Whin Sill

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Whin Sill is a tabular layer of igneous rock, or sill, in County Durham and Northumberland, in the Northeast of England. This east-west running geological formation lies partly in the North Pennines Area of Outstanding Natural Beauty and partly in Northumberland National Park. One of the best places to view the Whin Sill is High Force, but good observation points are also available from Housesteads, an ancient Roman auxiliary fort.

Formed of dolerite (a basaltic rock) 295 million years ago (MYA), the Whin Sill is part of a sheet of rock stretching from Teesdale, home of various Waterfalls, along a northerly line to Berwick, home of the Farne Islands. Lindisfarne Castle strategically takes advantage of a high, rocky cliff line 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 flows but about the same time.^[1]

[edit] Great Whin Sill geology

Towards the close of Carboniferous Period, crustal extension caused by movement of the Earth’s Tectonic Crustal Plates allowed the emplacement across much of northern England of the suite of tholeiitic dolerite intrusions collectively known as the Great Whin Sill. The igneous intrusions of magma were emplaced into and concordantly along the strata or layers of the country rock. This molten rock then cooled and crystallised and solidified to form the Great Whin Sill, the remains of which we see exposed on the surface today.

The huge body of dolerite underlies much of south and east Northumberland and the Durham Coalfield. Its maximum known thickness of around 70 metres occurs in the North Pennines. The Great Whin Sill is one of the key natural features of the North Pennines. There is also an associated smaller sill to the north in Weardale known as the Little Whin Sill

The study of geology began in the British Isles and many of the terms to describe British geological features are still used throughout the world. Sill is one of those terms. 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, and the term ‘sill’ was quickly and permanently adopted by geologists for concordant, horizontal intrusive bodies.

Sills, by definition are concordant, sheet-like bodies of injected igneous rocks. The Great Whin Sill of northern England often quoted as a primary example.

Although 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 of the dolerites of the Whin complex have revealed significant differences between the Little Whin and the Great Whin. The Little Whin Sill is olivine-bearing and believed to be composed of an early differentiate of the Whin dolerite magma.

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 reflectance over the Alston Block 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 suite of intrusions has been dated to around 295 Ma. The lithology is notable in many respects, including well developed pegmatite segregations which can be found in Upper Teesdale. Late stage hydrothermal mineralisation has filled the joints with Pectolite. 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 marbles within the aureole in a small number of places in Teesdale. T

The resultant regolith 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 of the dolerite gives rise to a number of striking geomorphological features such as High Force; Low Force; Cauldron Snout; High Cup Nick & Holwick Scars.

Disused roadstone quarries along the course of the sill offer sections for interpretation of the geological history of the Great Whin Sill.

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