Whin Sill
Encyclopedia
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...
and Northumberland
Northumberland
Northumberland is the northernmost ceremonial county and a unitary district in North East England. For Eurostat purposes Northumberland is a NUTS 3 region and is one of three boroughs or unitary districts that comprise the "Northumberland and Tyne and Wear" NUTS 2 region...
in the northeast of 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 lies partly in the North Pennines
North Pennines
The North Pennines is the northernmost section of the Pennine range of hills which runs north-south through northern England. It lies between Carlisle to the west and Darlington to the east...
Area of Outstanding Natural Beauty and partly in Northumberland National Park
Northumberland National Park
Northumberland National Park is the northernmost national park in England. It covers an area of more than 1030 km² between the Scottish Border in the north to just south of Hadrian's Wall. It is one of the least populated and least visited of the National Parks...
and stretches from Teesdale
Teesdale
Teesdale is a dale, or valley, of the east side of the Pennines in England. Large parts of Teesdale fall within the North Pennines Area of Outstanding Natural Beauty - the second largest AONB in England and Wales. The River Tees rises below Cross Fell, the highest hill in the Pennines, and its...
northwards towards Berwick
Berwick-upon-Tweed
Berwick-upon-Tweed or simply Berwick is a town in the county of Northumberland and is the northernmost town in England, on the east coast at the mouth of the River Tweed. It is situated 2.5 miles south of the Scottish border....
.
It is one of the key natural features of the North Pennines. A major outcrop is at the High Force
High Force
High Force is a waterfall on the River Tees, near Middleton-in-Teesdale, Teesdale, County Durham, England. The waterfall is within the North Pennines Area of Outstanding Natural Beauty and European Geopark....
waterfall in Teesdale. Bamburgh Castle
Bamburgh Castle
Bamburgh Castle is an imposing castle located on the coast at Bamburgh in Northumberland, England. It is a Grade I listed building.-History:...
, Dunstanburgh Castle
Dunstanburgh Castle
Dunstanburgh Castle lies on a spectacular headland on the coast of Northumberland in northern England, between the villages of Craster and Embleton....
, Lindisfarne Castle
Lindisfarne Castle
Lindisfarne Castle is a 16th-century castle located on Holy Island, near Berwick-upon-Tweed, Northumberland, England, much altered by Sir Edwin Lutyens in 1901. The island is accessible from the mainland at low tide by means of a causeway.-History:...
and stretches of Hadrian's Wall
Hadrian's Wall
Hadrian's Wall was a defensive fortification in Roman Britain. Begun in AD 122, during the rule of emperor Hadrian, it was the first of two fortifications built across Great Britain, the second being the Antonine Wall, lesser known of the two because its physical remains are less evident today.The...
all strategically take advantage of high, rocky cliff lines formed by the sill.
The Whin Sill complex is usually divided into three components: Holy Island Sill, Alnwick Sill and the Hadrian's Wall-Pennines Sill, which were created by separate magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
flows but about the same time.
The Little Whin Sill is an associated formation to the south 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...
.
Derivation of name
Much of the early study of geology began in the British Isles, from whence much of the terminology is derived. Quarrymen of Northern England used the term 'sill' to describe a more or less horizontal body of rock. 'Whin' was applied to dark, hard rocks. As the intrusive igneous origin of the Whin Sill was determined in the 19th century, the term 'sillSill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
' was adopted by geologists for concordant, horizontal intrusive bodies.
Geology
Towards the close of Carboniferous Period some 295 million years ago (Ma), crustalCrust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
extension caused by movement of the Earth’s tectonic plates allowed the emplacement of igneous intrusions of magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
across much of northern England of a suite of tholeiitic dolerite intrusion
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
s. These were largely concordant with the strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
of the existing country rock
Country rock (geology)
Country rock is a geological term meaning the rock native to an area. It is similar and in many cases interchangeable with the terms basement and wall rocks....
. On cooling, these crystallised and solidified to form the Great Whin Sill.
