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36-582: Moat Mount is a large, hilly open space with extensive views from the top of the hill. Most of it lies over London Clay . Much of the Countryside Park is grassland, which has a diversity of wild flowers and many butterflies. The Park also has Leg of Mutton pond and several small woods, such as Target Wood and Nut Wood. Moat Mount also includes Moat Mount Outdoor Centre and Campsite, which has a range of activities for local schools and youth organisations. The Dollis Brook rises on Mote End Farm. The site

72-491: A matrix or base rock, but can commonly be found as entire free-floating crystals, often in clay beds (and as can desert roses). Satin spar is almost always prismatic and fibrous in a parallel crystal habit. Satin spar often occurs in seams, some of them quite long, and is often attached to a matrix or base rock. Desert roses are most often bladed, exhibiting the familiar shape of a rose, and almost always have an exterior druse. Desert roses form in wet sand, unattached to

108-564: A fingernail (hardness: 2 on Mohs Scale ). The rosettes are not quite as soft due to their exterior druse; nevertheless, they too can be scratched. Selenite crystals that exhibit in either reticular or acicular habits, satin spar, in general (as fibrous crystals are thin and narrow), desert roses that are thinly bladed, and gypsum flowers, particularly acicular gypsum flowers, can be quite brittle and easily broken. All four crystalline varieties can range in size from minute to giant selenite crystals measuring 11 meters long such as those found in

144-403: A fingernail) and its three unequal cleavages. Other distinguishing characteristics include its crystal habits, pearly lustre, easy fusibility with loss of water, and solubility in hot dilute hydrochloric acid . Though sometimes grouped together as "selenite", the four crystalline varieties have differences. General identifying descriptions of the related crystalline varieties are: Satin spar

180-432: A matrix (can be gypsum) or base rock. Gypsum crystals are colorless (most often selenite), white (or pearly – most often satin spar), or gray, but may be tinted brown, yellow, red, or blue by the presence of impurities, such as iron oxides or clay minerals . Gypsum crystals can be transparent (most often selenite), translucent (most often satin spar but also selenite and gypsum flowers), and opaque (most often

216-623: A matrix or base rock. Gypsum flowers are most often acicular , scaly , stellate , and lenticular . Gypsum flowers most often exhibit simple twinning (known as contact twins ); where parallel, long, needle-like crystals, sometimes having severe curves and bends, will frequently form “ram’s horns”, "fishtail", "arrow/spear-head", and "swallowtail" twins. Selenite crystals can also exhibit “arrow/spear-head” as well as “duck-bill” twins. Both selenite crystals and gypsum flowers sometimes form quite densely in acicular mats or nets; and can be quite brittle and fragile. Gypsum flowers are usually attached to

252-475: Is also used to line exhausted quarries. This is because old quarry holes are generally refilled with waste material and by lining it with London Clay (which is virtually impermeable) it prevents waste and hazardous substances from entering the groundwater. London Clay is an ideal medium for boring tunnels, which is one reason why the London Underground railway network expanded very quickly north of

288-561: Is also well developed in the Hampshire Basin , where an exposure 91 metres (299 ft) thick occurs at Whitecliff Bay on the Isle of Wight and around 101 metres (331 ft) is spread along 6 kilometres (4 miles) of foreshore at Bognor Regis , West Sussex . The clay was deposited in a sea up to 200 metres (660 ft) deep at the eastern end. Up to five cycles of deposition (representing transgression followed by shallowing of

324-435: Is more soluble and mobile than iron oxides and can further recrystallize to form larger crystals sometimes called selenite (coming from the moon , but not related to selenium , although the etymology is the same), or "waterstones". Large septarian concretions , produced by microbial activity (oxidation of organic matter ) in the ancient seafloor during clay early diagenesis , are also common. These have been used in

360-460: Is not frequently exposed as it is to a great extent covered by more recent Neogene sediments and Pleistocene gravel deposits. One location of particular interest is Oxshott Heath , where the overlying sand and the London Clay layers are exposed as a sand escarpment, rising approximately 25 metres (82 feet). This supported a thriving brick industry in the area until the 1960s. The London Clay

396-436: Is private land and there is no public access. 51°38′02″N 0°15′09″W  /  51.6338°N 0.2526°W  / 51.6338; -0.2526 London Clay The London Clay Formation is a marine geological formation of Ypresian (early Eocene Epoch, c. 54-50 million years ago) age which crops out in the southeast of England . The London Clay is well known for its fossil content. The fossils from

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432-411: Is recognised to be covered by the interpretation of subsidence in buildings insurance policies, and the periods of dry weather in 1976/77 and 1988/92, in particular, led to a host of insurance claims. As a result, many insurance companies have now increased the cost of premiums for buildings located in the most susceptible areas where damage occurred, where the clay is close to the surface. London Clay

468-426: Is shown separately on many geological maps, and often caps hills. It is up to 15 metres (49 ft) thick at Claygate , Surrey. It is now believed to be diachronous , with the formation at Claygate for example being the same age as the end of the fourth cycle of deposition further east. The presence of a thick layer of London Clay underneath London itself, providing a soft yet stable environment for tunnelling ,

