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88-653: The Pipp Brook is a left-bank tributary of the River Mole , Surrey , England. It rises at two main springs north of Leith Hill on the Greensand Ridge , then descends steeply in a northward direction, before flowing eastwards along the Vale of Holmesdale . It runs to the north of Dorking High Street, before discharging into the Mole at Pixham . The Milton Brook, which rises at a tri-forked source lower than that of

176-408: A Local Nature Reserve . The Mole Gap forms part of a Special Area of Conservation and is an SSSI of European importance. The river has captured the imagination of several authors and poets , particularly since in very hot summers the river channel can become dry between Dorking and Leatherhead, most recently in 2022. In John Speed 's 1611 map of Surrey, this stretch of the river is denoted by

264-534: A Special Area of Conservation and is an SSSI of European importance. The stretch of river between Thorncroft Manor (1 km [0.6 mi] south of Leatherhead ) and River Lane in Fetcham has been designated a Local Nature Reserve. Although much of the surrounding land has been taken by residential and commercial development, this section of the Mole supports 20 different mammal species, 20 butterfly species and 15 species of dragonfly . The geology of

352-453: A 'Dorking Flood Area' comprising the sewage works, a handful of properties in Pixham and much of Brockham and Betchworth well upstream along the Mole to the east which are separated by more than a mile from Dorking and are narrowly in its post town. By the 19th century six grain mills stood on the river: Rookery, Westcott, Milton Court, Parsonage, Pippbrook and Pixham Mills. All were defunct by

440-429: A 3 by 3 tensor. The tensor is realised using a 3 by 3 matrix being both symmetric and positive definite (SPD matrix): The permeability tensor is always diagonalizable (being both symmetric and positive definite). The eigenvectors will yield the principal directions of flow where flow is parallel to the pressure gradient, and the eigenvalues represent the principal permeabilities. These values do not depend on

528-582: A Grand Imperial Ship Canal from Deptford to Chichester passing through the Mole Gap, however he was unable to attract sufficient financial interest in his scheme. Today the Mole is navigable for the 400 m (440 yd) from the confluence with the River Thames to Molember Weir at East Molesey where there is a private mooring facility. The river may be paddled all year by canoe between Brockham and Fetcham with an appropriate licence from

616-504: A diverse fish population, including chub, dace, roach, bleak , large pike and barbel. At the confluence of the Mole and the River Thames at Hampton Court Palace it is possible to catch a wide variety of coarse fish species. The geographical distribution of many species of invertebrate in the river reflects the geology of the catchment area. Viviparid snails and water scorpions (of the genus Nepidae ) are commonly found where

704-666: A flat-bottomed dug-out canoe were found at the confluence of the Mole and Thames in 1877 by a local boatman. The canoe is preserved at the Pitt Rivers Museum in Oxford . The Mole Gap and North Downs do not appear to have been settled until the late Stone Age: A large axe, typical of a "rough-out" axe produced during the Neolithic period, which was discovered in 1952 during building work in Westhumble, A flint mine of

792-468: A fraction to several thousand millidarcys. The unit of square centimetre (cm ) is also sometimes used (1 cm = 10 m ≈ 10 d). The concept of permeability is of importance in determining the flow characteristics of hydrocarbons in oil and gas reservoirs, and of groundwater in aquifers . For a rock to be considered as an exploitable hydrocarbon reservoir without stimulation, its permeability must be greater than approximately 100 md (depending on

880-639: A gravel spawning area to encourage chub and dace in addition to roach . In 1974 zander , a non-indigenous coarse fish native to Europe, were introduced legally to Old Bury Hill Lake which supplies the Pipp Brook. Zander have been caught in the Lower Mole below Dorking since the 1980s. In the Mole Gap between Dorking and Leatherhead the river supports populations of chub, dace, barbel , and brown trout. Both barbel and brown trout are extremely sensitive to water quality and pollution. Below Leatherhead

968-416: A lower flood plain, which broadens as the river turns in an axehead meander. At the east end of Painshill Park, the flood plain narrows into a trench about 60 m (66 yd) wide, in which the river runs northwards for 6.5 km (4.0 mi) towards Hersham, where the river enters the flood plain of the River Thames. Between Cobham and Esher, the Mole's historic courses have deposited gravel on top of

