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
Dorset United Kingdom. However, Dorset maintains higher summer temperatures than Devon and Cornwall, with average highs of . Excluding hills such as the Dorset Downs, the average annual temperature of the county is . The south coast counties of Dorset, Hampshire, West Sussex, East Sussex and Kent enjoy more sunshine than anywhere else in the United Kingdom, receiving 1,541–1,885 hours a year. Average annual rainfall varies across the county—southern and eastern coastal areas receive per year; the Dorset Downs receive between per year; less than Devon and Cornwall to the west but more than counties to the east. The 2011 Census