ruined Cistercian abbey, Ryedale district, administrative county of North Yorkshire, historic county of Yorkshire, England. It lies in the seclusion of a deep valley to which it has given its name, in the North York Moors National Park. The monastery was the mother church of the Christianizing mission of St. Bernard of Clairvaux in northern England. Of the great 12th-century church, 125 yards (115 metres) long, the chancel and transept walls still stand to full height and exhibit some of the best English Gothic architecture extant. On the eastern terrace overlooking the abbey are two classical temples built in 1750–60 by Thomas Duncombe, a local landowner, to decorate his estate.