Features caused by Deposition
Notes from Waugh plus other textbooks should have been made on:
Chesil Beach - perhaps the most famous example, and also used as a tombolo on occasions...
Dungeness: the word cuspate refers to its resemblance to a tooth - sediment ridges...
Where 2 sediment cells converge, sediment accumulates from 2 directions. At some areas this has created a tombolo e.g. Llandudno - the Great Orme...
Sediment Cell questions:
1. State the percentage of the sediment input at Cromer that moves a) West, b) East
Suggest where the rest of the input of sediment has gone
Why might the amount of longshore drift vary from year to year.
HOMEWORK
1. How is the sand which makes up many beaches actually formed ?
2. Describe Blakeney Point. What kind of feature is it ?
3. As far as coastal erosion management goes, is the 'do nothing' approach a realistic option ? What are the arguments for and against this policy ?
CHESIL BEACH
The Chesil Bank or Chesil Beach (Old English ceosol, cisel=shingle) stretches 29 km (18 miles) from Bridport Harbour (West Bay) to Chesil (or Chesilton or Chiswell) Bay in the Isle of Portland. It is a very large simple linear storm beach connecting the so-called Isle of Portland to the mainland. It is an example of a tombolo and faces the storm waves driven by the prevailing south-westerly winds up the English Channel from the Atlantic Ocean. The beach is linked to Portland at Chiswell and then north-westward it is backed for a stretch by Portland harbour and afterwards separated from the land by the 13 km of the Fleet lagoon.
Opposite the Fleet the Chesil Beach is between 150 and 200 m wide, but it is narrower both adjacent to the cliffs in the west and at its extreme eastern end. Its width is 155 m at Abbotsbury, 182 m at Portland. There is a generally increasing ridge height from northwest to southeast. Its height is 6.9 m above high water mark at Abbotsbury and with a maximum about 14m above mean sea level at Portland, where it forms a magnificent example of a storm beach (Davies, 1956). Measurements were made at an early date by Sir John Coode (1853) and more recently by Carr (1981) . On the seaward side offshore the beach drops at a broadly similar gradient to that of the seaward face above low water mark. The shingle extends to a depth below low-water mark of 11 m at West Bexington (some 270 m offshore) and also at Abbotsbury, to about 18 m at Wyke Regis (also at some 270 m offshore) and to 15 m at Portland. On the landward side, however, the shingle rests on a bed of clay 1 to 1.2 m below low water-mark.
The pebbles are well-graded, coarsest near Chesil (Chesilton) and diminishing in size towards Bridport, and it is said that fisherman landing on the bank at night can judge their position by the size of the pebbles. (I have been told that the local helicopter pilots can also do this in fog - but this sounds like a myth!)
From Bridport Harbour to Cliff End, the shingle is piled up against the cliff. A hollow appears on its landward side at Burton Mere and near Abbotsbury Coastguard Station, and for the 13 km from Abbotsbury to Small Mouth it is separated from the mainland by the shallow lagoon of the Fleet. The western end of the Fleet is brackish and almost tideless, the consolidated shingle being practically water-tight up to ordinary high water mark. In the East Fleet, however, wide stretches of mud appear as the tide ebbs through Small Mouth.
The north-eastern shore of the Fleet has clearly never met the full force of the waves, which would cut such soft materials back in a continuous curve. Significantly, it has no cliffs on the landward side. It shows the form of a river valley like that of the Wey, and represents the left bank of a drowned valley. The Fleet is widest, up to 0.8 km or more, on the Oxford Clay and Fullers Earth, narrowing to about 180 m where the shore is of Forest Marble or Cornbrash.
The pebbles of the Chesil Beach are mainly flint and chert, derived from Cretaceous rocks, perhaps by way of Tertiary gravels such as those at Blackdown and Bincombe. Limestone and chert from the Portland Group are also common, especially at the eastern end, and another striking constituent is the discoidal pebbles of quartzite, red, purple ("liver-coloured" or white) such as occur in the Triassic pebble beds at Budleigh Salterton in Devon. Much rarer are pebbles of porphyry of a type that can be matched in the Permian breccia of Dawlish. Various tourmalinised rocks, red and black chert, vein quartz, etc., have been identified with the Palaeozoic rocks of Cornwall; many of them may have come like the flints, out of Tertiary gravels (Davies, 1956).
The first and simplest explanation of this assemblage of pebbles was that they had been swept by the waves along the shore from west to east, the usual direction on the south coast, and the larger pebbles had travelled faster than the smaller ones. Then, it was suggested that the Bunter pebbles, for example, had been swept across Lyme Bay (West Bay), either from the Budleigh cliffs or from a submerged outcrop; that they had been diverted at Portland and carried by the waves north-westward along the shore, suffering attrition on the way. It was Prestwich in 1875 who first pointed out that the Raised Beach at Portland Bill contained these pebbles, which had therefore come into the district in Pleistocene times when conditions were very different from those of the present day. Baden-Powell stated (1930) that the Raised Beach marks the earliest known date at which pebbles from the Dawlish breccia, from the Budleigh Salterton pebble beds, and perhaps from the Tertiary gravels were assembled in one deposit in the Portland district. The Raised Beach is only the remnant of a much larger deposit, formed when the Portland and Purbeck rocks stretched far to the west and east of what is now Portland Bill. North of the extended Portlandian escarpment the Fleet river deposited gravels, remains of which are seen at Fleet Common, at Langton Herring, and near Wyke Regis. It is from the sweeping together of the materials of these ancient shingles and gravels that the Chesil Bank was formed.
At present the beach is practically stationary. The pebbles drift sometimes eastward, sometimes westward, as the storm winds blow from W.S.W. or S.S.W. There seems to be a general movement towards a point near Chesilton, where the bulk of the beach is greatest and the pebbles are largest. These larger pebbles can only be moved by large waves, while the smaller stones my travel before smaller waves; this may have a sorting effect. There is a tendency for the beach to be rolled over on itself, and so to retreat toward the northeast, and this movement is most marked at the Portland end. Near Chesil (Chesilton) the beach appears to have overwhelmed blown sand (Davies, 1956)
The beach pebbles are almost entirely composed of very resistant minerals, mostly quartz and chalcedony (with a hardness of 6 on Moh's Scale, i.e. harder than steel). The bulk of the beach pebbles, 98%, consists of flint and chert. This comprises grey to brown flint from the Cretaceous Chalk and light bluish grey chert from the Upper Greensand with, at the Portland end, some black chert from the Portland Cherty Series, the Portland Stone and the basal Purbeck Formation. Chert from the Portland Roach can be recognised easily because of the large molluscs and particularly the gastropod Aptyxiella portlandica .