The Durdle Door Arch

What you are looking at

Durdle Door is a natural arch of Portland Limestone, a rock laid down in a shallow, warm sea in the late Jurassic, around 150 million years ago. It is the same rock as the cliffs of Portland and the upper part of the Isle of Purbeck, and it is young enough in geological terms to still be nearly horizontal here — which is what makes the arch legible.

The sea has cut the arch along a fault: a fracture in the limestone where the rock on one side has slid a few metres relative to the other. That fault runs roughly at right angles to the coast, and the sea, coming in across the bay, found it and worked it. Waves compressed air into the crack, prised the blocks apart, and over thousands of years removed everything between the two headlands you now see joined by the arch.

The sequence, from the beach

Stand on the beach at Durdle Door and look up at the arch and the cliff behind it. The rock is bedded, and the beds are nearly horizontal. From the top of the cliff down to the shore you are looking at the upper part of the Purbeck Limestone Group:

The arch is formed entirely in the Hard Cockle Beds, which are the most resistant unit here. That is not a coincidence: the sea removes the soft rock first, and what is left standing is what refuses to be removed.

Logging tasks

Send answers to the five questions below through the message centre. Do not put them in your log. A photograph showing you and the arch is welcome but not required.

1. From the beach, look at the underside of the arch. Describe the shape of the opening, and state roughly how many metres separate the top of the arch from the sea at the point directly beneath it.
2. Find the fault line in the cliff behind the arch. Which way does it run relative to the shoreline, and how can you tell it is a fault rather than a bedding plane?
3. Explain in two or three sentences why the arch is in the Hard Cockle Beds and not in the softer beds above or below it.
4. On the shore platform, find a block of the Broken Beds. Describe its texture, and explain in one sentence what "brecciated" means in this context.
5. The arch is temporary. Give one process that will eventually destroy it, and say what evidence you can see on the beach today that this process is active.

Access and safety

• The path from Lulworth Cove to Durdle Door is steep in places and has a long flight of steps down to the beach. It is not accessible to wheelchairs or pushchairs.
• This is an active rockfall site. There was a significant collapse near the arch in 2019. Do not stand directly beneath the arch or the cliff face, and do not ignore warning signs.
• The beach is a shingle and sand strip that is almost entirely covered at high tide. Check the tide table before going down.
• Collecting fossils or rock samples here is prohibited; the whole coast is a World Heritage Site and a Site of Special Scientific Interest.

After the find

Lulworth Cove, twenty minutes west, shows the same rocks folded into a near-perfect circle by the Alpine earth movements that push up from the south. Man O'War beach on the far side of the arch is quieter and gives the best view back at the arch from sea level.