An impressive and scenic example of coast geology.
Chesil Beach is a shingle barrier ridge at the eastern end of Lyme Bay, extending 18 miles from Burton Bradstock to Fortuneswell on Portland. As far as Abbotsbury it is attached to the land, but beyond that the lagoon of The Fleet separates it from land, until it runs head-on into Portland. One can walk on the beach at Burton Bradstock, where there is a visitor centre, and get fine views from the coast road, the B3157, on the heights of White Hill just west of Abbotsbury, where there is a lay-by. This view is shown above. From the viewing area on the top at Portland, one looks to the north west along the whole length of the beach, and this view is shown below.
The beach consists of coarse gravel, called shingle, that is 98.5% flint and chert pebbles. The rest are quartzite, as from Budleigh Salterton, quartz, granite, porphyry, metamorphics and limestone. All are very smooth and well-rounded, and are of remarkably similar sizes. The sizes are graded, from pea-size on the west, to hen's egg-size near Portland. The berm is a maximum of 13 metres high. This beach of graded pebbles sits on an old beach of sand, silt and pebbles that is below low water level.
The origin of the beach has long been a subject of argument and uncertainly. Sir John Coode's paper of 1853 proposed that it had been built from gravel driven alongshore by waves from the western part of Lyme Bay. Sir Joseph Prestwich maintained in 1875 that it grew to the north from Portland. The grading of the pebbles suggested some sort of motion along the shore, but neither of these early theories satisfactorily explained what was observed. The winds and waves along the beach strike it at almost right angles, and the gravel could only be moved by storm waves breaking on the beach, which would also pile the pebbles into the observed berm. There was no evidence for movement along the shore.
The sea has risen steadily in the Holocene epoch, since the end of the last ice age. This rise is called Flandrian, and gave the British seacoast its intricate drowned topography. It began about 10,000 years ago with a rapid rise that then began to ease off about 8,000 years ago, gradually slowing to the modern slow rise. During this period, ancient beaches and gravels were driven before the waves, washing out the sand and silt, and rounding the pebbles. Chesil Beach is the current accumulation of this material, too heavy to be moved other than before the waves, and representing deposits from land eroded from what is now in the Channel up to 35 or 40 metres below sea level. This is a good reason for the gravel in the beach not being typical of the land near it, and for its peculiar grading. There is still not complete agreement, but this explanation seems to be the best available.
Chesil Beach has long been hazardous for sailing ships and swimmers. Even on a fine day, there is a strong sea breeze that would drive a sailing ship venturing too close onto the shore. In storms, with a southwestern gale, wreck would be certain. It is said that inhabitants of the coast sometimes showed lights in bad weather, tempting ships to their doom in hopes of salvage. The waves breaking on the beach would create a strong undertow that could drag even strong swimmers to destruction. On the other hand, The Fleet now gives protection to numerous pleasure craft.
The movement of shingle in the sea is a subject of some interest. Shingle can be moved by breaking waves, in which the water is moving rapidly forward, but this occurs only at quite shallow depths. Below 18 metres, apparently, shingle is not moved by wave action. A hole dredged in the sea bed below this depth does not fill again, except perhaps over a great length of time. Tidal currents, which can reach 4 to 5 knots, can move shingle and scour channels in the beds of estuaries. Most people mainly think of tides as a vertical motion, but strong, invisible horizontal motions are an essential part of the tides. Tidal currents have, apparently, played no role in forming Chesil Beach.
Longshore drift is also found on coasts. A wave striking the coast at an angle (waves are refracted to strike coasts more perpendicularly) drives sediment up the beach at an angle, but the material then slides directly down the slope. This sawtooth action moves sediment along the beach. The breaking waves also erode the coast, producing a sea cliff if conditions are right, above a shelving beach. Groynes built out from the shore in an attempt to arrest this motion create sand deposits on one side, but increase erosion on the other. These unlovely but popular seacoast features are largely futile. Chesil Beach seems to have been influenced very little by coastal drift, and there are no unsightly groynes defacing it.
Lyme Bay has a number of scenic and geological features that make a visit rewarding. It extends from the limestone reef of Portland to Start Point in Devon, with cliffs of changing character, from the chalk at the eastern end, lias, quartzite, red Triassic and Permian conglomerates and mudstones, to metamorphic rocks. The western end includes Babbacombe Bay, Tor Bay and Start Bay. Slapton Beach in Start Bay is a small Chesil Beach, also made of shingle. Lyme Regis features the ammonite, whose fossils weather out of the cliffs together with icthyosaurs. Lyme is one of the most attractive places along the coast, isolated and hilly. Its breakwater, the Cobb, is made of Portland stone, and there are fine views of the cliffs. Call in at Rose Cottage Fudge on the High Street for real fudge and excellent baguette sandwiches, and there is a good used book shop opposite. Weymouth and Exeter are the rail gateways to Lyme Bay at either end. Lyme Regis is accessible by frequent buses from Axminster, and there is a long-distance bus route from Exeter to Weymouth along the coast. Chesil Beach itself is accessible at its far eastern end on a bus route between Weymouth and Portland Bill. Cliffs and beaches can be found at Sidmouth, Dawlish and Teignmouth, all easily accessible by rail or bus.
A sign at Burton Bradstock warns against the crime of taking a few pebbles as souvenirs from the beach. If we assume that the height of the beach is 10 m, its width at the top 10 m and at the base 30 m, its length 30 km, and the average pebble to have a volume of 2 cc, there are about 3-5 x 1012 pebbles in the beach. If everyone in Britain came and took away a handful of pebbles, the beach would be depleted by 1 part in 10,000. So children taking souvenirs would not seem to be a very great danger to the beach. Such stupid officiousness is more than typical in such matters, harming efforts when protection is really necessary, and alienating those who could be supporters. We are very familiar with it in the case of fossils. Fossils weather out continuously, and if not found soon weather away. Those usually found are very common and of very little palaeontological or commercial value, like circulating coins to a collector. Without amateurs, most fossils would be lost, and rare specimens would never be found.
Chesil Beach is mentioned in guides to British coastal scenery, as well as in most textbooks in Physical Geology. It is a famous feature, and much has been written on it. However, it was not easy to find material to satisfy my curiosity in a casual recourse to the library.
J. A. Steers, Coastal Features of England and Wales (Cambridge: Oleander Press, 1981), pp. 20-22. Gives references to geological studies.
Composed by J. B. Calvert
Created 23 August 2000