Coastal environment and geology
During
the Mesozoic, from hundred and
fifty to sixty-five million years ago ,under a
warm and damp climate , not
only did the dinosaurs but many other species become extinct among
which
ammonites and sea urchins constituting the marine plankton. It is the
sedimentary accumulation of their calcareous remains that created the
high
cliffs that we can admire today.
The one hundred and thirty kilometres of chalk cliffs along the coast, called the alabaster coast , are a unique setting highly sensitive to erosion.
Under
the effect of the swell, frost, water seepage,
and other factors linked to the constitution of the chalk itself, the
cliffs are
eroded of about twenty centimetres a year. The blocks that fall down on
the
strand release chalk and flint.
The
chalk is rapidly dissolved in the sea while the flint is rolled and
crushed by
the sea and eventually becomes a pebble.
The
pebbles gather
into an offshore bar which constitutes a natural
barrier which protects the
bottom of the cliffs against direct
attacks from the waves.
This
phenomenon is common to all the cliffs along the Channel coasts
, so this
is what happens in Sussex with the Seven Sisters for example.
Man
has made this natural environment change.
Indeed, the economical development of ports induced the building of
piers and
sea walls in order to allow an easy access to bigger and bigger boats .
The
setting up of heavy equipment, such as the oil rig at Antifer or the
two nuclear
power stations around Dieppe, changed the coastal landscape and
accelerated the
erosion and the recession of the cliffs. ( from half a meter to a meter
a year)
. Huge sections fall down into the sea causing
the collapse of houses built on top of the cliffs .
Pebbles
are also a valuable industrial resource.
The traditional collecting of pebbles on the beaches of Dieppe and Le
Treport
was forbidden in 1985 in order to preserve the stock of pebbles and
ensure the
protection of the cliffs . This activity increased however in the area
of the
Somme mouth where the stock of pebbles is important.
The
pebble , made of ninety-nine per cent of silica , is exported to
eighty
countries over the world because it can be used in industry in three different ways :
-
Round
regular-shaped pebbles are used to crush fragile chemicals in
cosmetology and
pharmacology. Their purity do not alter the products.
- In spite of their hardness, pebbles can be crushed into grains of different sizes which are then used as gas or water filters or come into the making of glass-paper
-
When the pebble
is brought to a temperature of one thousand and six hundred degrees
centigrade,
it changes into a white powder which is used to make earthenware and
ceramic,
such as the famous English Wedgwood or the French Luneville. The burnt
pebble
also comes into the making of toothpastes, road-paints or architectural
coatings.