There are three main reasons why Lyme Regis is so good for fossils
Firstly, the area supported a rich diversity of life 200 million years ago and these creatures have been well preserved as fossils.
Secondly, the rocks containing these fossils were lifted back up to the surface about 15 million years ago.
Thirdly, massive natural erosion is constantly exposing a new crop of fossils.
Lyme Regis 200 Million Years Ago
During the Early Jurassic, 200 million years ago, what is now Britain was positioned at a latitude of about 30 degrees North. The climate was warm subtropical much like Bermuda today.
Most of Britain was covered by sea at the northern edge of the Tethys Ocean. Tethys was a vast ocean covering most of Europe and extending in to what is now North Africa, the Middle East and central Asia.
The sea over Britain was generally shallow with water depths of perhaps 200 metres during the Early Jurassic in the Lyme Regis area. The mountains of Scotland, Wales and Cornwall rise up as land out of the sea. They had extensive river systems which poured out into the Jurassic sea. These rivers brought vast quantities of fine grained mud out into the sea. They also brought a constant supply of nutrients for organisms to feed on.
This combination of warm, shallow, food rich, muddy sea in the Lyme Regis area supported a prolific collection of wildlife. The sea teemed with marine life. Ammonites and Belemnites were especially abundant swimming in the sea and they provided prey for the carnivorous marine reptiles such as the Ichthyosaurs and to a lesser extent the Plesiosaurs. The sea supported fish and sharks and millions of microscopic plankton.
Sea bottom conditions were not so good because of the constant rain of mud. Occasionally, large chunks of wood from the Welsh and Cornish mountains would be rafted out and provide a support above the muddy floor. They would soon be colonised by oysters and crinoids.
The other problem with the sea bottom was that the mud starved the environment of oxgen so life could not be supported. Periodically the supply of mud would be cut off and sea bottom conditions would become oxygenated. The sea bottom would become carpeted with crinoids or sea lilies. The return of the mud would remove oxgen and the crinoids would be killed off.
The muddy, oxygen starved bottom conditions were crucial for the preservation of fossils today. When organisms died, they were covered in the mud which preserved them and the lack of oxygen inhibited decomposition. So the remains of the creatures were preserved and took the first steps on the road to become fossils for us to find today.
The mud containing the preserved animals was buried deep within the earth and eventually covered by a pile of sediment about 3 miles thick.
We know the rocks have been buried this deep by reconstructing the thickness of rocks that have been eroded away. We can confirm this by maturation studies which look at the colour of a material called kerogen which changes colour as it is exposed to the greater temperatures reached at depth in the earth.
As the mud was buried to such great depths, the water was squeezed out of it and eventually it was compacted into rock in the form of mudstones and limestones that we see today. The preserved animal remains were also turned into rock to form fossils.
Uplift and Erosion
The rocks and their wonderful collection of fossils could have been lost forever still three miles deep beneath the surface. However, later upheavals in the earth’s crust have lifted the rocks back up to the surface. Particularly important in this was the collision of Italy with southern Europe which resulted in the uplift of the Alpine mountains. The shockwaves of the collision rippled northwards and were instrumental in bringing the fossiliferous Early Jurassic rocks back to the surface for us to see in the cliffs around Lyme Regis.
Lyme Regis suffers from massive natural erosion which is bad news for properties but very good news for the constant supply of new crops of fossils.
The erosion has been going on for the last 10 to 12 million years, continues to this day and will continue for the foreseeable future despite the attempts of man to contain it. The following are just a few examples of the many recorded episodes of movement.
In 1689, a “violent shock of an earthquake was experienced” followed by “alarming convulsions in the earth” in the cliffs to the west of Lyme Regis.
Major cliff falls from Church Cliffs to the east of Lyme Regis in 1844, 1849 and 1862 became so serious that part of the churchyard fell away resulting in coffins falling on to the beach. A protective seawall was built in 1910 but this was rapidly decayed by the sea. A new 300 metre long concrete wall was built to protect Church Cliffs in 1957 which is on its last legs today.
There used to be a road from Lyme Regis to Charmouth which passed along the seaward slopes of Black Ven. The road dated back to the eighteenth century but had to be rebuilt in 1825 because of slippage. The road continued to suffer damage from slippage and had become so dangerous that it was closed in 1924. There is no trace of the road today.
In Spring 1971, a slip occurred on Marine Parade which caused the partial destruction of Library Cottage when a concrete retaining wall was fractured and pushed through the side of the house.
The cause of this instability is a combination of the rock types of the area and water brought in by rainfall. The rocks around Lyme Regis are varied with hard limestones, soft plastic mudstones and porous sandstones. This variety is naturally unstable and will move where soft rocks cannot take the strain of the weight of rock above them. This is accentuated when water percolates down into the rock lubricating the layers and allowing them to slide more freely and to lower the load bearing ability of individual layers.
The instability leads to numerous different types of failure including mudflows, mudslides and rotational sliding. They coalesce to produce an incredibly complicated structure in the cliffs. The structures vary with time so that an individual feature will move actively until it becomes stable and movement will then migrate to a new site.
Instability is strongly affected by rainfall with greatest movement taking place in the wetter winter months. Instability can also be affected by man when for example excavation allows water to seep into a previously stable body of rock, lubricating it and causing instability and movement.
The implications of this for fossil hunting are great. The natural instability is constantly exposing new rock which yields a new crop of fossils. Of particular interest are the mudflows and mudslides where rock flows down to the beach from high up the cliffs. This material is washed by the sea every high tide. This action washes away the rock and the fossils drop out onto the beach for us to pick up.