North Downs

The Downland of the North Downs consists of distinct lithostratigraphic units (the types of strata, the names of which are always capitalised):

• Shallow layers of sand and gravel are associated with areas of heathland, including at Headley and Netley Heaths in Surrey. These deposits have their origin in the Quaternary and indicate the position of the sea shore during this period.[9]

• Clay-with-Flints, a sandy clay with many flints is found on higher ground. The origins of this layer are uncertain, however the clay is thought to have been formed during several periods of glaciation and was produced by cryoturbation and decalcification of the chalk.[10] This hypothesis is supported by the presence of flint, which is also found in the underlying strata.[10][11]

The exposed chalk workings of the former Betchworth Quarry on the south-facing scarp slope of Box Hill.

• The Chalk Group, composed almost entirely of chalk, a soft, fine-grained limestone. These strata have their origins in the late Cretaceous (approximately 100 – 66 million years ago). For the entirety of this period, south east England was covered by a warm, shallow sea in which coccolithophores, single-celled algae with small calcite skeletons, thrived. As the phytoplankton died, their calcium-rich shells were deposited on the sea bed and, over time were compressed to form rock.[12] The chalk of the North Downs is generally divided into three distinct stata: The Upper Chalk, which has many flints; the Middle Chalk, with fewer flints; and the Lower Chalk or Coombe Rock, greyish, with few flints. The chalk is most commonly exposed on slopes or as cliffs, where the overlying acidic strata have been quarried or washed away. The buried upper surface of the chalk beneath the acidic strata is often eroded into pipes, gulleys and pinnacles, sometimes visible in road cuttings and quarries.

• The Upper Greensand Formation, a whitish, limy sandstone, often used for building, for which it has been mined from beneath the chalk (for example from the Godstone Baby Mines). The Upper Greensand of the North Downs is a thin bed of one or two metres thickness, and it is rarely visible at the surface (it is much thicker elsewhere).

A cross-section, showing the Weald Anticline, and relating it to the towns of Kent

Following the Cretaceous, the sea covering the south of England began to retreat and the land was pushed higher. The Weald (the area covering modern day south Surrey, south Kent and north Sussex) was lifted by the same geological processes that created the Alps, resulting in an anticline which stretched across the English Channel to the Artois region of northern France. Initially an island, this dome-like structure was drained by the ancestors of the rivers which today cut through the North and South Downs. The dome was eroded away over the course of the Cenozoic, exposing the strata beneath and resulting in the escarpments of the Downs.[13][14]

Chalk is a relatively soft rock that may be eroded or weathered in a number of different ways. It is porous, absorbing up to 20% of its dry weight in water,[15] and is therefore highly susceptible to weathering by freeze-thaw action, which may occur over repeated annual or diurnal cycles. This frost weathering produces a mix of rubble and viscous mud, which may be washed downhill, further eroding the landscape (a process known as solifluction).[16] Since chalk contains a very high percentage of calcium carbonate, it can be dissolved by both groundwater[16] and rainwater.[17]

Across much of the North Downs, the chalk dips below the London Clay at a relatively shallow angle (c. 18° near Dorking), however at the far west of the range, local faulting has produced a much steeper angle (up to 55°), resulting in the distinctive Hog's Back between Farnham and Guildford.[18]

Chalk grassland is found on the steep south-facing scarp slopes and the more gently graded north-facing dip slopes, where woodland is absent. The alkaline soils are thin and nutrient poor, which prevents deeper-rooted lush grasses (with a high water demand) from dominating.[19] Each square metre of chalk downland may support up to 40 different species. This semi-natural habitat is maintained through sheep, cattle and rabbit grazing which prevents scrub encroachment. Chalk grassland to the west of the Medway Valley is dominated by upright brome and fescue, whilst grassland to the east is dominated by tor-grass.[1] Owing to the close proximity of the North Downs to the European continent, the warm climate and the south-facing escarpment, several plant species survive on the chalk grassland which are scarce or not found elsewhere in the British Isles.

The nationally rare late spider orchid is limited within the British Isles to chalk grassland between Folkestone and Wye. One of the two native British sites for the nationally rare monkey orchid is on the North Downs. The nationally scarce man orchid is not uncommon on chalk grassland on the Downs in Surrey and Kent. Other scarce plant species such as early gentian, dwarf milkwort and bedstraw broomrape also occur on chalk grassland in the North Downs.

Marbled white (left) and Adonis blue (right) butterflies photographed at Denbies Hillside (part of the Hackhurst and White Downs SSSI) near Dorking.

Chalk grassland also supports a rich fauna, particularly insects. Notable butterfly species include Adonis blue and silver-spotted skipper, which may be found on warm, sheltered, south-facing slopes. The chalk downland above the Stour Valley is the only British site for the black-veined moth. Other notable moth species that occur on the North Downs include the fiery clearwing moth and the straw belle moth.

On the summit of the cliffs between Deal and Folkestone the early spider orchid occurs in large numbers, as well as the rare oxtongue broomrape. Naturally exposed chalk is rare inland with the exception of the river cliffs formed by the River Mole on the west face of Box Hill and at Ham Bank in Norbury Park. However, quarry lakes within chalk pits provide habitats for great crested newt. The scarce musk orchid has colonised disused chalk pits near Hollingbourne in Kent.[20]

Woodland was far more extensive on the North Downs prior to human clearance 5000 to 4000 years ago.[1] Fragments still remain particularly where the layer of clay-with-flints overlying the chalk has inhibited clearance.[21] Where this is the case species such as pedunculate oak predominate, although much woodland has been replanted with conifer and sweet chestnut.[1]

Lady orchids growing in calcareous woodland and scrub, Denge Wood.

Calcareous woodland occurs on thin soils where chalk is close to the surface. These conditions are most often found on the escarpment of the North Downs and on valley slopes, but may also occur on the plateau of the dip slope. Calcareous woodland is typically dominated by beech, hornbeam, yew and ash. Box Hill has one of the largest areas of native box woodland in England. One notable species characteristic of calcareous woodland is the nationally scarce lady orchid, which is found in more than 100 sites on the Kent stretch of the North Downs, but is confined to just two sites elsewhere in the UK.[22]

Although its spread is often considered undesirable, scrub provides valuable habitat for a range of invertebrates, including the Roman snail, rufous grasshopper and the dark green fritillary.

In Surrey, superficial deposits from the Quaternary are found overlying the chalk. These sands and gravels indicate the position of a former sea shore. Where these deposits occur, they are thin and the chalk also comes to the surface in the same areas (e.g. Headley Heath on the north east side of Box Hill), allowing acid-loving plants to thrive alongside those that prefer alkaline conditions, producing the rare chalk heath habitat.