The channel gravels of the Dunwich and Minsmere cliff

77

THE CHANNEL GRAVELS OF THE DUNWICH AND MINSMERE CLIFF H. B. MOTTRAM On the 6th of October 1996 the Society's Geological Group walked along part of the beach between Dunwich and Minsmere and observed that the Norwich Crag in the cliff was mainly composed of sand, Two large Channels containing gravel were seen cutting down into the sands; a relatively simple one at Dunwich and a more elaborate one at Minsmere.

These two major Channels are considered to represent tidal inlets, that is to say, the gaps between and slightly seaward of barrier islands. The geographically nearest string of modern day examples is found among the barrier islands which skirt around the coast from northern Holland to western Denmark. These inlets are much wider than either the Dunwich or Minsmere inlets but examples of comparable width occur among the numerous inlets along the eastern seaboard of the U . S A . The Dunwich and Minsmere inlet Channels would have been aligned roughly parallel to the one which runs through Henham Park. Wangford and Southwold; i.e., approximately westnorth-west to east- south-east. Fig. 1 illustrates the likely Situation.

Although the Suffolk Channels have not been afforded S.S.S.I. or R.I.G.S.

Trans. Suffolk Nat. Soc. 33 (1997)

78

Suffolk Natural History, Vol. 33

status they are unique, no gravel occurrences of this nature have been described from anywhere eise in the world.

Dunwich Cliff From the car park at Dunwich the Group followed the cliff southwards. For the first 250m the cliff was seen to be composed of sand, but then a thin layer of gravel appeared near the top and gravel was present for about 450m. The cliff here is often marred by rain-washed spoil. Fortunately, exposures in the autumn of 1996 were good enough to allow Observation of a major Channel up to 12m deep, (see Plate 4). The infill showed concentric layering of the beds which were composed of gravel, of sand and occasionally of mud. It was noted that the southern flank was steeper than the northern flank, a direction of asymmetry which is in agreement with an approximately southward flowing long-shore current. In 1993 it appeared that the Channel had undergone 3 cycles of cut and fill, each time having drifted slightly south of its previous position (Mottram, 1994). Unfortunately, this detail could not be confirmed. It was explained that by measuring the alignment of pebbles it had been determined that flood and ebb tidal flow were equally important (Fig. 2).

Before the Group returned to their cars, it was pointed out that the gravel in the cliff at Dunwich fizzles out near a timber post in the beach (a Suffolk Coastal District Council survey Station). The cliff is then composed of sand for 1.75km until gravel reappears at Minsmere.

Minsmere Cliff The Group drove down to the car park beside the former Coastguard Cottage at Minsmere and walked along the beach until virtually below the O.S. trig point.

Channel infill up to 10m thick was very clearly visible. The impressive northern section extends for 350m and was notable for broad-scale somewhat southward dipping gravel beds, Work on pebble alignments has proved the dominance of flow in the ebb direction, There was also an area of sand beds dipping at low angles to the south-west. Although some gravel beds appeared to interdigitate with the sand beds, there was no gradation, the boundaries were found to be sharp and the bases of gravel beds which overlay sand were often cut irregularly into those sands. Within the major Channel many ancillary, i.e. smaller, Channels were seen. Analysis of pebble orientations from a massively bedded ancillary Channel has also shown the prevalence of ebb flow, but not perhaps to the total exclusion of flood flow, while analysis of pebbles from a more thinly bedded ancillary Channel suggested flood tidal flow was definitely influential in this instance (see Fig. 2).

It is likely that there was some segregation or concentration of ebb and flood tidal currents into ancillary Channels, At times of powerful ebb current flow, any sand in that flowing water would have probably been washed away leaving some gravel to accrete onto the flanks. The resulting dip of the gravel beds combined the effects of the slope of the flank and the direction of ebb tidal flow.

If the sediment had not been subjected to a streng ebb current then sand would have had the opportunity to accumulate into beds. The orientation of foresets in these sands suggests that deposition was from the longshore current which had been forced slightly west of its original course as it tried to cross the inlet Channel.

Trans. Suffolk Nat. Soc. 33 (1997)

79

C H A N N E L GRAVELS AT DUNWICH & MINSMERE

s s c »'

o

WATER FLOWNG OVER

UNSTABLE PEBBLES

CAN GET WBRtCATE

SUBMERGED PEBBLES

MOV ED INTO MORE

ARRANGEMENT

SOMETMES IMPEDED

STABLE POSITIONS (NOTE, PEBBLES NOW TEND TO D1P UPSTREAM)

COMPASS HSTOGRAMS C)F50 ELONGATE

FOR 23 PEBBLES MEAN = 319° (L = 80%)

PEBBLE ORENTATIONS AT EACH LOCÄTION SHOW ANY TRENDS.

I—I—

ALSO SHOWN IS EACH VECTORAL MEAN AND ITS L-VALUE (EXPRESSES HOW

FOR 27 PEBBLES MEAN = 146° (L = 80%)

STRONGLY THE MEAN REPRESENTS THE DUNWICH

OVERALL PATTERN)

s.

J.

MEAN = 70° (L = 9% 5)

MEAN = 320° (L = 24%)

ANC1LL. CHANNEL (MASSIVELY BEDDED) MINSMERE - NORTH FLANK

I—I—I-

ANOLLARY CHANNEL (THWLY BEDOED) MINSMERE - NORTH FLANK

MEAN = 304° (L =61%) —I—I—I

BROADSCALE BED MINSMERE - NORTH FLANK

Fig. 2

MEAN = 76° (L = 30%)

MMSMERE - SOUTH FLANK

PEBBLE DI PS/ORIENTATIONS

The central cliff section is approximatelv 150m wide and showed a continuation of the somewhat southward dip in the "master' bedding planes. 1t was composed of thin beds of sand in broadly concave and in troughed forms. Gravel w a s seldom present. The southern section is approximately 100m wide, It was made up of thin, somewhat northward dipping gravel beds in which the pebbles were smaller in size than those in the beds of the northern section. Mud lumps also occurred.

Düring the early life of the inlet Channel deposition was probably restricted to the northern flank with erosion taking place along the southern flank. In the later stages, as the effect of the ebb tidal current waned, the Channel started to

Trans. Suffolk Nat. Soc. 33

(1997)

80

Suffolk Natural History, Vol. 33

choke up with Sediment. Analysis of the orientation of pebbles from the southern flank suggests the importance of the longshore current, again forced slightly west of its original course.

The last 50m of cliff at Minsmere showed a return to sand. However, adjacent to another timber survey post in the beach, there was a small Channel mostly infilled with sand, although mud and gravel were also present. The pebbles have indicated eastward flow, This type of Channel is like those at Easton Bavents and Covehithe which have been interpreted as the product of rip currents (Mottram, 1989; Mathers & Zalasiewicz, 1996).

References Mathers, S. & Zalasiewicz, J. (1996). A gravel beach-rip Channel system: the Westleton Beds (Pleistocene) of Suffolk, England. Proc. Geol. Assoc., 107 57.

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.

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

Trans. Suffolk Nat. Soc. 33 (1997)

Plate 4: Cliffs at Dunwich showing gravel Channel (p. 78).