Fossil Coast Explores Chesil Beach Along the Jurassic Coast
- Wayne Munday
- Jul 27, 2022
- 3 min read
Updated: 3 days ago
Chesil Beach is one of Britain’s most distinctive coastal landforms, stretching 29 kilometres along Dorset’s Jurassic Coast. Formed after the last ice age, this naturally graded shingle tombolo links the Isle of Portland with the mainland near West Bay. Separated by the Fleet Lagoon, the UK’s largest tidal lagoon, the site is a protected Site of Special Scientific Interest. Chesil Beach demonstrates Holocene sea level change, longshore drift, pebble grading, and flint and chert geology. These features make it a destination for geology-focused coastal tourism. Its shingle ridge, lagoonal setting, and glacial history provide accessible insights into coastal processes, sediment transport, and environmental change for visitors exploring Dorset’s evolving shoreline and natural heritage.
Between Abbotsbury and the Isle of Portland Chesil Beach is separated from the mainland by Fleet Lagoon, the largest tidal lagoon in the UK stretching 19 km and one of the most important Marine Protected Areas in the country. The Fleet is home to many wading birds and Abbotsbury Swannery, and fossils can be found in the sand. Both Chesil Beach and the Fleet Lagoon are a Site of Special Scientific Interest.
Chesil Beach was formed during the Holocene Epoch. This is the name to the most recent interval of the Earth’s history dating back 11,700 years ago until present day. The sediments of the Holocene, both continental and marine, cover the largest area of the globe of any epoch in the geologic record.
The pebbles of Chesil Beach are mainly flint and chert from the Cretaceous and Jurassic rocks. Chert is a biological sedimentary rock composed of the mineral form of silicon dioxide (SiO2) that can form when microcrystals of silicon dioxide grow within soft sediments that will become limestone or chalk. The silicon dioxide in chert is biological origin sourced from diatoms and radiolarians that live in the shallow sea. As they died their silica skeletons fall to the sea bed bottom, dissolve, recrystallize, and over time form chert nodules.
During the last glacial period known as the Devensian the sea level was up to 120 meters lower than at present and a series of sand and gravel deposits accumulated on what is now the sea floor and potentially a large submerged bar on Lyme Bay.
It is believed that the initial formation of Chesil Beach began at the end of the Devensian between 20,000-14,000 years ago when a rapidly rising sea-level caused erosion of these seabed deposits and wave action drove the sands and gravels onshore forming a barrier beach.
An interesting feature of Chesil Beach is that the transportation of the shingle has had a lateral sorting or grading effect on the beach. Chesil Beach pebbles are graded from small to large in a south-eastward direction caused by the increasing impact from the wave energy derived from longshore drifting. Longshore or littoral drift is the movement of material along the shore by the forces of wave action.
When waves approach a beach at an angle the “swash” or waves moving along the beach carries material up and along the beach. The “backwash” of waves moving back down the beach carries this material by gravity back down the beach at right angles. This process slowly moves material along the beach and provides a link between erosion and deposition.
Then between 4,000-5,000 years relict cliffs of East Devon and West Dorset were eroded by the sea and supplied large quantities of gravel to the shore and drifted to the east. Over the last 500 years a combination of coastal recession and human intervention have now depleted the beaches to the west of West Bay, resulting in increased prominence of the headlands. Chesil Beach is now regarded as a closed shingle system with a finite volume and sensitive to future environmental changes.
