Geography of Dorset is the Blackmore Vale, a relatively low-lying area of clays and limestones of Jurassic age. The south-east of the county forms part of the Hampshire Basin, an asymmetric syncline covered in sands and gravels of Paleogene age. The main rivers in the county are the Stour, Frome and Piddle. The Stour is the county's largest river, although its source is in the neighbouring county of Wiltshire. The Frome and Piddle both lie wholly within the county. The whole of Dorset's coastline is designated part of the Jurassic Coast World Heritage Site, and contains examples of many different coastal landforms, reflecting
Geology of Dorset around the Dorset and Devon border, although these may have been transported here with the other river deposits. Chesil Bank a long barrier beach stretching from West Bay to Portland, was created 10,000 years ago, when sea levels rose at the end of the last ice age. The Variscan orogeny is the oldest event for which there is evidence in Dorset, forming a major series of thrust faults in Carboniferous and older rocks. These thrusts trend west–east, dip moderately towards the south and have controlled later basin development and subsequent phases of inversion. During the Permo-Triassic the area was affected