North Downs
The North Downs are a ridge of chalk hills and an area of downland in south-east England, that stretch from Farnham in Surrey to the White Cliffs of Dover in Kent. Much of the North Downs comprises two Areas of Outstanding Natural Beauty : the Surrey Hills and the Kent Downs. The North Downs Way National Trail runs along the North Downs from Farnham to Dover. They form the northern boundary of the Weald, which is correspondingly bounded to the south by the South Downs.
The highest point in the North Downs is Botley Hill, Surrey. The County Top of Kent is Betsom's Hill, which is less than 1 km from Westerham Heights, Bromley, the highest point in Greater London at an elevation of.
Toponymy
This term is used to describe the characteristic landscape of 'elevated rolling grassy hills' in southern England, where chalk and limestone is exposed at the surface, acquiring this sense around the 14th century. The name "downs" is derived from the Celtic word "dun", meaning "fort" or "fastness", though the original meaning would have been "hill", as early forts were commonly hillforts - compare Germanic "burg" and "berg". These hills are prefixed 'North' to distinguish them from a similar range of hills – the South Downs – which runs roughly parallel to them but some away on the southern edge of the Weald that divides the two ranges of hills.Geography
The narrow spine of the Hog's Back between Farnham and Guildford forms the western extremity of the North Downs, whilst the cliffs between Folkestone and Deal terminate the ridge in the east. The North Downs cuesta has a steep south-facing scarp slope and a more gentle north-facing dip slope. Its southern boundary is defined by the low-lying Vale of Holmesdale at the foot of the escarpment, in which the underlying stratum is primarily gault clay. The northern boundary is less apparent but occurs where the chalk submerges below the more recent Paleocene deposits.The Downs are highest near the Kent–Surrey border, often reaching heights in excess of above sea level at the crest of the escarpment. The highest point is Botley Hill in Surrey at. The County top of Kent at Betsom's Hill, with a height of, is nearby, and the highest point in Greater London, Westerham Heights, at, is on the northern side of the same hill. East of the Medway Valley the Downs become broader and flatter, extending as far as the Isle of Thanet.
The ridge of the North Downs is intersected by the valleys of a series of rivers: the Wey, Mole, Darent, Medway and Stour. These drain much of the Weald to the south. The western rivers are tributaries of the Thames; they have carved steep valleys through the chalk and provide natural corridor routes. In addition to existing rivers, the Downs are crossed by a number of wind gaps – prehistoric river valleys no longer occupied by rivers – including those at Farnham, Betchworth, Caterham, Lyminge and Hawkinge. Except for the river valleys and wind gaps, the crest of the escarpment is almost continuous along its length. The dip slope is dissected by many small dry valleys, and in the broad eastern part in Kent, by further river valleys such as that of the Little Stour.
Leith Hill is sometimes incorrectly referred to as part of the North Downs, but it is located on the parallel Greensand Ridge and does not consist of chalk.
Geology
Rock types
The Downland of the North Downs consists of distinct lithostratigraphic units :- 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.
- 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. This hypothesis is supported by the presence of flint, which is also found in the underlying strata.
- The Chalk Group, composed almost entirely of chalk, a soft, fine-grained limestone. These strata have their origins in the late Cretaceous. 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. 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. 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.
Wealden uplift and erosion
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, 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. Since chalk contains a very high percentage of calcium carbonate, it can be dissolved by both groundwater and rainwater.
Across much of the North Downs, the chalk dips below the London Clay at a relatively shallow angle, however at the far west of the range, local faulting has produced a much steeper angle, resulting in the distinctive Hog's Back between Farnham and Guildford.
Ecology
The North Downs support several important habitats including chalk grassland, scrub, woodland and chalk heath.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 from dominating. 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. 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.
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.
Quarries and chalk cliffs
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.Woodland
Woodland was far more extensive on the North Downs prior to human clearance 5000 to 4000 years ago. Fragments still remain particularly where the layer of clay-with-flints overlying the chalk has inhibited clearance. Where this is the case species such as pedunculate oak predominate, although much woodland has been replanted with conifer and sweet chestnut.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.