Igneous rocks crystallise from molten lava, or magma, generated within the Earth’s mantle or crust. The buoyant magma rises and much of it may be intruded into rocks at higher levels in the crust to cool and crystallise as igneous intrusions.
Large bodies of magma cool slowly forming coarsely crystalline rock such as granite, while smaller masses typically cool more quickly to form finer grained rocks. Kilometre-scale intrusions, with a rounded outline on the map, are referred to as plutons.
Sheet-like intrusions that are mainly concordant with bedded sedimentary strata are called sills, and those that cross-cut strata are called dykes.
Magma may also reach the surface at volcanoes, where it is erupted as lava or ejected explosively as fragments (including volcanic ash and volcanic bombs) that ultimately form pyroclastic rocks.
Igneous rocks may be classified based on their silica content: those with low silica, for example basalt and dolerite, are termed basic rocks, whereas those with abundant silica, such as rhyolite and granite, are acid.
Intermediate compositions include andesite and trachyte. Examples of all of these compositions and forms of igneous rock are represented within Northumberland National Park.
Influence on the landscape
The Devonian lavas and granitic rocks form the massive rounded hills of the Cheviot massif, much of which lies above 300 metres, with the highest point on The Cheviot at 815 metres.
The valleys are deep with steep-sided convex slopes. The rocks are not well exposed, except in the valley bottoms and on the tors, which characterise the hilltops on the granite, and where the volcanic rocks have been metamorphosed in contact with the granite.
Crags are few, though notable exceptions are Bizzle Crags and Hen Hole to the north and west of The Cheviot respectively. In the eastern half of the massif, the drainage pattern that has developed on the igneous rocks is relatively simple and the density of streams is low.
This contrasts markedly with that on the western side where the drainage pattern is intricate and dense. The wide, strikingly linear valley of the Harthope Burn has been carved along a major fault through the massif.
The lavas of the Cottonshope Volcanic Formation are marked by small, dark-coloured craggy outcrops which contrast with the surrounding outcrops of Dinantian sedimentary rocks. Small, long-abandoned quarries mark the outcrops. Because of their very small surface outcrop these rocks have only a modest effect of the district’s landscape.
The Whin Sill-swarm is one of the best known features of Northumberland geology and is responsible for some of the county’s finest, and most distinctive scenery. The north-facing cliffs, with the long, gentle southerly slopes, provided the Roman civil engineers with a natural defensive site for the construction of Hadrian’s Wall.
The massive, hard and resistant columnar-jointed dolerite imparts a distinctive character to these outcrops, which contrasts strikingly with the generally lower ridges and crags (cuestas) formed by parallel outcrops of Carboniferous sandstone and limestone.
The Great Whin Sill was formerly worked on a large scale in quarries at Walltown and Cawfields. Landscaping of the former site has significantly lessened its visual impact as a man-made feature. In contrast, the profile of Cawfields Quarry stands as an obvious interruption to the line of Whin Sill crags.
Both quarries offer opportunities to appreciate the nature of the Whin Sill and its role in creating the distinctive landscape of the Hadrian’s Wall country. The crags at Steel Rigg and elsewhere are popular rock climbing localities. East and north of Sewingshields the Whin Sill outcrops are locally concealed beneath spreads of superficial deposits.
However, steep escarpments with bare crags of dolerite can be seen at Teppermoor Hill, around Gunnerton Nick and in the Swinburne, Thockrington, Sweethope, Bavington, Fontburn and Kyloe areas.
Large working quarries at Keepershield, Barrasford, Swinburne, Divethill, Howick and Longhoughton are conspicuous features in the local landscape, though their visual impact is subject to strict planning and environmental constraints. There are also abandoned quarries at Thockrington, West Whelpington, Ward’s Hill and Ewesley.
Influence on Biodiversity
The steep-sided valleys on the Cheviot volcanic rocks typically host grassland dominated by bent and fescue grasses or bracken, which alternates with areas of broken rock; much of this current pattern of vegetation results from modification by sheep grazing. The screes have sparse vegetation dominated by ferns including lemon-scented and parsley ferns.
Soils derived from the volcanic rocks are usually base-rich and support some uncommon species such as maiden pink, Jacob’s ladder, common rockrose and hairy rock cress. At higher levels where slopes have lower angles, this gives way to a heather heath/acid grassland mosaic.
On the flatter summits extensive areas of blanket bog have formed in the cool wet climate. Here species such as Sphagnum mosses, cotton-grasses, cross-leaved heath and cloudberry are characteristic. Arctic alpine species and communities persist on The Cheviot, particularly on ungrazed ledges.
Upland lichen species such as Umbilicaria torrefacta, Melanelia hepatizon and Sphaerophorus fragilis occur on Cheviot itself, the Bizzle and Henhole have the richest flora and Cladonia rangiferina has been recorded from Braydon Crag.
