ridge of limestone hills extending for about 50 miles (80 km) across south-central England. The Cotswolds are part of the Jurassic uplands that cross the country from southwest to northeast. The Cotswolds escarpment rises steeply from the clay vale of the lower River Severn and its tributary, the River Avon (Avon, River) (Upper Avon), and slopes gradually eastward toward the clay vale of Oxford. Its crest is generally 600 to 700 feet (180 to 210 metres) high but reaches 1,083 feet (330 metres) in Cleeve Cloud above Cheltenham. The oolitic limestones provide fine building stone, which is much in evidence in the district. In the Middle Ages the Cotswolds were open sheep runs. The wealth obtained from the sale of wool and later from the domestic cloth industry is evident in the substantial buildings, especially the churches, that grace the villages and market towns. Enclosure of the sheep walks, typically by dry stone walls, subsequently accompanied the change to arable farming.