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Jurassic Coast
Old Harry Rocks - The end of the story
Location: Old Harry Rocks, near Studland
Rock type: Chalk
Age: Cretaceous, 65 million years old
Look out for: Sea stacks
The geology in detail
The striking white chalk of Old Harry Rocks formed towards the very end of the cretaceous period over 65 million years ago. Chalk is almost pure limestone, made up of trillions of microscopic skeletons from plankton as well as abundant fossil material from larger creatures. Calcium Carbonate (CaCO3), the chemical that limestone is made of, is readily available in sea water and is used by many forms of sea life to build their shells and skeletons. The most important source of the CaCO3 in chalk are cocolithophores, tiny structures formed by plankton. Trillions and Trillions of cocolithophores slowly settled on the sea bed at the end of the cretaceous creating the think deposits of chalk that form such characteristic cliffs today. Chalk also contains fossils of ammonites, gastropods, sea urchins, sponges and bivalve sea shells.
A distinctive feature of chalk is that it often contains nodules of hard, black flint. In fact flint is only found in chalk. Flint is very different from chalk, it is made from silica, not calcium carbonate and is very, very hard. Sponges, and a type of plankton called radiolaria, both form their hard structures from silica instead of calcium carbonate, and this is what eventually forms the flint. As the skeletons of radiolaria and sponges break down the silica dissolves in groundwater, which carries the chemical into spaces in the surrounding chalk where it crystallises to form flint nodules. These spaces are normally old burrows with irregular branching shapes and often appear in particular layers in the chalk. This gives the banding of flint nodules often seen in chalk cliffs.
The ancient environment
The presence of chalk indicates a warm sea around 100m deep. The water would have been teeming with life, especially plankton, and the sea bed would have been calm and tranquil.Loading...
Orcombe Point - The beginning of the story
The western end of the World Heritage Site and the oldest rocks on the Jurassic Coast. Striking Red sandstones and mudstones can be seen in the cliff and the ‘geoneedle' on top of
Orcombe point marks the start of the ‘walk through time'.
Budleigh Pebble Beds - Mountains and rivers
An incredible river deposit of pebbles from
across the channel and sandstones with ancient plant roots.
Ladram Bay - Sea stacks and the E.Devon AONB
Ladram Bay is a picturesque beach characterised by several
high sea stacks. It is one of the highlights of the East Devon Area of
Outstanding Natural Beauty (AONB)
Sidmouth - Desert dweller in the Otter Sandstone
Sidmouth is a traditional British seaside town bounded
within a wedge of red sandstone.
Beer - A story of stone Part 1
Beer is a picturesque fishing village and a source for
the famous Beer Stone.
Lyme Regis - World famous fossil site
Lyme Regis is famous worldwide for the abundant and
well preserved fossils that are found there eroded from the local cliffs.
Charmouth - World famous fossil site
Charmouth is famous worldwide for the abundant and
well preserved fossils that are found there eroded from the local cliffs.
Golden Cap - The highest view on the south coast
Golden Cap is the highest point on the south
coast and gives a fantastic viewpoint of this western part of the World
Heritage Site.
Chesil Beach - Finest barrier beach in Europe
Chesil Bank is one of the finest examples of a barrier
beach in Europe. Its formation is a direct
result of impact of the Ice ages on the geology and coastal processes of West Dorset.
Osmington Mills - Traces of life
Fantastic trace fossils can be seen on the beach at
Osmington Mills. Some of the best along the whole of the Jurassic Coast.
Durdle Door - The natural arch
Durdle Door, a huge natural rock arc, is an internationally
famous landmark and one of the most characteristic and spectacular features
along the Jurassic
Coast.
Lulworth Cove & Crumple - a geography pilgrimage
Lulworth Cove is a quintessential location for
the study of different rates of erosion in the formation of bays and headlands.
It is also a beautiful and popular tourist destination.
Old Harry Rocks - The end of the story
This is the eastern end of the Jurassic Coast
and where the youngest rocks on the world Heritage Site are exposed. Brilliant
white chalk has been eroded into steep cliffs and the famous collection of sea
stacks known as ‘Old Harry Rocks'.
Copyright 2006 Jurassic Coast
