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.
A Geological ‘Walk Through Time’.
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.
Jurassic Coast Fossils
Fossils have been found along this coast for over 200 years and continue to be uncovered to this day.
Fossils are found easiest in the Blue Lias of the Jurassic Period, most of Britain was covered by sea during the Jurassic, so dinosaur fossils are rare. However, the Jurassic seas were full of life, including ammonites and marine reptiles like Ichthyosaurs and Plesiosaurs.
The level of preservation is excellent and many of the finds are important to science and, in some cases, are actually new to science. Dinosaur footprints, ichthyosaur (a dolphin like type of dinosaur) remains, fossilised wood, ammonites and beautifully preserved dragon flies are just a few of the fossils which are frequently uncovered.
Ammonites, with their easily preserved coiled shells, were related to such creatures as squid, octopus and nautilus. Also to be found are Belemnites, with their interior shell, Belemnites looked similar to modern squids and were members of the same group as the ammonites. Crinoids, or sea lilies, are relatives of starfishes and sea urchins. Other fossils frequently found are urchins, bivalves, and gastropods (shells).
The best beaches to find fossils are at Lyme Regis and Charmouth in Dorset. Fossils are exposed in the rocks which have fallen into the sea where the seas action washes and breaks the rocks revealing the fossils. Each low tide after a period of rough seas exposes a new crop of fossil bearing with the stormy winter months being particularly fruitful.
The chalk cliffs of Beer are the most westerly chalk cliffs in England and were formed 90 million years ago towards the end Cretaceous period. Since then movement of the earth’s crust has compressed the chalk and raised it above sea level to form the chalk cliffs we see today.
At low tide it is possible to walk close to the bottom of the cliffs of Beer Cove and examine the chalk formation. Underneath the chalk is a bed of stone darker in colour to the chalk above. This is a band of Greenstone, a sandstone, pale yellow or brown in colour, and is only ‘green’ when it is freshly exposed to the air.
Between the layer of Upper Greensand and the Chalk is a narrow layer of Beer Head Limestone.
This limestone was deposited about 120 million years ago and by finding this narrow seam of creamy-white chalk limestone, Roman Stonemasons would have known that a valuable source of building stone could be quarried at Beer.
By walking along the beach at the foot of the cliffs on the west side of the cove it is possible to see that the rock formations slope in a different direction to that of the cliffs on the eastern side of the cove.
This layering or bedding was originally level on both sides of the cove, but a movement in the earth’s surface has caused a ‘U’ shaped fold known as a syncline.
On the eastern side of the cove the layers slope gently westwards whilst on the western side of the cove the layers slope gently eastwards.
From Beer walking eastwards towards Seaton along the Coastal Path you walk over the 85m high chalk cliffs to Seaton Hole.
It is at Seaton Hole that the geology changes dramatically with the white chalk and Upper Greensand to the west and the lower red Mercia Mud stone cliffs to the east.
The Mercia Mud stones cliffs are 225 million years old and were formed by mud being deposited on the beds of shallow lakes in the Triassic period. This mud has been hardened into the ‘mudstone’ we see today.
The Cretaceous sea flooded over the area 100 million years ago, firstly depositing sand which hardened into the ‘Upper Greensand’ followed by lime mud’s which became the strata of Beer Limestone and then over millions of years the algal coccoliths.
Both of these types of rock strata can be seen at the Seaton Hole ‘fault’ where the continuity of the rock layers have been ‘broken’ with the younger white Cretaceous rocks dropping down 60 metres to the level of the much older red Mercia mudstone rocks of the Triassic period.
Eastwards from Seaton Hole the red Mercia Mud stone cliffs disappear beneath the flood plain of the River Axe emerging once more in the lower layers of Haven Cliffs on the east bank of the estuary.
Here the older Mercia Mud stone of the Triassic period is capped with younger layers of Fox mould or Gault, Upper Greensand and chalk from the Cretaceous period.
In the past 65 million years the Jurassic Coastline has been uplifted and eroded to reveal the rock formations we see today.