Lulworth Cove was formed approximately 10,000 years ago by the sea opening up a stream valley through a narrow gap in the hard resistant Portland Stone.

Once the sea had been able to surge through the gap in the Portland Stone the tidal action extensively eroded the softer Wealden Clays and harder chalk to form the almost circular bay or basin we see today.

The unique shape is caused as a result of the narrow entrance which bends the waves into an arced shape as they enter the cove.

The entrance to the bay through the Portland stone is 120 metres wide with the bay opening out to 500 metres in diameter.

Stair Hole on the western side of Lulworth Cove, is a small cove with natural arches cut through the steep Portland and Purbeck limestone cliffs on the sea ward side.

Through these arches and a gap where a cave collapsed, the sea can flood in at high tide to erode the softer limestones and shale's behind perhaps eventually forming a new cove.