district, administrative and historic county of Buckinghamshire, England, in the southern part of the wooded Chiltern Hills. The River Thames forms its southern boundary. The predominantly rural district overlaps the designated Chilterns Area of Outstanding Natural Beauty. Prehistoric burial mounds and earthworks are visible. Towns include High Wycombe, Marlow, and Princes Risborough. Area 125 square miles (325 square km). Pop. (2001) 162,108.