The animation to the left shows how fossils were formed
in the Jurassic Sea.
The example shows an icthyosaur alive and then dying
on the sea floor. The flesh rots away, leaving the skeleton which becomes
covered in deep marine sediments raining down from above. These sediments
create an anoxic environment, one which is ideal for the preservation
of bones as there is no oxygen present to support the bacteria or creatures
which may feed on the skeletal remains.
Over time the sediments built up, compressing into rock. These rocks were later
uplifted during the mid Cretaceous Period and became exposed to erosion, shown
in this example by the sea.
Eventually the bones become exposed on a modern day sea shore where they can
be found by fossil hunters.
Fossils and Fossil Hunting
The Jurassic Coast is a great place to find fossils
and the best place to find them is around Charmouth and Lyme Regis.
The Jurassic clays here
formed in a deep tropical sea, the soft muddy bottom of which was
often stagnant, creating a sulphurous and anoxic environment, the best
for preserving the shells, bones and even soft tissue of dead prehistoric
It is these stagnant seabed conditions which have
given the cliffs around Charmouth their dark colour (and smell!), and
it explains why the sediments
found here, the 195 million year old Green Ammonite beds and 197
million year old Belamnite Marls, are some of the richest fossil bearing
to be found anywhere along the Jurassic Coast.
Many fossils can be found
loose on the beach, and this is the best place to find them, where
they have been washed
clean by the
sea. It isn’t
safe to look for fossils in the cliffs as they are unstable and any digging
may cause the cliff to collapse. The famous Lyme Regis fossil hunter, Mary
Anning, lost her dog when she left it guarding a fossil spot. The cliff
crashed down on him and in her own words “twas but a moment between
me and the same fate”.
Always hunt for fossils on the beach on a falling tide tide, and be aware
that if you stay too long, you can be cut off when the sea comes back in.
Keep away from cliff falls, landslides and mudflows, particularly during
or after wet weather when they may still be active.
Take care when walking on wet rocks, algae and seaweed make them slippery.
The Triassic sediments in East Devon were formed in
a vast sandy desert which had rivers, flood plains and salt pans, an
environment far from
ideal for the formation of fossils.
The fresh water of the river would have attracted a lot of life and been
home to fish and amphibians. Most of the larger life here was terrestrial,
living along the edges of the river where the vegetation grew.
The richest fossil bearing sediment from this time is that of the Otter
Sandstone Formation which was deposited on the bottom of a shallow, oxygen
rich river flowing through a desert. These are far from ideal conditions
for creating fossils and carcasses only had a chance of becoming preserved
if they were buried very quickly, such as when the river deposited vast
amounts of sediment when it was in flood. The reason many of the fossils
found are only fragmentary is because carcasses were preyed upon by scavengers
and became dispersed as they floated down stream.
Despite this the Otter Sandstone Formation is regarded by many as the
richest reptile locality from the Mid-Triassic Period to be found any where
in the world. Finds include fish, invertebrates and 10 reptile and amphibian
It must be remembered the fossils here are fragmentary and a more fruitful
fossil hunting expedition can be carried out in other parts of the Jurassic
The Jurassic sediments were laid down on the bottom of a warm ocean floor.
The water was often deep allowing soft, deep clays to form which excluded
oxygen. These are the best conditions for forming fossils. The sulphurous
environment created by the anaerobic bacteria that thrived here combined
with the soft, deep clays meant that even very delicate creatures or soft
tissue had a chance of being preserved.
Once a carcass had sunk to the seafloor it would be quickly covered by
the clay. Over time more and more fresh seafloor deposits would build up
increasing the weight and pressure on the sediments deep below. Over millions
of years the old seabed and whatever lay trapped within the clay would
be turned to rock, becoming fossilised.
Fossils from the Jurassic Period mainly consist of sea creatures, though
the remains of insects, flying reptiles and dinosaur carcasses have also
Towards the end of the Jurassic Period sea levels dropped forming pockets
of land where swampy forests grew.
The Cretaceous sediments were mainly deposited on the bottom of a shallow
sea and are dominated by the skeletal remains of plankton blooms which
lived in the warm, clear water. These solidified with the application of
pressure over millions of years to form the white chalk cliffs found within
the World Heritage Site today.
Most of the fossils from Cretaceous Period originate from the marine environment,
though the seabed conditions were less favourable for creating fossils
than during the Jurassic. Significant fossils from the Early Cretaceous
when the environment was more terrestrial, a time when dinosaurs roamed
through the swamplands, are very rare.