The evolution of Chesil Beach and the Fleet has been the subject of much discussion and remains the focus of considerable research today.

Understanding where the beach came from is essential in order to determine what may happen to it and the land behind it in the future.

Chesil Beach, the Fleet and Portland

A number of features provide important clues to the formation of the beach:

• The beach runs from West Bay to Portland and is 29km in length.
• It increases in height towards Portland.
• The overall grading of the beach is from pea sized at West Bay to cobbles at Portland (although there are local variations that may be significant).
• Most of the pebbles are locally derived flint and chert with some exotic material, notably quartzite pebbles from Budleigh Salterton in Devon.
• The core of the beach consists of a sand and pebble matrix of low permeability.
• The slope behind the beach is ancient and shows some signs of a raised shore line comparable to the Portland Bill raised beaches.
• If the beach were not there, the sea would be further inland and high cliffs would cut through the underlying geology.
• The Fleet lagoon contains sediments, including peats, that are at least 6 to 7,000 years old.
• The peat is being eroded on the seaward side of the beach and thrown onto it as large blocks while storm events push pebbles onshore, indicating that the beach is moving onshore.

The formation of Chesil Beach

Long shore drift

It has long been recognised that waves hitting a beach at an angle drive the beach materials in the direction that the waves are moving.

The beach is exposed to the full force of the Atlantic and the long shore drift is predominantly to the east. This explains the occurrence of pebbles from Budleigh Salterton and more importantly, demonstrates that at least a proportion of the material that forms the beach came from the west.

An early theory to the pebble gradation was that the strong south westerly gales carried all of the material to the east and that less strong waves from other directions carried the smaller pebbles back to the west. More recently it has been suggested that the larger pebbles move more rapidly due to their larger surface area. The local variations in pebble size both along and across the beach are thought to be significant but no satisfactory theory can yet explain it.

Ice Age history

The raised beaches on Portland Bill are 210,000 and 125,000 years old and clearly indicate a period when sea levels were up to 15 metres higher than today. The benches behind the Fleet lagoon are likely to be from the same periods of higher sea level. Sea levels then dropped to minus 45 metres below the present sea level 10,000 years ago.

Since the end of the Ice Age, sea level has risen to its present position.

Theories for beach formation

There is no full agreement on the formation of the beach or any theory that can account for all of the observable phenomena. The traditional view is that Chesil Beach was driven onshore with rising sea levels at the end of the last Ice Age. As the sea advanced, material from what is now Lyme Bay was swept up to form the beach, trapping the Fleet lagoon in the process.

More recently it has been proposed that a large volume of the beach was derived from the landslides of East Devon and West Dorset. During the last Ice Age, these landslides would have been very active indeed and as sea levels rose, erosion cut into the debris, sending a huge pulse of material, by long shore drift, to the east. This might explain the older, finer beach that underlies what we see today. This beach moved onshore, trapping the Fleet lagoon and reaching its present position some 4 to 5,000 years ago. It was then buried by the pulse of shingle that forms the beach that we see today.

Some of the questions still to be answered include:

• How did the finer beach survive to protect the Fleet with its smaller volume and lower height?
• Why has the beach apparently migrated so slowly in the last 4 to 5,000 years?
• Were there other geographical features along this coast that may have had an effect on long shore drift when sea levels were lower? (There are a number of submarine rocky reefs exposed on the floor of Lyme Bay that must have played some part on the control of sediment movement).