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THE EXTINCTION OF THE DINOSAURS
The end of the Cretaceous period saw the extinction of the dinosaurs and
the gradual disappearance of the chalk sea as sea level fell throughout
the world. The next chapter in our story involved further inundations
in later geological periods which deposited a great thickness of other
rocks over the whole area. Much of Essex is therefore built on these younger
rocks but the chalk is, of course, still present beneath the surface,
in some cases as deep as 200 metres below ground level. In many cases
the younger rocks consist of pebbles, sand or gravel, the remains of countless
billions of flint nodules that have been removed from the chalk and ground
down by erosion.
The first rocks to be laid down on the eroded chalk surface are a variety
of sands clays and shell beds deposited in shallow marine or estuarine
conditions. These have been reclassified many times but they are best
known as five distinct types of sediment, namely the Oldhaven, Blackheath,
Woolwich, Reading and Thanet Beds. Thanet Sand can be clearly seen in
many of the Thurrock quarries, lying on top of the Chalk. These deposits
are grouped under the name 'Lambeth Group' on the accompanying geological
map.
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Cliff of London Clay, River Crouch, Athorne
Photo: Chris Gibson
Shark's tooth, London
Clay, Essex coast
Photo: Gerald Lucy
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THE LONDON CLAY
The environment of the region was now to change dramatically, leading
to the deposition of the London Clay, a mud laid down on the floor of
a subtropical sea some 50 million years ago.
The London Clay has yielded fine fossils of the sea's inhabitants such
as molluscs, lobsters, crabs and sharks. There are also fossilised fruits,
seeds and twigs which provide us with valuable information about the rain
forest vegetation which existed on the land at this time.
Most remarkable of all are probably the fossil turtles and mammals from
the workings of the Harwich cement industry in the 18th and 19th centuries.
The cement was made from hard limestone nodules that occur in the clay
and in one of these, in 1856, a workman found the skeleton of Hyracotherium
(also known as Eohippus). Hyracotherium was the earliest ancestor of the
horse and was no larger than a fox, with toes instead of hooves. This
animal lived on the river banks and its bones must have been washed down
a river into the sea. This was a time of rapid evolution of mammals following
extinction of the dinosaurs.
London Clay fossils, particularly sharks' teeth, turn up all around the
Essex coast but the most famous site is at Walton-on-the-Naze where the
beach is very popular with collectors. The London Clay here is one of
the reasons Walton is a site of international importance, mostly because
in it are the best preserved bird fossils of Tertiary age to be found
anywhere in the world. The London Clay also contains layers of volcanic
ash which may have originated in Scotland where there were active volcanoes
around this time.
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