504-789: Is sometimes cut into cabochons to best display its chatoyance . Crystal habit refers to the shapes that crystals exhibit. Selenite crystals show a variety of habits, but the most common are tabular, prismatic, or acicular (columnar) crystals, often with no imperfections or inclusions . Twinned crystals are common, and often take the form of "swallow tail" twins. Selenite crystals sometimes form in thin tabular or mica -like sheets and have been used as window panes as at Santa Sabina in Rome. Selenite crystals sometimes will also exhibit bladed rosette habit (usually transparent and like desert roses) often with accompanying transparent, columnar crystals. Selenite crystals can be found both attached to

540-423: Is the most common of all the sulfate minerals. Gypsum is formed as an evaporative mineral, frequently found in alkaline lake muds, clay beds, evaporated seas, salt flats, salt springs, and caves. It is frequently found in conjunction with other minerals such as, copper ores, sulfur and sulfides, silver, iron ores, coal, calcite, dolomite, limestone, and opal. Gypsum has been dated to almost every geologic age since

576-613: The Isle of Sheppey in Kent and Walton-on-the-Naze , Essex , in the London Basin, and Bognor Regis in the Hampshire Basin. Selenite (mineral) Selenite , satin spar , desert rose , and gypsum flower are crystal habit varieties of the mineral gypsum . All varieties of gypsum, including selenite and alabaster , are composed of calcium sulfate dihydrate (meaning that it has two molecules of water), with

612-546: The River Thames . However, south of the Thames, the stratum at tube level is composed of water-bearing sand and gravel (not good for tunnelling) with London Clay below, which partly explains why there are very few tube tunnels south of the Thames. London Clay has a stand-up time long enough to enable support to be installed without urgency. It is also almost waterproof , resulting in virtually no seepage of groundwater into

648-471: The Silurian Period 443.7 ± 1.5 Ma . In dry, desert conditions and arid areas, sand may become trapped both on the inside and the outside of gypsum crystals as they form. Interior inclusion of sand can take on shapes such as an interior hourglass shape common to selenite crystals of the ancient Great Salt Plains Lake bed, Oklahoma, US. Exterior inclusion (druse) occurs as embedded sand grains on

684-490: The lower Eocene rocks indicate a moderately warm climate, the tropical or subtropical flora. Though sea levels changed during the deposition of the clay, the habitat was generally a lush forest – perhaps like in Indonesia or East Africa today – bordering a warm, shallow ocean. The London Clay is a stiff bluish clay which becomes brown when weathered and oxidized. Nodular lumps of pyrite are frequently found in

720-589: The 15th century, it has named the transparent variety that occurs in crystals or crystalline masses. The name derives through Middle English selenite from Latin selenites , ultimately from Greek selēnítēs líthos ( σεληνίτης λίθος , lit.   ' moon stone ' ). It got this name because people historically believed the mineral waxed and waned with the cycles of the Moon. The main distinguishing characteristics of crystalline gypsum are its softness (hardness 2 on Mohs scale , soft enough to scratch with

756-527: The London Clay for almost 300 years. Some 350 named species of plant have been found, making the London Clay flora one of the world's most diverse for fossil seeds and fruits. The flora includes plant types found today in tropical forests of Asia and demonstrates the much warmer climate of the Eocene epoch , with plants such as Nypa (Nipah palms) and other palms being frequently encountered. Notable coastal exposures from which fossils can be collected are on

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792-460: The caves of the Naica Mine of Chihuahua , Mexico . The crystals thrived in the cave's extremely rare and stable natural environment. Temperatures stayed at 58 °C, and the cave was filled with mineral-rich water that drove the crystals' growth. The largest of those crystals weighs 55 tons, is 11 meters (36 ft) long, and is over 500,000 years old. Gypsum occurs on every continent and

828-461: The chemical formula CaSO 4 ·2H 2 O. Selenite contains no significant selenium  – The similar names both derive from Greek selḗnē ( σελήνη ' Moon '). Some of the largest crystals ever found are of selenite, the largest specimen found in the Naica Mine 's Cave of the Crystals being 12 metres long and weighing 55 tons. "Selenite" is mostly synonymous with gypsum, but from

864-443: The clay layers. Pyrite was produced by microbial activity ( sulfate reducing bacteria ) during clay sedimentation. Once clay is exposed to atmospheric oxygen, framboidal pyrite with a great specific surface is rapidly oxidized. Pyrite oxidation produces insoluble brown iron oxyhydroxide ( FeOOH ) and sulfuric acid leading to the formation of relatively soluble gypsum (CaSO 4 ·2H 2 O, calcium sulfate dihydrate). This latter

900-536: The clay. London Clay is highly susceptible to volumetric changes depending upon its moisture content. During exceptionally dry periods or where the moisture is extracted by tree root activity, the clay can become desiccated and shrink in volume, and conversely swell again when the moisture content is restored. This can lead to many problems near the ground surface, including structural movement and fracturing of buildings, fractured sewers and service pipes/ducts and uneven and damaged road surfaces and pavings. Such damage

