The biosphere reserve area covers 842ha. This is still in operation but is controlled by very strict lease agreements. The archipelago comprises four islands, namely Hirta, Dun, Soay and Boreray which are the remains of a Tertiary ring volcano, which have been weathered and glaciated to produce dramatic precipitatious cliffs. The cliffs of Boreray and Soay rise sheer to over 370m and the north face of Conachair hill, on Hirta is 430m high. Stac an Armin 191m and Stac Lee 165m.
Rocks are predominantly gabbro, granophyre, dolerite and basalt.
The climate is oceanic, of a cool Atlantic type with extremely high humidity and strong winds. Northern Atlantic species include Botiychium lunaria , Ophioglossum vulgatum , Gentianella campestris , Ligusticum scoticum and Sedum rosea with Silene acaulis and Saxifraga oppositifolia representing the Arctic-Alpine montane element. Fossombonia angulosa also occurs here, and is at its northernmost locality. The full range of habitats includes dwarf shrub heath, dwarf shrub moss tundra, low bog, meadow, freshwater marsh lair grassland, maritime communities and 12 grassland associations. Over 130 species of flowering plants have now been recorded from the island group.