Celebrating our Jurassic Heritage 10th Anniversary
PUBLISHED: 11:32 17 June 2011 | UPDATED: 19:33 20 February 2013
On its 10th anniversary, Edward Griffiths and geologist Alan Holiday take a close-up look at England's first Natural World Heritage Site
Celebrating our Jurassic Heritage
On its 10th anniversary, Edward Griffiths and geologist Alan Holiday take a close-up look at Englands first Natural World Heritage Site
Weve always known that our coastline is special but few suspected that, one day, it would be given the status of Englands first Natural World Heritage Site, selected by UNESCO (United Nations Educational, Scientific and Cultural Organisation), and ranking alongside the Grand Canyon and the Great Barrier Reef. The Jurassic Coast covers 95 miles (155km) from Orcombe Rocks on the Exe Estuary in East Devon, to Old Harry Rocks between Swanage and Studland in Purbeck, Dorset. Its designation as a World Heritage Site, awarded ten years ago this December, means that the coast should be protected, conserved and passed intact to future generations. The Site was recognised for its many outstanding coastal features, its importance for current education, and its role in the development of scientific knowledge, especially in the 19th century, but the main reason for its international recognition is the geology. Strictly speaking, the Heritage Site covers just a narrow ribbon between the low water mark and the tops of the cliffs but the Jurassic Coast is a living laboratory of coastal change.
Local Geology with a Global Perspective
Alan Holiday is Chairman of Dorset Geological Association Group (DGAG) and Dorsets Regionally Important Geological Sites Group (DIGS) and a retired Lecturer in Geology and Geography at Weymouth College. Alan explains the full geological importance of the site as he walks you through 185 million years in 95 miles.
The rocks exposed along this coast are not just Jurassic, as is suggested by the Sites convenient title. Jurassic is the term for just one of the three geological periods in the Mesozoic Era which consists of the Triassic, Jurassic and Cretaceous periods. A more or less complete sequence of the rocks from the Mesozoic is seen along the coast, representing a globally important record of this very significant part of geological history.
The oldest rocks occur in the west, where the Orcombe Rocks are around 250 million years old, at the top of the Permian or the base of the Triassic. Moving eastwards, the rocks generally get progressively younger until, at Old Harry Rocks, the chalk is around 70 million years old, towards the top of the Cretaceous period.
Unfortunately, the situation is not all as simple as this. The Devon coastal section is distinctive, with its red coloured Triassic rocks seen almost as far as the Dorset/Devon border just west of Lyme Regis. The red colour results from the rocks having been formed in a desert environment similar to the modern-day Sahara Desert but 210-250 million years ago. Capping these in places are yellow coloured Cretaceous Upper Greensand sandstone and chalk. The chalk is particularly obvious around Beer Head, between Branscombe and Seaton, and Beer stone from the chalk was an important building stone. So, the third Mesozoic period Upper Greensand and chalk is resting on the first Mesozoic period Triassic rocks but where has the Jurassic gone?
The sequence is as it should be further inland, but near the coast we see the results of earth movement between 140-65 million years ago, when the entire rock system was tilted to the east, the top Jurassic layer was eroded away and the Cretaceous sandstone and chalk were deposited on top of the Triassic. Geologists call this feature an unconformity, a gap in geological time where the geological record has been removed by erosion before the younger rock was deposited.
Moving eastwards, the contrast between the red rocks of Devon and grey clays of west Dorset is obvious. The lower Jurassic rocks, called lias, which are rich in fossils, were formed in a shallow subtropical sea, the sea level having risen and flooded the area at the beginning of the Jurassic. The gentle easterly dip of the strata is again apparent, bringing progressively younger rock down to sea level. In many places there is a capping of Cretaceous rocks, most famously at Golden Cap but also at other high points between Lyme Regis and Seatown. This geological situation is important because it results in extensive cliff falls, the mass-movement exposing more fossils on the beaches of west Dorset. Water percolating through the permeable sandstone soaks into the clay below, which loses strength and slumps under the influence of gravity.
Further east the yellow sandstone cliffs at West Bay and Burton Bradstock shine out from the coastline. The sandstone results in spectacular cliffs, which are liable to rock falls as they are eroded at the base by the sea. The change in the cliffs colour and shape, near the Hive Caf at Burton Bradstock, is due to a geological fault, with a displacement of around 60 metres down to the south. Further east of Burton Bradstock, the geology is less clear because of relatively weak rocks and the protection of Chesil Beach. Beaches are the best natural coastal defence and, without Chesil Beach, the area south of the Ridgeway, called Weyland, would look very different.
The variation in south Dorsets geology is best seen from the Hardy Monument or the hills above Portesham where the ridges and valleys of harder and softer rocks are apparent. The rocks are part of a large dome-shaped fold created by earth movements associated with the formation of the Alps 25 million years ago. Portland is on the south of this structure and the Ridgeway to the north. On reaching Portland the youngest of the Jurassic rocks are seen with the Portland stone and lowest Purbeck Beds. These are also seen along the coast near White Nothe, and they are then covered by Cretaceous chalk which extends to Lulworth. The rocks are complicated by folding, as mentioned for the Weymouth area, but around Lulworth the effects are more intense. In Stair Hole, the folding is dramatic, and at Durdle Door the rock layers are tipped almost vertically.