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1056-613: A north-easterly direction via Richmond to meet the proto-Thames near Ware in Hertfordshire . Today the Mole and Wey are less than 2 km (1.2 mi) apart at their closest point near Painshill Park. During the Middle Pleistocene period, a large ice sheet built up across much of the East of England, reaching as far south as St Albans and Chelmsford , blocking the path of the proto-Thames. Glacial meltwater from

1144-450: A series of hills accompanied by the legend "The river runneth under". However the river's name is unlikely to have derived from this behaviour: The Oxford Dictionary of English Place Names suggests that Mole either comes from the Latin mola (a mill) or is a back-formation from Molesey (Mul's island). Domesday Book lists twenty mills on the river in 1086, of which Sidlow Mill

1232-543: A small lake at Baldhorns Park, before running eastwards through a largely rural area towards Crawley. The first tributaries to join the young river drain the northernmost part of St Leonard's Forest , between Horsham and Crawley, although much of the forest is in the catchment area of the River Arun . The Mole skirts the northern suburbs of Crawley where it is joined by its first major tributary, Ifield Brook, which drains Ifield Mill Pond . The first gauging station on

1320-473: A smaller 50-metre (160 ft) cliff at Ham Bank in Norbury Park . The sudden change from impermeable Weald Clay to permeable chalk and the increased gradient of the river, allow the water table to drop below the bed of the river. Water is therefore able to flow out of the river through swallow holes in the bed and banks, decreasing the volume of water carried in the main channel. The course of

1408-459: A survey in 1958, the geologist C. C. Fagg identified 25 active swallow holes between Dorking and Mickleham ; the majority were only a few centimetres in diameter and were located in the vertical banks of the river below the water line. Most holes were difficult to observe in times of normal or heavy flow and were susceptible to silting up as new holes were continually being formed. A few much larger swallow holes were also observed separated from

1496-498: Is a "charming Surrey steam" and "If Lord Lytton went to fish in the River Mole – and there are still angling clubs there – he would find that he would still be able to catch a trout ". Highly polluting discharges have become less common but have taken place since 2000. In 2003, Gatwick Airport Ltd pleaded guilty to charges of allowing chemical pollution to enter the River Mole after a detergent, used to clean rubber and oil from

1584-579: Is also found along the river, and the downy emerald ( Cordulia aenea ) is found between Box Hill and Leatherhead. The Mole is one of only three locations in England where the river shingle beetle ( Meotica anglica ) is found. A nationally scarce species which is locally common on the River Mole is the greater dodder ( Cuscuta europaea ), a parasitic plant . The Mole catchment area includes twelve Sites of Special Scientific Interest (SSSIs) that include wetland habitats, The Mole Gap forms part of

1672-449: Is an indication of the ability for fluids (gas or liquid) to flow through them. Fluids can more easily flow through a material with high permeability than one with low permeability. The permeability of a medium is related to the porosity , but also to the shapes of the pores in the medium and their level of connectedness. Fluid flows can also be influenced in different lithological settings by brittle deformation of rocks in fault zones ;

1760-789: Is first recorded in the Red Book of Thorney in AD 983 as Emen and in the AD 1005 Cartulary of the Abbey of Eynsham as both Emen and Æmen . Variations in the name are recorded throughout the Middle Ages and the river appears as Amele in the Domesday Book of 1086, and subsequently as Emele in 12th- and 13th-century Court Rolls. This name is probably derived from the Old English word æmen meaning misty or causing mists , and

1848-415: Is important in petroleum engineering , when considering the optimal extraction of gas from unconventional sources such as shale gas , tight gas , or coalbed methane . To model permeability in anisotropic media, a permeability tensor is needed. Pressure can be applied in three directions, and for each direction, permeability can be measured (via Darcy's law in 3D) in three directions, thus leading to

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1936-582: Is measured at a fourth gauging station, located at Castle Mill (39 m [128 ft] above OD). At Pixham the Mole meets the Pipp Brook , a tributary draining the northeastern slopes of Leith Hill. Between Dorking and Leatherhead the Mole cuts a steep-sided valley (the Mole Gap) though the North Downs , carving a 170-metre-high (560 ft) river cliff on the western flank of Box Hill and

