| North Downs | |
|---|---|
 The North Downs near the entrance to theChannel Tunnel inFolkestone | |
| Highest point | |
| Peak | Botley Hill |
| Elevation | 270 m (890 ft) |
| Naming | |
| Etymology | Old English dūn, meaning 'hill' |
| Geography | |
 | |
| Country | England (United Kingdom) |
| Region(s) | Surrey,Kent |
| Parent range | Southern England Chalk Formation |
| Geology | |
| Orogeny | Alpine orogeny |
| Rock age | Cretaceous |
| Rock type | chalk |
TheNorth Downs are a ridge ofchalk hills in south eastEngland that stretch fromFarnham inSurrey to theWhite Cliffs of Dover inKent.[1] Much of the North Downs comprises twoAreas of Outstanding Natural Beauty (AONBs): theSurrey Hills and theKent Downs. TheNorth Downs Way National Trail runs along the North Downs from Farnham toDover.
The highest point in the North Downs isBotley Hill, Surrey (270 m (890 ft) above sea level). TheCounty Top of Kent isBetsom's Hill (251 m (823 ft) above sea level), which is less than 1 km fromWesterham Heights,Bromley, the highest point inGreater London at an elevation of 245 m (804 ft).
'Downs' is from Old Englishdun, meaning, amongst other things, "hill". The word acquired the sense of "elevated rolling grassland" around the 14th century.[2] The name contains "North" to distinguish them from a similar range of hills – theSouth Downs – which runs roughly parallel to them but some 50 km (31 mi) to the south.
The narrow spine of theHog's Back between Farnham andGuildford forms the western extremity of the North Downs, whilst the cliffs betweenFolkestone andDeal terminate the ridge in the east. The North Downscuesta has a steep south-facingscarp slope and a more gentle north-facingdip slope. Its southern boundary is defined by the low-lyingVale of Holmesdale at the foot of the escarpment, in which the underlying stratum is primarilygault clay. The northern boundary is less apparent but occurs where the chalk submerges below the more recentPaleocene deposits.[1]
The Downs are highest near the Kent–Surrey border, often reaching heights in excess of 200 m (660 ft) above sea level at the crest of the escarpment. The highest point isBotley Hill in Surrey at 269 m (883 ft).[3] TheCounty top of Kent atBetsom's Hill, with a height of 251 m (823 ft), is nearby,[4][5] and thehighest point in Greater London,Westerham Heights, at 245 m (804 ft), is on the northern side of the same hill.[4][6] East of theMedway Valley the Downs become broader and flatter, extending as far as theIsle of Thanet.
The ridge of the North Downs is intersected by the valleys of a series of rivers: theWey,Mole,Darent,Medway andStour. These drain much of theWeald to the south. The western rivers are tributaries of theThames; 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 ofwind gaps – prehistoric river valleys no longer occupied by rivers – including those atFarnham,Betchworth,Caterham,Lyminge andHawkinge. Except for the river valleys and wind gaps, the crest of theescarpment is almost continuous along its length. Thedip slope is dissected by many smalldry valleys, and in the broad eastern part in Kent, by further river valleys such as that of theLittle 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 parallelGreensand Ridge[7] and does not consist of chalk.[8]
TheDownland of the North Downs consists of distinctlithostratigraphic units (the types of strata, the names of which are always capitalised):
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 bythe same geological processes that created the Alps, resulting in ananticline which stretched across theEnglish Channel to theArtois region of northern France. Initially an island, thisdome-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 theCenozoic, 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 byfreeze-thaw action, which may occur over repeated annual ordiurnal cycles. This frost weathering produces a mix of rubble and viscous mud, which may be washed downhill, further eroding the landscape (a process known assolifluction).[16] Since chalk contains a very high percentage ofcalcium carbonate, it can bedissolved by both groundwater[16] and rainwater.[17]
Across much of the North Downs, the chalk dips below theLondon 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 distinctiveHog's Back between Farnham and Guildford.[18]
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The North Downs support several important habitats includingchalk grassland,scrub,woodland andchalk 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 (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 rabbitgrazing which prevents scrub encroachment. Chalk grassland to the west of theMedway Valley is dominated byupright brome andfescue, whilst grassland to the east is dominated bytor-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 rarelate spider orchid is limited within the British Isles to chalk grassland betweenFolkestone andWye. One of the two native British sites for the nationally raremonkey orchid is on the North Downs. The nationally scarceman orchid is not uncommon on chalk grassland on the Downs in Surrey and Kent. Other scarce plant species such asearly gentian,dwarf milkwort andbedstraw broomrape also occur on chalk grassland in the North Downs.
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Chalk grassland also supports a rich fauna, particularly insects. Notable butterfly species includeAdonis blue andsilver-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 theblack-veined moth. Other notable moth species that occur on the North Downs include thefiery clearwing moth and thestraw belle moth.
