At the southern margin of the Isle of Purbeck lies Durlston Head. It is a headland composed of Portland Limestone. The geological structure changes from West Dorset to the east, with the dip of the beds gradually becoming less steep. From here northwards to Studland the Isle of Purbeck coast becomes discordant, with the geological structure meeting the coastline at right angles (the geological structure is perpendicular to the coastline). This arrangement of the geology allows for the development of headlands and bays, depending on the relative resistance of the rocks to weathering and erosion. The headlands of Peveril Point (Purbeck Beds) and Ballard Point (Chalk) can be seen in the distance towards the north. Swanage lies in a bay formed from the relatively less-resistant Waelden Clays. The topography of the landscape on the Isle of Purbeck is also related to the geology with Swanage lying in a valley between the limestone ridges of the Portland Limestone to the south and the Chalk to the north.