Old Harry Rocks
The chalk of Old Harry Rocks used to be part of a long stretch of chalk between Purbeck and the Isle of Wight, but remained as a headland after large parts of this seam were eroded away. As the headland suffered hydraulic action (a process in which air and water is forced into small cracks by the force of the sea, resulting in enlarging cracks), first caves, then arches, formed. The tops of the arches collapsed after being weakened by rainfall and wind, leaving disconnected stacks. One of these stacks is known as Old Harry. Old Harry's Wife was another stack which was eroded through corrosion and abrasion, until the bottom was so weak the top fell away, leaving a stump. Hydraulic action is the main cause of erosion (sheer force of the wave) that damaged the rock.
There are two stories about the naming of the rocks. One legend says that the Devil (traditionally known euphemistically as "Old Harry") had a sleep on the rocks .
Another local legend says that the rocks were named after Harry Paye, the infamous Poole pirate, who stored his contraband nearby. These could be linked as Harry Paye could have been considered as the devil and could well have slept on these rocks.