936-738: The fibers and polished on the ends, satin spar exhibits an optical illusion when placed on a printed or pictured surface: print and pictures appear to be on the surface of the sample. It is often called and sold as the “television stone” (as is ulexite ). Some selenite and satin spar specimens exhibit fluorescence or phosphorescence . All four crystalline varieties are slightly flexible, though will break if bent significantly. They are not elastic, meaning they can be bent, but will not bend back on their own. All four crystalline varieties are sectile in that they can be easily cut, will peel (particularly selenite crystals that exhibit mica-like properties), and like all gypsum varieties, can be scratched by

972-501: The mid nineteenth century the field was part of the park of Highfield House, which was then owned by Lady Raffles, the widow of Stamford Raffles . Most of it is now used as pasture for cattle from the Royal Veterinary College . It has a varied topography with a pond and small streams in boggy areas, while grass areas have typical pasture herbs. Some of the oldest oaks mark eighteenth century field boundaries. The field

1008-763: The past for making cement. They were once dug for this purpose at Sheppey, near Sittingbourne , and at Harwich , and also dredged, off the Hampshire coast. The clay is still used commercially for making bricks , tiles, and coarse pottery in places such as Michelmersh in Hampshire. The London Clay is well developed in the London Basin , where it thins westwards from around 150 metres (492 feet) in Essex and north Kent to around 4.6 metres (15 feet) in Wiltshire . It

1044-578: The rosettes and flowers). Opacity can be caused by impurities, inclusions, druse, and crust, and can occur in all four crystalline varieties. Selenite typically shows vitreous luster , but may show pearly luster on cleavage surfaces. Satin spar shows characteristic silky luster. Luster is not often exhibited in the rosettes, due to their exterior druse; nevertheless, the rosettes often show glassy to pearly luster on edges. Gypsum flowers usually exhibit more luster than desert roses. Fibrous satin spar exhibits chatoyancy (cat's eye effect). When cut across

1080-611: The route of the Greenwalk and the Loop, and from Bedford Road. The Dollis Valley Greenwalk starts at the car park, and the London Loop goes through the reserve. Scratchwood is on the other side of Barnet Way, with access by an underpass at Courtland Avenue. Bruno's Field is a Site of Borough Importance for Nature Conservation , Grade II, at Grid Ref TQ215 934 , south of Moat Mount between Worcester Crescent and Marsh Lane. In

1116-548: The sea) have been found, most markedly at the shallower, western end. Each cycle begins with coarser material (sometimes including rounded flint pebbles), followed by clay which becomes increasingly sandy. The final cycle ends with the Claygate Beds. The youngest part of the London Clay, known as the Claygate Beds or Claygate Member forms a transition between the clay and the sandier Bagshot Beds above. This

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1152-469: The surface such as, commonly seen in the familiar desert rose. When gypsum dehydrates severely, anhydrite is formed. If water is reintroduced, gypsum can and will reform – including as the four crystalline varieties. An example of gypsum crystals reforming in modern times is found at Philips Copper Mine (closed and abandoned), Putnam County, New York, US where selenite micro crystal coatings are commonly found on numerous surfaces (rock and otherwise) in

1188-520: The tunnel. It is over-consolidated, which means that it was once subject to an overburden pressure higher than it is subjected to today, and expands upon excavation, thus gradually loading the support, i.e. it is not necessary to stress the support against the ground. Due to its impermeability especially when exposed by ploughing, London Clay does not make good agricultural soil. In Middlesex , ploughing London Clay land so deep that it brings up clay has historically been called "ploughing up poison". It

1224-711: Was also disliked for building houses on, though inevitably most of Greater London is built on it. Jane Ellen Panton in her Suburban Residences and How To Circumvent Them (1896), remarks: "I do not believe clay is or even can be fit for anyone to reside upon ... though roses flourished magnificently children didn't, and coughs and colds [lasted through autumn and winter, this at Shortlands ". Many London buildings have ultimately been constructed with London Clay. When compressed and burnt, London Clay can be fashioned into brick known as London stock , identifiable by its yellowish brown hue. Plant fossils, especially seeds and fruits, are found in abundance and have been collected from

1260-492: Was instrumental in the early development of the London Underground , although this is also the reason why London had no true skyscraper buildings, at least to the same degree as many other cities throughout the world. Erecting tall buildings in London required very deep, large and costly piled foundations. This has changed in recent decades due to the development of 'plunge piles'. London's skyscrapers float on rafts embedded in

1296-617: Was once owned by Irwin Cox , the Conservative MP for Harrow from 1899 to 1906. The main entrance to the site is the permanently closed Moat Mount car park in the Countryside Park on the southbound lane of Barnet Way, a dual carriageway section of the A1 , between Stirling Corner and Apex Corner . There are also entrances to the reserve from the back of Mill Hill County High School , Barnet Road (near Stirling Corner), from Hendon Wood Lane along

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