range of hills in the geographic county of Somerset, England, extending 23 miles (37 km) northwest from the Frome valley. The Eastern Mendip is comparatively low, but the Western Mendip forms a plateau 6 miles wide and more than 800 feet (244 metres) high. Farther west the Wavering Down and Bleadon Hill continue the trend of the upland toward the Bristol Channel. Swallet holes and drainage (the sudden disappearance of streams and rivers underground) characterize the easily dissolved rocks of the limestone region. There are many caves such as those at Wookey Hole and Cheddar Cliffs. Important traces of prehistoric settlement have been found in the area.