2024-577: Is the scalar hydraulic permeability, and 1 is the identity tensor . Permeability is typically determined in the lab by application of Darcy's law under steady state conditions or, more generally, by application of various solutions to the diffusion equation for unsteady flow conditions. Permeability needs to be measured, either directly (using Darcy's law), or through estimation using empirically derived formulas. However, for some simple models of porous media, permeability can be calculated (e.g., random close packing of identical spheres ). Based on

2112-708: Is used. In common with much of the rest of the Weald, the earliest evidence of human settlement along the Upper Mole is from the Mesolithic Period (20,000–7000 BC). Mesolithic sites at Wonham, Flanchford and Sidlow. Finds at Wonham include arrowheads and a plano-convex knife. The Lower Mole appears to have been settled during the same period and a flint axe dating from Mesolithic period found on spit of land close to River Mole in Cobham in 1965. Remains of

2200-694: The British Canoe Union . During the Second World War several sites along the course of the Mole were fortified for the defence of London against invasion from the south. GHQ Line B ran along the North Downs from Farnham and Guildford, crossing the Mole Gap to the north of Dorking and following the river to Horley. Between Betchworth and Box Hill, the north bank of the River Mole was stabilised and made steeper to prevent wheeled vehicles from crossing. At Boxhill Farm, where access to

2288-466: The Hagen–Poiseuille equation for viscous flow in a pipe, permeability can be expressed as: where: Absolute permeability denotes the permeability in a porous medium that is 100% saturated with a single-phase fluid. This may also be called the intrinsic permeability or specific permeability. These terms refer to the quality that the permeability value in question is an intensive property of

2376-540: The Island Barn Reservoir near Molesey : the northern (and smaller) branch continues as the River Mole and the southern branch is known as the River Ember . The two rivers flow either side of the reservoir, before running side by side in a northeasterly direction, merging 400 m (440 yd) before the confluence with the River Thames, on the reach above Teddington Lock . (For the purposes of

2464-538: The Thames at Hampton Court Palace . The river gives its name to the Surrey district of Mole Valley . The Mole crosses the North Downs between Dorking and Leatherhead , where it cuts a steep-sided valley, known as the Mole Gap, through the chalk. Much of the catchment area lies on impermeable rock (including Weald Clay and London Clay ), meaning that the river level responds rapidly to heavy rainfall. During

2552-472: The Wey Gap is significantly higher than might be expected from natural rainwater percolation alone. It has been suggested that a proportion of the excess water originates from the Mole Gap. At Leatherhead the river leaves the chalk bedrock, moving onto London Clay . The river meanders across an alluvial plain between 400 and 800 m (440–870 yd) wide towards Cobham, where it begins to descend to

2640-721: The Anglian ice sheet caused the Thames to divert southwards and flood the valley of the Mole–Wey river, thus adopting its present route through London . Standard water quality of Mole and its tributaries has improved markedly since the 1990s. In 1990 the Environment Agency assessed 23% of the watercourses as Grade B (good) or better. In 2002 this figure was 60%. Investment in the Sewage Treatment Works in

2728-465: The Earlswood Brook. From Sidlow, the Mole turns northwest towards Brockham . A number of minor tributaries join the river from the west and are typically second order streams draining the woodland and arable land between Horsham and Dorking. The 18th-century weir at Betchworth was modified in 2004 to facilitate the installation of two 27.5 kW low-head hydro turbines . About 90% of

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2816-493: The London Clay. The depth of the deposits generally varies from 2.5 to 7 m (8–23 ft); the lower layers are generally highly compacted and cemented together with brick-red iron oxide, whereas the upper layers are loosely packed with angular flints and sand. Remains of a further gravel terrace, containing cherts and flints to a depth of 4 m (13 ft), line the east side of St George's Hill . The river

2904-497: The Mole Gap becomes or became subterranean, depending on amount of sedimentation. The Pipp Brook itself has no sewage treatment works, few plains which it drains and as such an extremely narrow and low risk area of riparian flooding. These factors are instead present as to a few parcels of land of east Pixham sometimes considered Dorking by the national media based on a 2010s shorthand by the Environment Agency which has

