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May 24th, 2006 admin Leave a comment Go to comments

The History and Geomorphology of the East Devon Coast

World Heritage Sites are locations of "outstanding universal value"' chosen by UNESCO (the United Nations Educational, Scientific and Cultural Organisation). The Dorset and East Devon Coast forms one of England's World Heritage Sites. Called The Jurassic Coast, this area comprises 95 miles of truly beautiful coastline from East Devon to Dorset. The time span of the rocks along this coast covers a period of 185 million years of the Earth's history.

World Heritage status was granted because the coast offers a unique insight into a geological "time line" spanning the Triassic, Jurassic and Cretaceous periods of the Earth's history. Very distinct and different sections of coast formed over millions of years through geological events and later by coastal processes unfold before your eyes as you walk through this beautiful area.

Orcombe Point marks the west edge of the World Heritage Site, and you can start your journey by seeing the Geoneedle, unveiled by the Prince of Wales in 2002 to commemorate granting of World Heritage Status to the Devon and East Dorset coast. The Geoneedle is built from stones taken from the coast in the sequence in which the rocks were deposited along the coast during its development.

The rocks of this area show us the period known as the Mesozoic era - which is broken down into the Triassic, Jurassic and Cretaceous Periods of the Earth's history. These represent the period from 250 million years ago to 65 million years ago. All along the coast, this amazing geology is clearly exposed and easily accessible.

In Triassic times, which were between 250 and 200 million years ago, the World Heritage Site was an element of the super-continent called Pangaea, a landmass which later divided into the continents of our current world. Dorset and East Devon was somewhere in the desert-like, dry centre of this unimaginable super-continent. The Triassic was a vital time in the evolution of life on Earth. Those animals which survived a mass extinction at the end of the previous period evolved and diversified; for example, the dinosaurs evolved around this time and later became dominant during the Mesozoic Era. By the end of the Triassic, most of the groups of four legged animals which we know today had evolved, including the first true mammals.

Pangaea started to split up during the Jurassic Period between 200 and 140 million years ago. The Atlantic Ocean formed to the west of Britain and the Americas moved away from Europe. The Earth was warm and sea levels were high, with almost no polar ice caps. The Jurassic rocks of East Devon and Dorset record these marine conditions - although the depth of the oceans varied from relatively deep seas to coastal swamps. The geology of this area indicates that sea levels rose and fell in cycles, with the deposition of deep water clays, then sandstones and last of all shallow water limestones. The oceans were relatively shallow in the middle of the Jurassic, which created a series of islands raised slightly above the shallow shoals, rather like the Caribbean of today. The seas deepened as the Jurassic period progressed, before finally becoming more shallow at the end of this period, which created a tropical swamp environment. This may be hard to imagine in the weather of our current climate!

Jurassic animals included Ammonites, a type of mollusc related to the squid, but with hard spiral shells. These are one of the most common fossils you can find on the Dorset and East Devon Coast; and in fact, Portland and its limestone and chalk is where the giant ammonite is found. As the shallow seas expanded, there was an explosion of life during which many animals evolved rapidly. Dinosaurs were abundant on Earth and the dominant animals in the oceans included ichthyosaurs, plesiosaurs and crocodiles.

During the Cretaceous Period, which extended from 140 to 65 million years ago, America continued to drift away from Europe, and the Atlantic became more like it is today in form. The landscape on our current World Heritage Site was rather like the Gulf of Arabia today, with lagoons and salt flats. As the rocks under what is now south-west England tilted to the East, the warm waters of the Atlantic expanded, and sea conditions became more hospitable, allowing billions of microscopic algae to bloom in the clear waters. As their exo-skeletons sank to the sea floor, they gradually formed the pure, white chalk we see in the area today.

Right across the World Heritage Site you can see the "Great Unconformity", a time gap between rocks of different ages. In the mid-Cretaceous the rocks tilted eastwards, and were then gradually eroded by seas and rivers, especially in the west of the area. And so, all the Jurassic and Lower Cretaceous rock history is absent from the geological timeline in this "fault", and the Cretaceous rocks are deposited on the eroded rock surfaces of the Triassic period. As you walk along the coast, this makes interpretation of the time line more difficult, because the oldest and the youngest rocks on the coast are found near each other in East Devon.

The Cretaceous saw the largest and most fearsome dinosaurs on the Earth, but it was also the period when the first flowering plants evolved. A mass extinction took place at the end of the Cretaceous period which was critical to the form and animal population of the modern world (although this is not explicitly recorded in the World Heritage Site). Certainly it was around this time that the reign of the reptiles - including dinosaurs - as the predominant life on Earth came to an end; dinosaurs, marine reptiles and ammonites were some of the species which became extinct. After their time, the present style of life on Earth evolved, dominated by mammals, flowering plants and grasses. The earliest rocks from the Cretaceous period of the World Heritage coast are the Purbeck Beds, which coincidentally represent one of the most challenging rock sequences along this part of the Devon coast. They have given us many fossils including dinosaur footprints and the microscopic animal teeth. Chalk is the youngest Cretaceous rock in the Devon and Dorset World Heritage Site - it is found all through the area, containing fossils of animals such as the sea urchin. All in all, the varied geology of this beautiful coast has formed an amazing laboratory for geomorphology - the study of the land and the geological processes that created it. Coastal land is never stable; it changes as the sea and frost mould it, as rain and human activity subtly alters it. But geomorphology is looking at longer time periods than that which represents the hand of man, even though small changes, repeated often enough over long periods of time, can be powerful agents for change as well. As we all know, storms and landslips have both formed the shape of the coast and revealed millions of fossils, which are abundant and easy to find in this astonishing natural laboratory of geomorphology!

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Atlantic Coast