The geological maps of the Bath area indicate that most of the valley sides below the level of the Great Oolite are characterised by landslipped or foundered strata containing Fullers Earth and the inferior Oolite and Midford Sands limestone.
The maps show the stronger, more resistant Great Oolite to be the cap rock which forms the hill crests in the area and the Lower Lias Clays forming the valley floor.
The slopes of the Combe Down plateau are over-steepened as a result of erosion by the River Avon and its tributaries which has led to land slippage on many of the slopes. Furthermore, softening of the Fullers Earth clays has led to cambering of the overlying strata, especially near the edge of the plateau. This has resulted in stress fractures, known as gulls, being a feature across the plateau. Some of these features remain empty (or void), others are filled with clay or rock fragments.