2992-601: The Mole catchment is located on the Gatwick Stream 3 km (1.9 mi) upstream of the confluence with the Mole: Crawley STW discharges 15,000 m (530,000 cu ft) of water per day, and in prolonged dry periods it accounts for up to 75% of the flow of the Mole downstream of the confluence. The mean flow measured at Horley gauging station (52 m [171 ft] above OD) is 1.40 m /s (49 cu ft/s). The Mole passes Horley to

3080-417: The Mole is south of Gatwick Airport (57 m [187 ft] above OD ). The mean flow is 0.33 m /s (12 cu ft/s) and the river ran dry at this point for the first time in the summer of 1995. The Mole runs under the airport runway in a culvert completed in 1985. The course of the Mole within the airport perimeter has been altered several times since commercial flights began in 1945; however

3168-412: The Mole leaves the chalk and turns northwestwards to flow across impermeable London Clay , winding towards Cobham. The water table rises at this point and much of the water which drained out of the channel through the chalk returns through springs in the riverbed. The aquifer at Fetcham is the only one in the entire catchment area. The next major tributary, The Rye , (which drains Ashtead ) joins to

3256-462: The Mole was used for navigation in the past: In the late 13th century, Thorncroft Manor (south of Leatherhead) purchased a shout , a type of boat up to 16 metres (52 ft) in length used to carry produce to market and it has been suggested that stone cut from quarries in Reigate was transported to London via the river. During the 17th century, two bills came before parliament to make sections of

3344-543: The Pipp Brook, is the principal tributary. The geological varying depth of greensand top layers caused by erosion towards the underlying clay means that there is no uniform spring line in this part of the Greensand Ridge . The tower is the second highest point in the South-East of England. The Pipp Brook's main tributary, the Milton Brook rises at three close points in farmland on the indented north-east edge of

3432-410: The River Mole in 2019: The marsh frog ( Pelophylax ridibundus , a non-native species introduced from Europe in the 1930s) is now commonly found in the upper Mole and its tributaries around Newdigate and Gatwick. There is no evidence that the presence of the frogs has had a deleterious effect on indigenous amphibians. A second non-native species, the edible frog ( Pelophylax kl. esculentus )

3520-422: The bed and banks. The amount of water lost from the river is significant and in very hot summers the channel can become dry between Mickleham and Thorncroft Manor; this was recorded most recently in 1949, 1976 and 2022. At Leatherhead , the river leaves the chalk and flows across impermeable London Clay . At this point, the water table rises enough for the water to flow back into the main river channel. In

3608-401: The brook drains entirely) and much larger Wotton. The Pipp Brook runs almost due east along its Dorking stretch north of much of the town and south of the steep scarp of the North Downs at Ranmore Common before joining the Mole at Pixham . The area around the confluence is the subject of occasional riparian (river) flooding as the rate of descent of the Mole decreases significantly and at

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3696-473: The canalisation of a short stretch of the River Mole between Betchworth and Dorking to facilitate the movement of chalk from quarry to market. In 1810 the engineer John Rennie proposed a canal linking the River Medway to Portsmouth which was to have a branch to London following the Mole for much of its length. Between 1825 and 1828 the architect and civil engineer Nicholas Wilcox Cundy proposed

3784-400: The catchment area has improved the quality of the discharges into the river, and modifications to the runway and apron drainage systems at Gatwick Airport mean that surface water is diverted to aerated pollution control lagoons and balancing ponds for treatments, including acid/alkali neutralisation, before release into the river. In 1972 a sub-debate on the future cleanliness of the Mole

3872-537: The cutting at the south end of Box Hill & Westhumble railway station , suggesting that even in its early history, the river had swallow holes. The author Daniel Defoe , who attended school in Dorking and probably grew up in the village of Westhumble , described the swallow holes in the River Mole in his book A tour thro' the Whole Island of Great Britain (first published in 1724): ...the current of

