The Chiltern Hills or the Chilterns are a chalk escarpment in southern England, northwest of London, covering 660 square miles across Oxfordshire, Buckinghamshire, Hertfordshire, and Bedfordshire, stretching 45 miles from Goring-on-Thames in the southwest to Hitchin in the northeast. The hills are 12 miles at their widest. In 1965, almost half of the Chilterns was designated as an Area of Outstanding Natural Beauty. The northwest boundary is clearly defined by the escarpment.