Located in the Green Belt, most of it forms part of the Farthing Downs and Happy Valley Site of Special Scientific Interest.

The 67-hectare (170-acre) site is a steeply sided valley with large areas of grassland and wooded slopes.

[1] The diverse chalk grasslands on the eastern and north-western sides are rich in many species of herb which are uncommon in London, and the nationally scarce man orchid and round-headed rampion have also been recorded.

Devilsden Wood on the western side has a canopy of ash, oak and hazel.

Trees such as midland hawthorn and plants such as sweet woodruff are characteristic of ancient woodlands.