3960-414: The effect of temperature on the viscosity of the fluid flowing though the porous medium and to address other fluids than pure water, e.g. , concentrated brines , petroleum , or organic solvents . Given the value of hydraulic conductivity for a studied system, the permeability can be calculated as follows: Tissue such as brain, liver, muscle, etc can be treated as a heterogeneous porous medium. Describing

4048-539: The energy generated is fed into the regional electricity grid , while the remainder is used to supply the Betchworth Park Estate, where the weir is situated. The river leaves the Weald Clay at Brockham, passing Betchworth Castle and flowing briefly across greensand and Gault Clay to Pixham , 1 km (0.6 mi) northeast of Dorking . A mean flow of 3.74 m /s (132 cu ft/s)

4136-402: The entry to the Mole Gap. Above Meath Green (near Horley ), the terrace corresponds to the flood plain of the river, however from this point downstream to Brockham, the river cuts a narrow trench no more than 150 m (160 yd) wide. A similar trench is runs up several of the Mole's tributaries, notably along Deanoak Brook which follows a local strike . The presence of the trench protects

4224-450: The flow of biofluids (blood, cerebrospinal fluid, etc.) within such a medium requires a full 3-dimensional anisotropic treatment of the tissue. In this case the scalar hydraulic permeability is replaced with the hydraulic permeability tensor so that Darcy's Law reads Connecting this expression to the isotropic case, κ = k 1 {\displaystyle {\boldsymbol {\kappa }}=k\mathbb {1} } , where k

4312-422: The flow of water through a porous medium is called the hydraulic conductivity ( K , unit: m/s). Permeability, or intrinsic permeability, ( k , unit: m ) is a part of this, and is a specific property characteristic of the solid skeleton and the microstructure of the porous medium itself, independently of the nature and properties of the fluid flowing through the pores of the medium. This allows to take into account

4400-463: The forest. During the early nineteenth century the sulphate-rich (chalybeate) waters of the Mag's Well spring were attributed by a countryside travel writer as having aided a person's recovery. Both the Pipp Brook and the larger River Tillingbourne drain the northern slopes of Leith Hill and the watershed between the two rivers (which runs roughly north–south, marks the boundary between Westcott (which

4488-540: The ground conditions of a site are suitable for construction. Permeability is part of the proportionality constant in Darcy's law which relates discharge (flow rate) and fluid physical properties (e.g. dynamic viscosity ), to a pressure gradient applied to the porous media: Therefore: where: In naturally occurring materials, the permeability values range over many orders of magnitude (see table below for an example of this range). The global proportionality constant for

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4576-447: The impermeable Weald Clay . From the source to Dorking , the river drains an area of 340 km (130 sq mi), of which approximately 60% is on Wealden or Atherfield Clay, 20% is on Tunbridge Wells Sand and 20% is on greensand . Brickearth deposits are common in the valley around Betchworth and east of Dorking. The upper Mole catchment is dominated by a single broad terrace, which runs continuously from Tilgate Forest to

4664-453: The interface with the solid when the gas mean free path is comparable to the pore size (about 0.01 to 0.1 μm at standard temperature and pressure). See also Knudsen diffusion and constrictivity . For example, measurement of permeability through sandstones and shales yielded values from 9.0×10 m to 2.4×10  m for water and between 1.7×10  m to 2.6×10  m for nitrogen gas. Gas permeability of reservoir rock and source rock

4752-414: The land from flooding, rendering it suitable for agriculture . Between Dorking and Leatherhead the Mole cuts a steep-sided valley through the North Downs, creating a 170-metre-high (560 ft) river cliff on the western flank of Box Hill. The bedrock is permeable chalk and the water table lies permanently below the level of the riverbed, allowing water to drain out of the river through swallow holes in

4840-530: The local area is complex, since the river leaves the chalk of the Mole Gap at this point and flows over gravel and clay, creating a patchwork of different habitats including scrub , woodland , hedges , banks, and meadows as well as the water itself. The river also provides a corridor for wildlife through the centre of Leatherhead. The West End Common forms part of the Esher Commons , owned and managed by Elmbridge Borough Council . The River Mole forms

