Purn Hill is an outlier of the western Mendip Hills, rising just over 100 feet above its surroundings, a mix of flat fields, marshland, and two villages - Purn to the south and Oldmixon to the north, a southern suburb of Weston-super-Mare. Like all of the Mendips, the hill is underlain by limestone, mostly covered by grass but exposed in a few locations, and here are found three nationally rare plant species; white rock-rose (helianthemum apenninum), honewort (trinia glauca) and Somerset hair grass (koeleria vallesiana), while elsewhere are a selection of more common species such as the early purple orchid, yellow-wort, carline thistle and the common (yellow) rock-rose.
The top of the hill is protected as a nature reserve, a small site just 1,500 feet long and 300 feet wide, mostly grassland, with just a few trees around the perimeter, and crossed by a half mile loop trail. Views from the top are slightly obscured by trees in some directions, but otherwise good, of the Somerset Levels, the higher Mendips, and westwards towards the coast and the two westernmost peaks, Uphill Hill and Brean Down.
The hill is narrowly oval in outline, orientated north-south. The northern end is crossed by the West Mendip Way, intersecting a minor road from Oldmixon (Purn Lane), but the usual access to the nature reserve is from the south, starting in Purn village, parking in front of a group of shops, reached by Bleadon Road, just off the A370. A grassy path heads west then north, alongside a garden, up a slope and into a belt of woodland, where it reaches the entrance, marked with an information board. The path continues through the trees to the north end of the hill, then meets the Mendip Way. The limestone exposures, where the majority of the interesting plants are found, are on the southwest side, and the main area extends for about 100 feet. Of the three rare species, hair grass is difficult to identify, honewort is a low growing, small-flowered species of the carrot family, also not particularly distinctive, but the white rock-rose is much more noticeable, with hundreds of flowers during the peak blooming month of May. A few rock-rose plants produce deep pink flowers; these are hybrids of the two species, helianthemum x sulphureum.