West Rigg Open Cutting
Encyclopedia
West Rigg Open Cutting is a Site of Special Scientific Interest
in the Wear Valley
district of west County Durham
, England
. It is a disused ironstone
quarry
, located just over 1 km north of the village of Westgate
, in Weardale
.
At West Rigg, the Slitt Vein, a 20 km-long quartz intrusion in the Namurian
Great Limestone
, is exposed at the surface; the exposure is up to 5 m wide over a distance of some 200 m. During Late Permian times, extensive mineralisation occurred in the vicinity of the Slitt Vein: high salinity fluids, rich in iron, permeated the Great Limestone which, throughout its full thickness, was mineralised, producing siderite
and ankerite
minerals which were subsequently oxidised, yielding a limonitic
ore with an iron content of over 40 percent. At West Rigg, the iron mineralisation extends up to 60 m on each side of the vein.
The Slitt Vein is predominantly composed of quartz
, with smaller amounts of fluorite
and uneconomic amounts of galena
lead ore. The earliest mining activity at the site was the extraction of fluorite and galena from the Slitt Vein itself: the miners worked from levels driven into the vein from the west. In the late 19th century, quarrying of the ironstone began, the Slitt Vein being left untouched so that today it protrudes as a vertical 'rib' along the centre of the quarry.
The Geological Conservation Review
has identified the site as being of national importance because it provides an accessible illustration of the formation of iron ore by replacement and oxidation. The site also offers a rare surface view of the Slitt Vein.
Site of Special Scientific Interest
A Site of Special Scientific Interest is a conservation designation denoting a protected area in the United Kingdom. SSSIs are the basic building block of site-based nature conservation legislation and most other legal nature/geological conservation designations in Great Britain are based upon...
in the Wear Valley
Wear Valley
Wear Valley was, from 1974 to 2009, a local government district in County Durham, England. Its council was based in Crook.The district covered much of the Weardale area. In the west it was parished and rural, whereas in the east it was more urban...
district of west County Durham
County Durham
County Durham is a ceremonial county and unitary district in north east England. The county town is Durham. The largest settlement in the ceremonial county is the town of Darlington...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
. It is a disused ironstone
Ironstone
Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical repacement, that contains a substantial proportion of an iron compound from which iron either can be or once was smelted commercially. This term is customarily restricted to hard coarsely...
quarry
Quarry
A quarry is a type of open-pit mine from which rock or minerals are extracted. Quarries are generally used for extracting building materials, such as dimension stone, construction aggregate, riprap, sand, and gravel. They are often collocated with concrete and asphalt plants due to the requirement...
, located just over 1 km north of the village of Westgate
Westgate, County Durham
Westgate is a village in County Durham, in England. It is situated in Weardale between St John's Chapel and Eastgate.Westgate is also the entrance to Slitt wood and an old abandoned lead mine. Other features of the village include a caravan site and a football and basketball court.-External links:...
, in Weardale
Weardale
Weardale is a dale, or valley, of the east side of the Pennines in County Durham, in England. Large parts of Weardale fall within the North Pennines Area of Outstanding Natural Beauty - the second largest AONB in England and Wales. The upper valley is surrounded by high fells and heather grouse...
.
At West Rigg, the Slitt Vein, a 20 km-long quartz intrusion in the Namurian
Namurian
The Namurian is a stage in the regional stratigraphy of northwest Europe with an age between roughly 326 and 313 Ma . It is a subdivision of the Carboniferous system or period and the regional Silesian series. The Namurian is named for the Belgian city and province of Namur where strata of this age...
Great Limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, is exposed at the surface; the exposure is up to 5 m wide over a distance of some 200 m. During Late Permian times, extensive mineralisation occurred in the vicinity of the Slitt Vein: high salinity fluids, rich in iron, permeated the Great Limestone which, throughout its full thickness, was mineralised, producing siderite
Siderite
Siderite is a mineral composed of iron carbonate FeCO3. It takes its name from the Greek word σίδηρος sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus...
and ankerite
Ankerite
Ankerite is a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral carbonates with formula: Ca2. In composition it is closely related to dolomite, but differs from this in having magnesium replaced by varying amounts of iron and manganese.The crystallographic and...
minerals which were subsequently oxidised, yielding a limonitic
Limonite
Limonite is an ore consisting in a mixture of hydrated iron oxide-hydroxide of varying composition. The generic formula is frequently written as FeO·nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide.Together with hematite, it has...
ore with an iron content of over 40 percent. At West Rigg, the iron mineralisation extends up to 60 m on each side of the vein.
The Slitt Vein is predominantly composed of quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, with smaller amounts of fluorite
Fluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
and uneconomic amounts of galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
lead ore. The earliest mining activity at the site was the extraction of fluorite and galena from the Slitt Vein itself: the miners worked from levels driven into the vein from the west. In the late 19th century, quarrying of the ironstone began, the Slitt Vein being left untouched so that today it protrudes as a vertical 'rib' along the centre of the quarry.
The Geological Conservation Review
Geological Conservation Review
The Geological Conservation Review is produced by the UK's Joint Nature Conservation Committee and is designed to identify those sites of national and international importance needed to show all the key scientific elements of the geological and geomorphological features of Britain...
has identified the site as being of national importance because it provides an accessible illustration of the formation of iron ore by replacement and oxidation. The site also offers a rare surface view of the Slitt Vein.