4928-613: The main river by a channel of about a metre. About six of these larger swallow holes were found to the west of the Burford Bridge Hotel , along the course of the A24 Mickleham Bypass during its construction in 1936. Initially the surveyors tried to fill the holes with rubble to prevent the foundations of the new road subsiding. However this proved to be impractical and they were instead covered by concrete domes, up to 18 m in diameter, each fully supported by

5016-471: The meanders visible on the 1839 tithe map in the 1.5 km (0.93 mi) stretch immediately north of the runway were reinstated in 1999, in a £1.2 million project to facilitate airport expansion. The Mole enters Surrey to the south of Horley, where it meets the Gatwick Stream , a tributary draining Worth Forest to the southeast of Crawley. The second-largest Sewage Treatment Works (STW) in

5104-606: The mechanisms by which this occurs are the subject of fault zone hydrogeology . Permeability is also affected by the pressure inside a material. The SI unit for permeability is the square metre (m ). A practical unit for permeability is the darcy (d), or more commonly the millidarcy (md) (1 d ≈ 10 m ). The name honors the French Engineer Henry Darcy who first described the flow of water through sand filters for potable water supply. Permeability values for most materials commonly range typically from

5192-405: The medium, not a spatial average of a heterogeneous block of material equation 2.28 ; and that it is a function of the material structure only (and not of the fluid). They explicitly distinguish the value from that of relative permeability . Sometimes permeability to gases can be somewhat different than those for liquids in the same media. One difference is attributable to "slippage" of gas at

5280-514: The mid-20th century when bread production was widely commercialised and the product nationally was transformed to slow its staleness. Pippbrook Mill and Pixham Mill survive and are Grade II listed buildings . The Environment Agency measures the water quality of the river systems in England. Each is given an overall ecological status, which may be one of five levels: high, good, moderate, poor and bad. There are several components that are used to determine this, including biological status, which looks at

5368-625: The name of the River Ember probably has its origins in this name. The name Mole does not appear until the 16th century, first occurring as Moule in Harrison's Description of Britain of 1577. The antiquarian William Camden uses the Latinized form Molis in the 1586 edition of Britannia and Michael Drayton is the first to use Mole in his poem Poly-Olbion published in 1613. In John Speed 's 1611 map of Surrey this stretch of

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5456-593: The nature of the hydrocarbon – gas reservoirs with lower permeabilities are still exploitable because of the lower viscosity of gas with respect to oil). Rocks with permeabilities significantly lower than 100 md can form efficient seals (see petroleum geology ). Unconsolidated sands may have permeabilities of over 5000 md. The concept also has many practical applications outside of geology, for example in chemical engineering (e.g., filtration ), as well as in Civil Engineering when determining whether

5544-456: The north of Leatherhead, before the river is crossed by the M25 motorway . At Cobham the river swings round in a pronounced axehead meander skirting Painshill Landscape Garden , where a 11-metre (35 ft) diameter waterwheel raises water 5 metres (16 ft) from the river to feed the ornamental lake in the park. From Painshill Park the river flows northeastwards to the Thames, passing to

5632-551: The quantity and varieties of invertebrates , angiosperms and fish. Chemical status, which compares the concentrations of various chemicals against known safe concentrations, is rated good or fail. Water quality of the Pipp Brook in 2019: Download coordinates as: River Mole The River Mole is a tributary of the River Thames in southern England . It rises in West Sussex near Gatwick Airport and flows north-west through Surrey for 80 km (50 miles) to

5720-537: The remainder of this article, the River Mole and the River Ember are treated as a single entity.) Prior to the last ice age , the River Thames followed a more northerly route to the North Sea , from Reading via Marlow , Chorleywood , St Albans , Hertford and along the present Suffolk – Essex border. During this period, the Mole is thought to have merged with the River Wey near Byfleet and then flowed in

5808-495: The river navigable . The first, in 1663, was passed by the House of Commons but was defeated in the House of Lords . During the following year (1664), an act was passed by both Houses of Parliament to make the River Mole navigable from Reigate to the River Thames, but was never executed. The only Surrey river to have been made fully navigable is the combined lower section of the River Wey . In 1798 William Marshall advocated