It underlies much of south and east Northumberland and the Durham Coalfield. Its maximum known thickness of around 70 metres occurs in the North Pennines.
Surface and subsurface records of the Great Whin reveal it is not always concordant over wide areas and often rises and falls in the stratigraphical succession in marked leaps and gentle transgressions different levels.
Studies of the petrology
Petrology
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....
of the dolerites of the Whin Sill complex have revealed significant differences between the Little Whin and the Great Whin. The Little Whin Sill is olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
-bearing and believed to be composed of an early differentiate
Igneous differentiation
In geology, igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement or eruption.-Primary melts:...
of the Whin dolerite magma. On the other hand, the Great Whin, non-olivine-bearing and slightly density graded, is a later differentiate of the Whin magma.
Two separate periods of Whin dolerite injection are confirmed by studies of vitrinite
Vitrinite
Vitrinite is one of the primary components of coals and most sedimentary kerogens. Vitrinite is a type of maceral, where "macerals" are organic components of coal analogous to the "minerals" of rocks. Vitrinite has a shiny appearance resembling glass . It is derived from the cell-wall material or...
reflectance over the Alston Block
Alston Block
The Alston Block is a term used by geologists to describe the geological structure of the North Pennines of northern England and which forms a part of the Pennine Block & Basin Province which originated during the Carboniferous period. It is defined by the Stublick and Ninety Fathom Faults to the...
where two periods of Whin contact metamorphism have been recognized. The two periods of Whin dolerite emplacement form part of the end-Carboniferous earth movements in northern England. They can be shown to have occurred between a period of compression from a W-SW direction and later gentle doming of the Alston Block near the Westphalian-Stephanian boundary, dated about 300-295 Ma.
The lithology
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
is notable in many respects, including well developed pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
segregations which can be found in Upper Teesdale. Late stage hydrothermal mineralisation has filled the joints with pectolite
Pectolite
Pectolite is a white to gray mineral, NaCa2Si3O8, sodium calcium inosilicate hydroxide. It crystallizes in the triclinic system typically occurring in radiated or fibrous crystalline masses. It has a Mohs hardness of 4.5 to 5 and a specific gravity of 2.7 to 2.9. The gemstone variety, larimar, is a...
. The dolerite has been carbonated as metasomatic selvages into veins and joints and other flaws to produce 'White Whin'. Thermal metamorphism of the country rock in the contact zone produced coarse grained marble
Marble
Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite.Geologists use the term "marble" to refer to metamorphosed limestone; however stonemasons use the term more broadly to encompass unmetamorphosed limestone.Marble is commonly used for...
s within the aureole in a small number of places in Teesdale.
Disused roadstone quarries
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...
along the course of the sill offer sections for its interpretation.
Effects on landscape and geography
The resultant regolithRegolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
supports rare alpine/Arctic flora which includes Spring Gentian (Gentiana verna); Bird’s Eye Primrose (Primula farinosa); Mountain Avens (Dryas octupetala) and Teesdale Violet (Viola rupestris). This is an example of the intimate interaction and dependence of geology and ecology.
The resistance to erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
of the dolerite gives rise to a number of striking geomorphological
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
features such as High Force
High Force
High Force is a waterfall on the River Tees, near Middleton-in-Teesdale, Teesdale, County Durham, England. The waterfall is within the North Pennines Area of Outstanding Natural Beauty and European Geopark....
; Low Force
Low Force
Low Force is a 5.5m high set of falls in the Tees Valley, England. Further upstream is the High Force waterfall. Low Force is also the site of the Wynch Bridge, completed in 1830. It is suggested that only one person at a time should cross the bridge as it may be unstable.Low Force is within the...
; Cauldron Snout
Cauldron Snout
Cauldron Snout is a waterfall on the upper reaches of the River Tees in Northern England, immediately below the dam of the Cow Green Reservoir. It is well upstream of the High Force waterfall, and is on the boundary between County Durham and Cumbria, England...
; High Cup Nick and Holwick Scars.