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.

It is possible, however, 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. The Yellow Ledge Stone Band is best seen round the corner to the east where it starts high up in the cliff below Clavell's Tower and then dips slowly down to reach the shore at Cuddle.

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

Fossils are abundant in the Kimmeridge shales and can be easily seen as white marks in the rocky ledges stretching out from the shore and in the shale pebbles that lie on the beach. Ammonites are the most easily found 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

Industrial Past

The history of Kimmeridge Bay and the surrounding area is very much tied in with its geology. The thin seam of blackstone or Kimmeridge Coal - a hard, bituminous shale which runs through the cliffs - has been worked since prehistoric times. Initially used as a fuel, albeit a rather foul-smelling one, it then became popular as a material for craftsmen. It can be worked and polished much like a hard wood and can be finished to resemble jet.

It was popular in pre-Roman times, but the Romans went into mass-production, turning out large numbers of rings and bangles on pole-lathes. Large numbers of the waste cores of shale left over from this industry have been found in this area - for many years these cores regularly washed out of the eroding cliff just west of Gaulter Gap.

Later, many attempts were made to base an industry on this locally available fuel. These ranged from alum and glass-works in the seventeenth century to the distillation of oils and gases from the shale in the last century. None of these ventures appears to have been particularly successful or long-lived and little obvious evidence of their existence has been left behind. You can still make out the remains of stone piers and jetties at the east end of the bay and the remnants of mine workings are just visible at some points along the cliffs.

Perhaps a more successful "industry", and one to which the area was certainly well suited, was that of smuggling. The irregular and barely accessible coastline provided perfect cover for contraband to be run in from across the Channel. This activity is echoed in the name of Brandy Bay.

Clavell's Tower on Hen Cliff was built in 1831 by the Reverend John Richards, who assumed the name Clavell after inheriting the Smedmore Estate. Initially built as a summerhouse, the tower was later used as a Coastguard lookout before falling into disrepair after it was gutted by fire. In 2006, work comenced to rebuild the tower and regain it's former glory, 25 metres landward under the Landmark Trust to become a inhabitable holiday property. The date for completion is set to be December 2008 and plans can be viewed within the marine centre.

Present day Kimmeridge hasn't completely shrugged off its industrial past - what was once Britain's most important oil-well is still quietly nodding away on the cliff-top, producing about a tanker of oil a day.

Much of the activity in Kimmeridge and the surrounding coastline today revolves around recreation. As one of the few access points to the Purbeck Coast, Kimmeridge Bay inevitably attracts many visitors. A great proportion come to stroll along the beach or wander along the cliffs, other more serious walkers pass through on the Dorset Coast Path.

Traditional water-based activities such as angling and, in warmer weather, paddling and swimming, remain popular alongside more "hi-tech" pursuits. Divers have known about Kimmeridge for many years and converge on the bay on summer weekends. Launching from Kimmeridge provides access to many excellent dive sites, such as the Underwater Nature Trail.

More recently, intrepid windsurfers have discovered that the bay provides and exciting combination of wind and wave and, given the right conditions, they can be seen flitting across the bay and leaping over waves. The shallow platform of Broad Bench and the ledges under Clavell's Tower also attract surfers.

Fishing has always been carried out from Kimmeridge, though it has never been a major industry. The reserve's Information Centre stands on the site of a former fishermen's hut, once belonging to the Marshall family, who have fished out of Kimmeridge for generations. Today only one boat operates full-time out of Kimmeridge, mainly potting for crabs and lobsters.