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 (AONBs): 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 ( above sea level). The County Top of Kent is Betsom's Hill ( above sea level), 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" (and by extension "fortified settlement", from which it entered English as "town", similar to Germanic "burg"/"burough"), though the original meaning would have been "hill", as early forts were commonly hillforts - compare Germanic "burg" (fort) and "berg" (mountain). 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
thumb|Geology of the South East, [[chalk is light green (6)]] thumb|right|View from North Downs towards [[Reigate]]
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 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 (the highest point in Surrey) 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 (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.
- 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.
thumb|The exposed chalk workings of the former [[Betchworth Quarry and Lime Kilns|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. 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).
Wealden uplift and erosion
thumb|right|A cross-section, showing the [[Weald-Artois Anticline|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.
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 (a process known as solifluction). Since chalk contains a very high percentage of calcium carbonate, it can be dissolved by both groundwater
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.
Ecology
The North Downs support several important habitats including chalk grassland, scrub, woodland and chalk heath.
Chalk grassland
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. 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.
Woodland
Woodland was far more extensive on the North Downs prior to human clearance 5000 to 4000 years ago. Where this is the case species such as pedunculate oak predominate, although much woodland has been replanted with conifer and sweet chestnut.
Scrub
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.
Chalk heath
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.
History
Worked flints and human and animal remains dating from the Lower Paleolithic, about 400,000 years ago, were discovered at Barnfield Pit near Swanscombe; this provides the earliest evidence of human occupation in the North Downs. It is suggested that human activity at that time coincided with intermittent warm phases during the last glacial period and that continuous occupation of the Downs did not occur until warming after the glaciation. Flint axes have also been found on river terraces at Farnham, on Walton and Banstead Heaths and on the crest of the escarpment above Folkestone.