now virtually covering the survey station (p. 81).

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