The Future of Durdle Door
On the 8th March 2017, the unthinkable happened to one of Malta’s most iconic natural monuments. The Azure Window, a natural limestone coastal arch, which formed the backdrop for scenes in Clash of the Titans, the Count of Monte Cristo and Game of Thrones collapsed after a violent storm. The Prime Minister, Joseph Muscat said the news was heartbreaking and he commented that “reports commissioned over the years indicated that the landmark would be hard hit by unavoidable natural corrosion”. From a geomorphologist’s perspective, “natural corrosion” and indeed weathering are part and parcel of why beautiful coastlines and features like this exist all over the world. Our very own and much loved coastal arch, Durdle Door, itself has become a famous in its own right featuring in a multitude of Hollywood and Bollywood films.
Durdle Door is a textbook example of a natural coastal arch which is located to the west of Lulworth Cove on the Dorset coast. The feature was probably named more than a 1000 years ago which reflects how slowly change happens on some parts of the coast. The name “Durdle” is derived from an Old English word “thirl”, meaning to pierce (as in “nostril”).
The rock layers here have been tilted and shifted so that the oldest rocks in the sequence (Portland Stone) have been pushed up so they face the sea. You can see this very clearly at Stair Hole; a dramatic visual expression of powerful tectonic forces crumpling the rock like they were pieces of paper. Equally fascinating is how the nature of the rocks at this location has defined the beauty of the landscape. Behind the resistant limestone layer of Portland Stone, are sequences of (weaker) clay, sandstone and then finally a strong headwall of chalk. Over time, the waves have broken through the resistant wall of limestone, carving into the weaker sands and clays to create a beautiful horseshoe shaped cove.
The origin of Durdle Door probably began as a notch in a point of weakness in the wall of Portland Limestone that faced the onslaught of the sea. Over time, this small notch was subject to constant battering by the sea and widened to form a cave. Once the waves broke through the back wall of the cave, they would have eroded away the weaker rocks behind and over time the sweeping beach would have formed in that space. The cavity in the wall of limestone would have become larger to eventually form the freestanding natural arch that we see today. Eventually physical weathering processes combined with chemical weathering processes (limestone is particularly prone to solution weathering in coastal environments) will weaken the structure so that it is unable to support the upper bridge. This will collapse and leave behind a freestanding limestone pillar which over time will erode away and leave a stump.
So, here is the million dollar question! Since erosion is the key process that maintains the beauty and scientific integrity of the Jurassic Coast, how long do we have left to enjoy this iconic landform? From a geomorphologists point of view, erosion of Portland stone features on the coast occur at a much slower rate than those features composed of weaker rocks such as sandstone and clays. Although you can never be definitive with nature, I would say that we can look forward to another thousand years or so of movies shot at this iconic location.
Anjana Khatwa Ford- Programme Manager