From here, late Jurassic and Cretaceous rocks are seen all the way to Old Harry, and because the rocks are arranged in west to east bands where the sea has cut through, different rocks are seen. This is especially well illustrated in Worbarrow Bay, whether approached from Lulworth Cove or from Tyneham. The dip of the rocks varies in Purbeck from steeply dipping or near vertical in the inland chalk ridge around Corfe Castle to near horizontal at St Aldhelms Head. The height of the cliffs varies with the strength of the rock type, from low cliffs on the weaker Kimmeridge Clay east and west of Kimmeridge, to high cliffs on the stronger Portland Limestone at St Aldhelms Head.
Northwards from Durlston Head the change in direction of the coastline exposes late Jurassic and Cretaceous rocks to Ballard Down and Old Harry. The bays are the result of erosion of the weaker rocks, and the headlands are the stronger rocks. At Old Harry Rocks there are excellent examples of caves, arches and stacks, some of the many classic features of the Jurassic Coast.
The white cliffs here are part of the Cretaceous chalk which includes the famous white cliffs of Dover. Beer is the only place on Devons South Coast where this chalk formation is exposed. A visit to the Roman Mines in Quarry Lane, where Beer stone has been mined for centuries, is highly recommended. Pay car parks in The Meadows, Commons Hill and Clapps Lane.
From Church Cliffs, behind St Michaels churchyard, there are fine views to the Black Ven landslip between Lyme Regis and Charmouth. Water seeping through the overlying Upper Greensand reaches the impermeable early-Jurassic blue lias, the resulting lubricated plane causes the landslips. Pay car park on Charmouth Road.
East of the harbour, the cliffs of Bridport Sands span the boundary between the Upper Lias and the Middle Jurassic. Chesil Beach starts at West Bay, first as a pebble bar, then separating into the unique shingle ridge which runs for 18 miles to Portland. Pay car park in West Bay Road.
Burton Bradstock and Hive Beach
Take the cliff-top path to Freshwater Beach and then walk back along the beach. Burton Cliffs Bridport Sands formations are stunning. National Trust Car Park at Hive Beach, signed off B3157 mile east of Burton Bradstock.
At 620ft (191m), Golden Cap is the highest point on the south coast. The climb from Seatown to the capping of Cretaceous Upper Greensand is rewarding for the fabulous views along the coast, but its easier starting from Langdon Hill. Pay car park near the beach at Seatown or Langdon Hill car park off the A35.
The sea has broken through the steeply dipping limestone and carved out the circular cove in the softer Wealden clay and chalk. The enormous forces which caused the dramatic folds of the Lulworth Crumple in Stair Hole can hardly be imagined. A good walk west from here leads to Durdle Door, where the sea has eroded the steeply dipping limestone beds into a stunning arch. A steep climb up Swyre Head is followed by lower Bats Head and the high chalk cliffs of White Nothe. Pay car park at Lulworth Cove.
A long walk to the bay is rewarded by the colourful cliffs of slipped chalk and greensand running down to the Wealden Beds. A quick look at Worbarrow Tout, the outcrop at the east end of the bay where the beach level has exposed segments of Purbeck marble, illustrates the general inland-dipping angle of the Purbeck and Portland Beds along this coast. Pay car park at Tyneham deserted village (open when the Army firing ranges arent in use), all Bank Holidays and summer holidays.
Until about 1890, oil was extracted from the Kimmeridge Oil Shale, but every year since 1960 about 16,000 tons of oil has been pumped by nodding donkeys from the Cornbrash, 520 metres below sea level. The Kimmeridge clay includes beds which form the Kimmeridge Ledges where children enjoy dipping into the rockpools, with Clavells Tower standing guard on top of nearby Hen Cliff. Toll car park overlooking the bay.
Durlston Country Park
Fine views westwards along the coast and east to the Isle of White, where the Needles and the chalk stacks of Old Harry mark the two remaining points of the old coastline before the sea broke through, swamping the land and absorbing the ancient River Solent. In Durlston Bay, the man-made cliff masks the Durlston Fault, which was threatening the buildings above. Pay and display parking in the park.
Formed by erosion of the valleys soft Wealden clay, Swanage Bay is flanked by Peveril Down, where the perilous Peveril Ledges of Purbeck marble head out to sea, and the chalk ridge of Ballard Down, with Old Harry marking the end of the Jurassic Coast. Pay car parks in Broad Road on Peveril Down and in Victoria Avenue.
Learn about Dorsets Geology The Dorset Important Geological Sites Group (DIGS) work to conserve and protect old quarries, cuttings and rock faces along the Jurassic Coast. By removing brambles and pulling out ivy, rock exposures are kept accessible. DIGS runs a conservation day on the third weekend of every month. Volunteers are always welcome. For more information, contact Alan on 01305 789643 or dorsetrigs.org.uk Alan is also available for talks on the Jurassic Coast. For more details e-mail firstname.lastname@example.org