Hackpen, Warren & Gramp's Hill Downs is a 71.4-hectare (176-acre) biological Site of Special Scientific Interest in Letcombe Bassett in Oxfordshire.The site is composed of three separate sloping areas of unimproved chalk downland, representing a type of habitat now uncommon in the Berkshire Downs. It was formerly notified as Hackpen Hill and Crowhole Bottom. Eleven species of butterfly have been recorded, including chalkhill blue, brown argus and marbled white. The site has extensive views over rolling countryside.
Gramp's Hill Down is the most easterly area, and the closest to Letcombe Bassett. The main grass is upright brome, and it has a rich variety of herbs, including yellow rattle, chalk milkwort and kidney vetch.The eastern half of the central area, Warren Down, is also dominated by upright brome, with herbs including bulbous buttercup and salad burnet. The main grass in the western half is meadow grass, with bare patches and rabbit damaged areas which have been colonised by plants of disturbed ground such as henbane.At the western end is Hackpen Down, the largest part of the site, which has a wide variety of aspects and gradients. The main grass is red fescue, and herbs include horseshoe vetch and common rockrose. A narrow strip to the east contains hawthorns and elder scrub.