Somerset has one of the numerous small karst areas of Great Britain, called the Mendips or Mendip Hills. An area of limestones and limstone conglomerates is heavily karstified. The water enters the area of soluble rock from the uphill side, and rivers and brooks flowing on the surface go down underground through numerous swallow holes into caves. A chain of dolines, where the caves collapsed, marks the underground path until the water emerges in a huge karst spring the the lowere end of the area, and again flows as a river on the surface of insoluble rocks.
The Mendips cover an area of about 200km² between Weston-super-Mare and Frome, south of Bristol and Bath.