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Rocks and Fossils

Geology - 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 water. There are in fact four principal bands of cementstone in the locality but they are not that easy to segregate because in places the strata have moved along geological fault lines and the bands have been pushed up or moved down.

Geology - Worbarrow Bay

In contrast to Kimmeridge Bay, the rocks which outcrop 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.

Originally, most of these rocks were marine sediments, laid down layer by layer in an ever-changing and subsiding shallow sea at a time when the climate was sub-tropical. After millions of yearsof compression they were eventuallyuplifted and folded into the form which we see today. The harder limestone resists the sea best and can be seen 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. Many of these fossils are fragmentary and difficult to collect but a search among the rocks beneath Worbarrow Tout is often rewarding.

Oil and oil shale

Small seepages of oil can be found in a variety of places in Purbeck, but their source remained a mystery for years. It was known that oil moves around or "migrates" within the rocks but it was not until the late 1950s that 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 oils today at a rate of about 100 barrels a day - compare that to the 80,000 barrels a day coming out of the Wytch Farm Oilfield!

It is probable that the Kimmeridge oil originates from even deeper, in the main oil-bearing strata that feed the Wytch Farm oilfield. These strata extend far out to sea and a number of oil companies have started to explore the sea-bed off the Dorset coast in recent years in an attempt to discover the extent of thse reserves.

The oil-shale, or "Kimmeridge Coal"that has been won from the cliffs to the east of Kimmerigdge since the early 17th century 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

 
Fossil bivalves (like today's sea-shells) are also commonly found. Unfortunately, Kimmeridge fossils do not make very good collector's specimens as they are often crushed and hence distorted.

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. As an alternative to collecting, many fossils are embedded in the surface of the flat shale bands crossing the beach and are excellent for taking rubbings.

For more detailed information on the geology of Kimmeridge and the rest of Dorset, visit Ian West's excellent website - http://www.soton.ac.uk/~imw/kim.htm

 
Original text by Malcolm Turnbull