5896-481: The river being much obstructed by the interposition of those hills, called Box Hill ... it forces the waters as it were to find their way through as well as they can; and in order to do this, beginning, I say, where the river comes close to the foot of the precipice of Box-Hill, called the Stomacher, the waters sink insensibly away, and in some places are to be seen (and I have seen them) little channels which go out on

5984-495: The river from the north bank was required for the herd of dairy cows, a row of twelve concrete cylinders were cast as an anti-tank measure. Gun mounts were also installed to protect both Boxhill and Deepdene bridges and several pillboxes were installed. An anti-tank ditch was dug from the Stepping Stones eastwards across the fields belonging to Bradley Farm (now Denbies vineyard ). The river crossing at Sidlow Bridge

6072-411: The river has historically supported larger predatory fish including chub, perch , pike , and eels, however in recent years chub and eel numbers have begun to decline. North of Esher the old river channel is dominated by floating pennywort , a highly invasive weed, which cuts off all light to the river bed, reducing oxygen levels and resulting in a poor habitat for fish. The Ember flood relief channel has

6160-401: The river is denoted by a series of hills accompanied by the legend "The river runneth under". However the river's name is unlikely to have derived from this behaviour: The Oxford Dictionary of English Place Names suggests that Mole either comes from the Latin mola (a mill) or is a back-formation from Molesey (Mul's island). In John Rocque 's 1768 map of Surrey, the name Moulsey River

6248-570: The river runs over the London Clay . Crayfish are common in areas associated with high alkalinity , particularly around Brockham, and the tributaries which run over the Weald Clay provide an excellent habitat for stoneflies , caddisflies , fast swimming mayflies and riffle beetles . The beautiful demoiselle ( Calopteryx virgo ) disappeared from the River Mole during the 1960s owing to deteriorating water quality, but has since recolonised. The white-legged damselfly ( Platycnemis pennipes )

6336-588: The river through Norbury Park was partially straightened when the Epsom to Horsham railway was built in 1837, with the removal of a small meander north of Westhumble . The meander was reinstated in 1997, in an attempt to enhance this area of the Park, although it has since become blocked by silt . The entirety of the Mole Gap lies within the Surrey Hills Area of Outstanding Natural Beauty . At Leatherhead,

6424-530: The runway, was washed into Crawters Brook by airport workers. The Environment Agency estimated that up to 5200 fish of 14 different species were killed as the pollution drained downstream. The airport was fined £30,000 by Lewes Crown Court . In May 2003, sewage leaking from a pump operated by Thames Water leaked into the Stanford Brook, killing coarse fish in the Gatwick stream. Water quality of

6512-483: The same period has been discovered at East Horsley along with Neolithic flakes of flint at Fetcham and Headley Heath . Significant Bronze Age finds include a bronze sword found close to the river north of Amberley Farm near Charlwood and a small hoard of weaponry consisting of two palstave axes and a scabbard chape was discovered in 2003 in Norbury Park close to Ham Bank. It is not clear to what extent

6600-431: The second half of the 20th century, pollution levels in the river were high; however, since 1995 the water quality has improved dramatically and the Mole now boasts the greatest diversity of fish species of any river in England. Twelve Sites of Special Scientific Interest (SSSIs) that include wetland habitats are located within the Mole catchment area , and the stretch of river through Leatherhead has been designated

6688-493: The sides of the river, where the water in a stream not so big as would fill a pipe of a quarter of an inch diameter, trills away out of the river, and sinks insensibly into the ground. In this manner it goes away, lessening the stream for above a mile, near two, and these they call the Swallows. Not all of the water removed from the river by the swallow holes is returned to the channel at Leatherhead . The chalk aquifer also feeds

6776-555: The southwest of Dorking . There is only one aquifer in the drainage basin, at Fetcham , which means that the majority of the water in the river is from surface drainage, particularly from Gatwick Airport and the urban areas of Horley and Crawley, and that the flow rate responds rapidly to rainfall. The Mole rises in Baldhorns Copse 700 m (0.4 mi) to the south of the village of Rusper in West Sussex . It flows initially southwards for 1 km (0.6 mi) to