On the summit of the cliffs betweenDeal and Folkestone theearly spider orchid occurs in large numbers, as well as the rareoxtongue broomrape. Naturally exposed chalk is rare inland with the exception of the river cliffs formed by theRiver Mole on the west face of Box Hill and at Ham Bank inNorbury Park. However, quarry lakes within chalk pits provide habitats forgreat crested newt. The scarcemusk orchid has colonised disused chalk pits nearHollingbourne 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 ofclay-with-flints overlying the chalk has inhibited clearance.[21] Where this is the case species such aspedunculate oak predominate, although much woodland has been replanted withconifer andsweet chestnut.[1]
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 thedip slope. Calcareous woodland is typically dominated bybeech,hornbeam,yew andash.Box Hill has one of the largest areas of nativebox woodland in England. One notable species characteristic of calcareous woodland is the nationally scarcelady 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 theRoman snail,rufous grasshopper and thedark green fritillary.
In Surrey,superficial deposits from theQuaternary 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), allowingacid-loving plants to thrive alongside those thatprefer alkaline conditions, producing the rarechalk heath habitat.
Worked flints and human and animal remains dating from theLower Paleolithic, about 400,000 years ago, were discovered atBarnfield Pit nearSwanscombe; 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[23] 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,[24] onWalton andBanstead Heaths and on the crest of the escarpment above Folkestone.[21] There is considerable evidence ofMesolithic activity in the Surrey Downs through the discovery ofpit-dwellings at Weston Woods nearAlbury and the quantity of discarded tools,microliths and other implements discovered.[21]
In about 3000 BC the emergence ofNeolithic culture saw the lifestyle of the Mesolithic hunter-gatherers shift to a more sedentary and communal lifestyle that relied upon the keeping of livestock and the growing of crops.[23] There is substantial evidence ofNeolithic activity within the North Downs, notably thelong barrows concentrated in the Medway and Stour valleys. TheMedway long barrows, which includeKit's Coty House andColdrum Stones, are constructed ofsarsen stone, locally found onBlue Bell Hill and in the valleys of the dip slope, whilst the Stour Valley long barrows are constructed of earth.[23]
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The landscape of the Downs has been greatly shaped by man. There has beendeforestation of the Downs for centuries. The woodland that remains today is largely located where deforestation has been inhibited by steep slopes or by the layer of clay with flints, which is difficult to plough. Because of the many small farms that have survived on the Downs, a network of narrow lanes and minor roads has developed. This has resulted in a landscape similar to that found in Normandy known asbocage. The predominant type of farming on the Downs is arable farming; this increased greatly during the 20th century. Pastoral farming also occurs but to a lesser extent. Human settlements within the Downs have generally formed in sheltered valleys and at the foot of the scarp slope (known as spring line settlements). In recent years vineyards have been planted along the southern slopes of the Downs, in particular theDenbies Wine Estate,Dorking, which is the largest vineyard in the country, accounting for 10% of the country's vines. The chalky soils are similar to those of theChampagne region.
There is plenty of evidence of chalk extraction on the Downs, particularly along the scarp slope. Oil has been drilled at the foot of the Downs in several locations in Surrey. TheKent Coalfield was established in the late 19th century after coal was found in 1890. Four successful collieries continued to be worked through much of the 20th century:Betteshanger,Snowdown,Tilmanstone andChislet; Bettershanger was the last to close in 1989. The east to west ridge of the Downs has provided a natural transport route for centuries. Much of the historicPilgrims' Way still survives at the foot of the scarp slope, and this has been joined much more recently by theM20 motorway. The scarp slope has also been used for fortification; many examples of this still exist, such as Thurnham Castle and on Castle Hill, Folkestone Castle Hill near Folkestone.
Many beauty spots on the North Downs are owned by theNational Trust, for instanceBox Hill andLangdon Cliffs. Other areas of interest areNewland's Corner,Wye Downs and theHog's Back. Much chalk grassland and ancient woodland on the Downs is open-access, and there are also many historic sites such asKit's Coty House (near Maidstone) andDover Castle.
The topography of the North Downs means there are relatively few summits withtopographic prominence. Below is a list of summits on the North Downs which are more than 180 metres in height with 30 metres of topographic prominence (a commonly used threshold in Britain). The summits are arranged in order along the range, from west to east.
| Hill | Height (m) | Prominence (m) | OS grid reference |
|---|---|---|---|
| Dunley Hill | 227 | 105 | TQ104490 |
| Box Hill | 224 | 53 | TQ203517 |
| Reigate Hill | 235 | 100 | TQ255520 |
| White Hill | 233 | 30 | TQ328532 |
| Gravelly Hill | 237 | 64 | TQ337532 |
| Winders Hill | 236 | 47 | TQ353540 |
| Botley Hill | 270 | 209 | TQ387551 |
| Wrotham Hill | 235 | 129 | TQ593600 |
| Detling Hill | 200 | 163 | TQ804586 |
| West Down | 186 | 73 | TR091453 |
| Tolsford Hill | 183 | 61 | TR159386 |
| Cheriton Hill | 188 | 150 | TR197396 |
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