Fossil Coast explores Durdle Door the iconic Portland Stone arch and world class landmark along the Jurassic Coast of Dorset.
Heading west from Lulworth Cove there is a steep walk along the South West Coast Path up Hambury Tout towards Durdle Door. As you walk the path and look down to your left is Dungy Head with its dangerous cliff edges and the start of St Oswald’s Bay.
As you reach the summit of Hambury Tout take a minute. On a clear day and looking out to sea you should see the 6 Km silhouette of the Isle of Portland running from the Port to Portland Bill’s lighthouse, the southernmost point of Dorset. The Isle of Portland is an impressive place for geology and is essentially an "island of limestone" with a mix of Kimmeridge Clay and Portland Sand. Each of these layers have a combined fossil record that includes giant ammonites (Titanites), bivalves, gastropods, ostracods, fossil trees, fossil soils, early mammals and dinosaur footprints.
As you sweep around the coastline past the seaside town of Weymouth you will see that the coastline ahead is again influenced by its geology including the steep chalk cliffs. In the distance you will notice a small natural arch known as Bat’s Hole and a series of offshore limestone outcrops and small rocky islets with the nearest called The Bull followed by The Blind Cow, The Cow, and The Calf.
Before you approach Durdle Door you will notice that it is flanked by a steep and rocky projecting headland with steps down to an enclosed cove known as The Man o’ War.
Apparently, this is a popular sandy beach in the Summer as well as good for fishing, swimming, snorkelling and diving, the water is protected from swells by a partially submerged offshore reef. Seals are known to visit.
Among the most iconic and photographed geological landmarks along the Jurassic Coast is Durdle Door (Durdle meaning "drill"). The softer Purbeck limestones and Wealden sandstones and siltstones have been eroded by the sea leaving a natural limestone arch of steeply dipping Portland Stone.
Approximately between 145 – 150 million years ago during the Jurassic period the limestone was deposited as sediment in the shallow warm seas. At this time the sea bed was flat until between 65 and 7 million years ago when the Alpine orogeny, or Alpine mountain building event occurred creating the Alps, Pyrenees and Carpathian Mountains in Europe. In fact, this event has never stopped by movements today are nominal.
View from the South Coast Path walking to Durdle Door
The Alpine orogeny led to intense metamorphism, warping and crumpling of stacked-up layers of sedimentary rock that became vertically uplifted creating geological features such as the Weald-Artois Anticline in Southern England/Northern France.
As modern-day Dorset was over 1,000 km from the collision zone of the Alpine orogeny it did not create any mountains but instead uplifted and folded rocks into what is knowns as the Purbeck Monocline or the northernmost 'ripple' of the Alpine orogeny. This ripple continues under the sea towards The Needles of the Isle of Wight as the Purbeck-Isle of Wight Disturbance.