Geology of the Jurassic Coast, Devon & Dorset

Geology of the Jurassic Coast

Dorset and East Devon Coast World Heritage Site has a wealth of exposed rock strata giving a nearly continuous geological record of the earth's geological history between 251 and 66 million years ago, the Triassic, Jurassic and Cretaceous periods. The coast is part of the Wessex Basin, one of the best known Mesozoic-Tertary sedimentary basins in Europe.

In the main, the stratae dip gently to the east. Therefore the oldest rocks are found in the west of this coastal strip, with younger strata outcropping to the east. As a result, the succession of aged rock stratae are to be found in sequential order along the cliffs, and the continuous processes of coastal erosion means that new material is continuously being exposed.

The extensive coastal exposures provided by the Dorset and East Devon coast have been put into detailed geological maps, and today this is one of the best understood sedimentary basins in the world.

Triassic. The Triassic Period (c. 250-203 million years ago) can be seen in a virtually continuous exposure of 1,100m of sediments in continental, terrestrial red-bed and, near the top of the sequence, shallow marine facies. These exposures record the gradual disintegration of mountains formed in the Variscan orogeny of 330-280 million years ago, and the widespread marine environment that formed with this Jurassic basin during the opening of the Atlantic Ocean.

Jurassic. The Jurassic Period (c. 203-135 million years ago) rocks in this coastal area provide one of the finest marine sequences of The Jurassic Period of anywhere in the World. All stages of the Jurassic are represented For example of the 74 ammonite zones within the Jurassic, only three are definitely absent. It provides excellent evidence of the history of the Earth by recording six major cycles of sea level change, seen by repeated rhythms passing from clay to sandstone and then limestone.

Cretaceous. The boundary between the Jurassic and Cretaceous has still to be internationally defined. However in Dorset, is expected to lie within the lowest beds of the Purbeck Formation. This coastal stretch includes rocks of all stages of the Cretaceous Period, with the exception of the uppermost stage. The Purbeck Formation in Dorset is one of the finest late Jurassic- early Cretaceous terrestrial sequences in the world. The overlying Wealden Group is the most complete sequence of this age available at a single site in north-west Europe.

Dorset localities have provided the names for internationally recognised stages for the Mesozoic. The Kimmeridge Clay unit gave its name to the Kimmeridgian Stage (D'Orbigny 1846-1849). Portland gives its name to the Portlandian Stage (named by Brongniart 1829). The Purbeckian, named after Purbeck, was, until recently, in international use for the lowermost stage of the Cretaceous.

Devon and Dorset Jurassic Coast