The geology of the Isle is complex. It has a discordant coastline along the east and concordant coastline along the south. The northern part is Eocene clay (Barton Beds), including significant deposits of Purbeck Ball Clay. Where the land rises to the sea there are several parallel strata of Jurassic rocks, including Portland limestone and the Purbeck beds. The latter include Purbeck Marble, a particularly hard limestone that can be polished (though mineralogically, it is not marble). A ridge of Cretaceous chalk runs along the peninsula creating the Purbeck Hills, part of the Southern England Chalk Formation that includes Salisbury Plain, the Dorset Downs and the Isle of Wight. The cliffs here are some of the most spectacular in England, and of great geological interest, both for the rock types and variety of landforms, notably Lulworth Cove and Durdle Door, and the coast is part of the Jurassic Coast World Heritage Site because of the unique geology.