Rocks and Fossils

The Jurassic Coast.

The coast between Exmouth in East Devon, and Studland Bay in Dorset is designated as a World Heritage Site for rocks, fossils and coastal landforms. The rocks formed one on top of another over millions of years. The animals and plants alive during this time also evolved so each rock layer contains a unique assemblage of fossils. The rock layers are rather like pages in a book and the fossil the page numbers.

What makes this place unique is that overall, the rocks dip or tilt gently to the east and as a result, the oldest are found in Exmouth and younger and younger layers form the cliffs through to Studland Bay. In fact this coast records a staggering 185 million years of geological time and provides the finest record through the Triassic, Jurassic and Cretaceous periods anywhere in the World.

The rocks at Kimmeridge belong to the Jurassic period and form a part of this amazing story through time.

Kimmeridge Bay

The soft, fragile, dark clays and shales that form the cliffs of Kimmeridge Bay are interspersed with bands of much paler, harder rock, known as cementstone. Occasionally, blocks of this cementstone are found on the shore below the cliffs, following erosion of the softer shales beneath, causing them to fall. In parts of the bay, these blocks form a boulder shore.

The cementstone bands stand out quite clearly in the cliffs, and where they reach the shore they form distinct ledges, which are prominent at low tide. It is possible to pick out the Flats Stone Band which forms Broad Bench and other ledges in the west of the bay, the Washing Ledge Stone Band which curves down to the beach just to the west of Gaulter Gap and the Maple Ledge Stone Band which stretches from 100yds to the east of Gaulter Gap to the slipway.

Worbarrow Bay

The rocks in Worbarrow Bay cliffs are very varied, with hard limestone and chalk as well as the much softer shales and sandstones. The rocks also show a remarkable range of colours ranging from whites and greys to reds, yellows and purples.

These rocks were laid down under the sea and in swamps and lagoons over about 50 million years. After millions of years of compression they were eventually uplifted and folded into the form which we see today. The harder limestone resists the sea best and forms the prominent features at Mupe Rocks and Worbarrow Tout. The multi-coloured sands and gravels of the Wealden Beds are rapidly eroded once the limestone barrier is breached, but the sea's erosive power is slowed again once it reaches the chalk.

The numerous fossils that are found in the rocks of Worbarrow Bay reflect the conditions that prevailed when the beds were first formed. The Portland limestone is famous for its giant ammonites, the Purbeck limestone for its fish and turtle remains, crocodile and shark teeth, oyster shells and dinosaur footprints, and the chalk for its sea-urchins.

Chapman's Pool

The Kimmeridge Clay is very thick and each layer records a moment in geological time. The rocks dips or tilt gently towards the east and as a result, the oldest rocks are found around Kimmeridge Bay and they become younger towards Chapman's Pool. This piece of coast provides probably the best record of geological time between 155 and 150 million years ago. Take care along this section of coast as the tide cuts off the beaches in several places and there is no way up the cliffs.

Oil and Oil shale

In the late 1950s a borehole at Kimmeridge showed that oil was seeping out of the Cornbrash - a layer of rock over 500m below the surface. In 1959 the Kimmeridge Oil Well of British Petroleum started production and it can still be clearly seen on the top of the cliffs in the north end of the bay. The nodding donkey still pumps oil at a rate of about 80 barrels a day. 

The oil-shale, or "Kimmeridge Coal "found in the cliffs to the east of Kimmeridge has nothing to do with the "free oil" being pumped by the nodding donkey. The "coal" is actually a shaly, bituminous stone that burns with a bright flame, gives off an offensive smell and leaves copious quantities of grey ash. It is no longer used commercially.

Fossils

Fossils are abundant in the Kimmeridge shales and can be easily seen in the rocky ledges of the shore and in the shale pebbles on the beach. Ammonites are the most easily found fossils. Fossil bivalves (like today's oysters and clams) are also commonly found.

Unfortunately, Kimmeridge fossils do not make very good collector's specimens as they are often crushed and distorted and soon crumble when exposed to the air.

The Kimmeridge cliffs are extremely unstable and it is highly dangerous to collect fossils from the cliffs themselves. Fossils can be obtained quite safely by searching through the shale fragments that are to be found on the beach, away from the bottom of the cliff.  

Worbarrow Bay

An Ammonite Fossil