Seaton Bay on Devon's Jurassic Coast
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Geology of the Jurassic Coast

The 95 mile long Jurassic Coast line displays a near continuous sequence of Triassic, Jurassic and Cretaceous rock features representing almost the entire Mesozoic era, together with outstanding geographical features such as landslides, a barrier beach and lagoon, cliffs and raised fossil beaches.

From Seaton it is possible to see, walk to, or easily visit all three eras of rocks in this 185 million-year ‘geological walk through time’.

The Chalk cliffs of Beer date from the Cretaceous period with the rock strata changing at Seaton to the red cliffs of the Triassic age.

This Triassic rock stratum stretches from Axmouth to Pinhay Bay, west of Lyme Regis when the exposed rock strata changes again the grey clays of the Jurassic period.

The Triassic Period

The Triassic Period is dated from 200 to 245 million years ago. At that time there were no continents as we know them today, only a vast super continent straddling the equator known as Pangaea.

Britain was located in the centre of Pangaea, the climate was dry and arid with the landscape consisting of extensive desert basins similar to those that can be seen today in Death Valley. Red-coloured sandstone and mud were the main features of the landscape.

Sediments consisting of desert sands and shallow lake mud stones accumulated in the large shallow basins. Huge rivers crossed the desert plains which were punctuated by mountainous areas formed by the older rocks of Dartmoor, the Mendips and the Malverns.

These vast rivers washed stones across Devon before their waters evaporated and these Triassic river deposits can still be seen today between Exmouth and Sidmouth.

Forests of conifers and cycads, a tree resembling a palm tree were the major plant life replacing earlier plant forms, such as ferns, and the reptiles started to attain supremacy, with the first dinosaurs evolving in the late Triassic.

By the end of the Triassic period and the beginning of the Jurassic Period the continental plates had started to drift apart the sea-level started to rise, and a warm, shallow sea flooded over what is now Dorset and East Devon.

The Jurassic Period

The extent of the oceans was far more widespread in the Jurassic then they had been in the Triassic period and warm shallow seas spread across Europe. Deeper waters occurred during times of fluctuating sea levels.

These warm shallow seas were home to a rich diversity of life, Ichthyosaurs, Pliosaurs, Ammonites, and Belemnites. On the land enormous dinosaurs such as the Brachiosaurus and the Diplodocus evolved.

The climate was warm and moister than during the Triassic and the rock stratas of the Jurassic period are characterised by deep-water clays, sandstone and shallow water limestone.

The Cretaceous Period

The start of the Cretaceous period sees much of the land masses covered by shallow continental oceans and inland seas. On what is now the south coast of England, the Jurassic Coast become first a gulf of salt lagoons covered with salt flats then lush swamps, before becoming a warm sea.

The period is characterised by pure white chalk, formed by the skeletons of the warm water algae and the shells of micro organisms.

The majority of the fossil debris comprising this chalk consists of the microscopic plates, which are called coccoliths, of microscopic green algae known as coccolithophores. The coccolithophores lived close to the surface. When they died, the microscopic calcium carbonate plates, which formed their shells settled downward through the ocean water and accumulated on the ocean bottom to form thick layer sediment on the sea bed which eventually became the Chalk Formation.

This period sees the birth of flowering plants and the reign of the most glamorous of the dinosaurs, T-Rex, Velociraptor and Triceratops. It also eventually saw their extinction.