A super-continent divides

Rocks record the history of our world and geologists can read this record like a story. The story of the Dorset and East Devon Coast begins 250 million years ago at the start of the Mesozoic Era. This was the ‘middle ages’ of life on Earth and consists of the Triassic, Jurassic and Cretaceous periods of geological time.

The Triassic World – 250 to 200 million years ago

In Triassic times theUnited Kingdom was part of a super-continent called Pangaea. Dorset and East Devon lay within the arid centre of this giant landmass and hot, desert conditions prevailed. A huge mountain chain lay to the south and west which was gradually eroded. Our Triassic rocks contain sand and pebbles carried by rivers flowing from the mountains out into the desert. These deposits right across what is now southern England. Thick layers of gypsum crystals reveal that huge lakes formed and evaporated in the intense heat leaving behind minerals that were dissolved in the water.

Triassic Life

The Triassic is a critical period of evolution. Life on Earth had been devastated by a large mass extinction event at the end of the previous period. During the Triassic the surviving groups of animals evolved and diversified. The first dinosaurs evolved and went on to dominate life during the Mesozoic Era. Most of our living groups of four legged animals had arrived by the end of the Triassic, including frogs, turtles and crocodiles. The first true mammals also evolved during this time.

The Jurassic World – 200 to 145 million years ago

The super-continent Pangaea started to break apart during the Jurassic Period. The Earth was relatively warm and sea levels were much higher than today. The Triassic deserts had been flooded in this part of the world but rocks continued to form at the bottom of the sea. The Jurassic rocks of Dorset and East Devon show us that sea levels rose and fell in a series of cycles. Deep water clays are followed by sandstones and finally shallow water limestones. When seas were shallower the environment would have resembled the Caribbean of today! Towards the end of the Jurassic sea levels dropped enough to create the conditions for a forest to flourish in a tropical swamp.

Jurassic Life

The expansion of shallow seas encouraged an explosion of life in the Jurassic, and many animals evolved rapidly in order to take advantage of the new habitats. Ammonites were common but reptiles were the 'top predators' on land, sea and in the air. While dinosaurs conquered the land and pterosaurs controlled the skies it was the ichthyosaurs and plesiosaurs that dominated the world's oceans, giving rise to the most terrifying creatures that ever lived - pliosaurs.

The Cretaceous world – 145 to 65 million years ago

During the Cretaceous Period the continents continued to spread apart. Repeating thin layers of limestone and clay form the Early Cretaceous show that conditions fluctuated between salt flats and lush swamps and lagoons. Dinosaurs roamed the salt flats while crocodiles hunted fish in the swamps. Through the middle of the period thick layers of sand and grit were laid down by rivers, which also carried the occasional dinosaur carcass that was laid down too and fossilised! Finally, towards the end of the period, a vast sea developed over the area. Within the clear, warm waters billions of microscopic algae bloomed, and their skeletons sank to the sea floor to form the pure, white Chalk.

Cretaceous Life

During the Cretaceous Period the continents continued to spread apart. Repeating thin layers of limestone and clay form the Early Cretaceous show that conditions fluctuated between salt flats and lush swamps and lagoons. Dinosaurs roamed the salt flats while crocodiles hunted fish in the swamps. Through the middle of the period thick layers of sand and grit were laid down by rivers, which also carried the occasional dinosaur carcass that was laid down too and fossilised! Finally, towards the end of the period, a vast sea developed over the area. Within the clear, warm waters billions of microscopic algae bloomed, and their skeletons sank to the sea floor to form the pure, white Chalk..

The rocks of the World Heritage Site are laid out from oldest to youngest moving west to east. This is a little unusual because these layers of rock would have originally formed one top of the other, and if they had stayed like that we would have to drill deep into the Earth to see them. Something truly epic must have happened to allow us to see these layers in continuous sequence along the coast.

Read more: The formation of a geological spectacle

The Jurassic Coast: A Mighty Tale

A light-hearted python-esque animated film by Tim Britton.