The geology of the Isle of Purbeck is very complex and this is shown very clearly along this stretch of coast. The northern side of Worbarrow Bay is mainly Upper and Middle Chalk. This is subject to frequent landslides because of the unstable underlying Gault Clay. At the top of the cliffs is the Iron Age hill fort on Flower's Barrow, which is threatened by erosion on its seaward side. The Flower's Barrow ridge forms the western end of the ridge which runs all the way to Ballard Point north of Swanage. The Chalk cliffs become lower to the west of Cow Corner and just beyond they form a knife-edge ridge that drops down to the little Bay of Arish Mell, where a stream has cut a gap in the Cretaceous hills.
The steep angular layers of rock are clearly visible in the cliffs of the bay. They reveal complex sedimentary folding, that deranged the geology of this neighbourhood about 30 million years ago, caused by the tectonic pressures as the African and European continents collided. The rock sediments were twisted horizontally and this is why the Chalks that are between 85 and 145 million years old are found at the rear of the bay.
The sediments that form Mupe Ledges and Worbarrow Tout are the 150-million-year-old Portland Limestones and the 147-million-year-old Purbeck Beds.[1]
