The higher parts of Hampstead town and all of Sandy Heath and West Heath are an outcrop, up to 25m thick, of Bagshot Sands, rare so near London. Much of East Heath and all of Parliament Hill Fields are on the clay of the Claygate Beds. In some places the sand appears coarse yellow, while in others it is finer and light in colour, mixed with loam and sandy clay. Below it lies at least 90m of London Clay, which is rich in fossils. Beneath that, lie the Woolwich and Reading Beds, Thanet Sands, Upper and Lower Chalk, Chalk Marl and Gault. In places the sand and gravel rest on spongy ground from which issue Hampstead’s many springs.
The cessation of grazing and the removal of sand and gravel during the war years has allowed trees and shrubs to take hold on former open heath. The effect has been to turn much of what was originally traditional heath into woodland. The Heath today contains areas of ancient woodland, bog, ponds, acidic grassland and other habitat types.
One of the Hampstead Heath Conservation Unit’s main tasks has been to restore the West Field Bog, a S.S.S.I. Encroaching birches have been removed, dams made to increase the saturated area and the site fenced to help re-establishment, protect the sphagnum moss and encourage bog plants to grow again.
Other important conservation projects include hedgerows and coppicing. Hedgerows are renewed and strengthened while new ones are established. Coppicing is an ancient method of woodland scrub management that involves cutting certain species to the ground to allow multi-stem regeneration and gives greater diversity of habitats.
The large number of visitors to the Heath causes erosion on some patches of land. These patches are reinstated by planting native species and keeping the areas fenced until the planting has become established.
New conservation projects, such as experiments to create wildflower meadows, have taken place and over-intrusive sycamores have been removed to allow new grass, flowers and other trees to grow.
Hampstead Heath offers a diverse range of habitats attractive to many birds such as sparrows, starlings, kestrels, nuthatches, tawny owls and woodpeckers, and small mammals, including voles, water rats, weasels, grass snakes, slow worms, badgers and hares.