6864-512: The springs at the southern end of Fetcham Mill Pond, which have never been known to run dry. A survey in March 1883 estimated that the Fetcham springs were producing about 3.6 million imperial gallons (16,000 m ) every day. A second survey in 1948 estimated that the same springs were yielding about 5 million imperial gallons (23,000 m ) a day. The water table in the chalk of

6952-469: The surrounding chalk and provided with a manhole and access shaft to allow periodic inspection. In the late 1960s the domes were reopened and inspected and the alluvium in the largest swallow hole was observed to have subsided by 1.5 metres (4 ft 11 in) under the centre of one of the domes. When the Dorking to Leatherhead railway was constructed in 1859, a fossilised swallow hole was discovered in

7040-487: The then Prime Minister Clement Attlee , replacing those destroyed during the Second World War as an anti-invasion measure. The location is popular with anglers and families, but swimming is strongly discouraged as the water is polluted in places. The stones give their name to the pub in the nearby village of Westhumble . Permeability (earth sciences) Permeability is a property of porous materials that

7128-607: The west bank of the Mole, west of the Island Barn Reservoir. The Heath is an area of rough scrubland and, despite its name, is a reclaimed landfill site hence its large but artificial mound and small plateau. The site is a rich habitat for birds including the redshank and little ringed plover . The Mole rises south of Rusper in West Sussex, where an outcrop of the Hastings Beds sandstone dips below

7216-459: The west of Esher Commons and then forming the boundary between Hersham and Esher , where the mean flow of the river is 5.43 m /s (192 cu ft/s). In response to heavy flooding of East Molesey and Thames Ditton in September 1968 , the river was modified downstream of Albany Bridge to the Thames and new flood defences were constructed. Finally the Mole splits into two branches at

7304-499: The west, flowing north towards Sidlow and entering a largely rural area. 0.7 km (0.4 mi) south of Sidlow the mean flow is measured as 2.21 m /s (78 cu ft/s) at Kinnersley Manor gauging station (48 m [157 ft] above OD). The Earlswood Brook, a tributary draining the urban area of Reigate and Redhill , joins the Mole at Sidlow. The largest STW in the Mole catchment (Reigate STW) discharges up to 118,500 m (4,180,000 cu ft) per day into

7392-458: The western boundary of the common, flowing past a steep sandy area known locally as The Ledges . Plant species typical of ancient woodland are found in this area, including bluebells ( Hyacinthoides non-scriptus ), marsh marigolds ( Caltha palustris ) and golden saxifrages ( Chrysosplenium sp.). large bittercress ( Cardamine amara ) and the non-native Himalayan balsam ( Impatiens glandulifera ). Molesey Heath Local Nature Reserve lies on

7480-551: Was had by the House of Lords , involving a member of the South-East Strategic Committee of the Thames Conservancy (succeeded by the Environment Agency ) and four others. In this it was said, "...I can see no future for [The Mole]. The Mole Valley has been polluted. The Mole where I used to tickle trout in my youth is a drain, and it will remain a drain." Rebuttals included that the Mole

7568-457: Was heavily defended and a line of pill boxes was constructed on the north bank of the river. Concrete anti-tank dragon's teeth were built on both sides of the river, a short distance upstream of the bridge, as an obstacle to armoured vehicles . The North Downs Way crosses the river at Box Hill via seventeen hexagonal stepping stones , which are frequently submerged after heavy rainfall. The current stones were dedicated in September 1946 by

7656-413: Was introduced to a site at Newdigate in the early 1900s. It has in 2008 been recorded in tributaries of the River Mole at Capel and Brockham . The River Mole has the most diverse fish population of any river in England. The Gatwick Stream is dominated by coarse fish such as brown trout , brook lamprey , and eel . In 2003, the upper River Mole near Meath Green Lane, Horley, was enhanced to create

7744-465: Was the oldest, dating from Saxon times . The drainage area of the Mole is 477 km (184 sq mi) and forms 5% of the River Thames 's catchment area above Teddington . The catchment area receives 761 mm (30.0 in) of rain each year; the greatest average level of rainfall is 800 mm (31.5 in) around Crawley . The Mole catchment reaches a maximum elevation of 265 m (869 ft) above Ordnance Datum at Leith Hill to

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