Dorset covers an area of and contains considerable variety in its underlying geology, which is partly responsible for the diversity of landscape. A large percentage (66%) of the county comprises either chalk, clay or mixed sand and gravels. The remainder is less straightforward and includes Portland and Purbeck stone, other limestones, calcareous clays and shales. Portland and Purbeck stone are of national importance as a building material and for restoring some of Britain's most famous landmarks. Almost every type of rock known from the Early Jurassic to the Eocene epochs can be found in the county.