Where free, or comparatively free, of superficial deposits, Whin Sill outcrops typically support rather thin, acid soils, which in places support a distinctive Whin Sill grassland flora including wild chives, biting stonecrop, rue-leaved saxifrage and mountain pansy. Craggy outcrops, including those in abandoned quarries, offer important nesting sites for birds including kestrels, raven and peregrines, and possibly roosting sites for bats.
Neither the Carboniferous, nor the Cainozoic dykes make any significant impact on the district’s landscape; small long-abandoned quarries are still visible in the Acklington Dyke near Cartington.
The physical properties of the Whin Sill dolerite make it a good source of roadstone, crushed rock aggregate, riprap and armour stone. ‘Northumberland Whinstone’ has long been exploited for these purposes from quarries across the outcrop. Its intractable nature generally precludes its use as a building stone, though it has been employed locally in a few buildings, notably the now abandoned cottages at Barrasford Quarry.
Large, abandoned quarries in the main sill at Walltown, Cawfields and West Whelpington were once important sources both of crushed rock for roadstone, and as shaped blocks or setts, for road paving and kerb stones.
Similar dolerite was formerly worked from the Haydon Bridge Dyke, at West Mill Hills, east of Haydon Bridge: this quarry was backfilled and the site completely landscaped in the early 1980s. It was also formerly worked from one of the Carboniferous dykes north of Bellingham. Very small pits have been worked for dolerite from the Cainozoic Acklington Dyke in the Cartington area, though there are no operating quarries within the present district.
‘Whinstone’ quarrying remains an important industry in Barrasford, Swinburne, Divethill, and, just outside the district, Howick and Longhoughton supplying crushed dolerite products for use widely across northern England.
On the south side of the Cheviot massif, the small intrusion of Devonian age near Biddleston has been worked for crushed rock and roadstone at Harden Quarry [NT 958 086]. The natural bright red colour of the rock makes it sought after for specialised uses such as surfacing the hard shoulders on Britain’s motorways and, perhaps most famously, for lining The Mall in London.
The Cottonshope basalts have been quarried on a modest scale, probably mainly for local use as roadstone and walling stone. There is extensive use of local volcanic stone for walling despite its round profile.
Future working of any mineral deposit is dependent upon a complex range of commercial and planning considerations, but it seems likely that demand for roadstone, crushed rock and the other products currently extracted within the district will continue for the foreseeable future. Substantial reserves of rock of satisfactory quality are understood to remain at several, or all, of the working quarries within the district.
Additional workable reserves of dolerite capable of meeting commercial specifications could no doubt be identified within the district, though planning and environmental conditions would have to be met in any proposals for working.
The natural exposures of igneous rocks within the district are generally robust and none appears to be under threat. Working quarries typically provide excellent representative sections of the geology, but by definition quarrying destroys the materials worked.
However, the continually changing nature of the sections can yield invaluable insights into the rocks exposed. Accurate recording of such sections, accompanied by the collection and curation of representative specimens, can play a vital role in maintaining and furthering knowledge and understanding of the local geology.
By contrast, abandoned quarries are potentially at risk of becoming degraded or overgrown, either due to natural deterioration or by inappropriate after-use and management.
The planting of trees adjacent to the fine Whin Sill section at Walltown Quarry may be cited as an example of inappropriate management of a valuable and instructive geological site. The use of old quarries for landfill may threaten to damage or totally obliterate important sections, though none is known to be under any such immediate threat within the district.
The quarry faces at Cottonshope Head Quarry are now rather weathered, and in places degraded. Consideration might be given to restoring this section. The district also includes several abandoned quarries which, because of the significant geological features exposed, merit consideration for protection.
Of particular note are the sections through the Whin Sill and adjacent country rocks at Ward’s Hill Quarry. In addition to the range of geological features exposed here, the site has considerable historical significance for its place in the development of ideas on the nature and origin of the Whin Sill.
Exposures of the Acklington Dyke and adjoining wall rocks at Cartington, though comparatively modest, offer a rare opportunity to examine this important, though otherwise poorly exposed, intrusion.
The Scroggs is an example of an SNCI listed for botanical interest that has geological links. This is an exceptional piece of grassland on the contact zone between the Whin Sill and limestones in the Tyne Limestone Formation. The pasture is among the richest found on any of the Whin Sill sites and its flora is outstanding with many species uncommon in north-east England.
The Cheviot volcanic rocks and Cheviot Granite are among the southernmost occurrences of a suite of igneous rocks of late Silurian to Devonian age, the members of which are distributed northwards to Orkney and Shetland.
Volcanic rocks of this suite form such notable areas as the Ochil and Sidlaw Hills in the Midland Valley of Scotland, and the caldera-volcano at Glencoe in the Highlands. Though modern studies of the Cheviot rocks are few, these rocks contribute to our wider understanding of this important phase of igneous activity in the evolution of the British Isles.
The Whin Sill is generally regarded as the original sill of geological science and therefore has to be regarded as one of the district’s most important natural heritage features. It takes its name from the north of England quarryman’s term ‘whin’, meaning a black, generally hard and intractable rock, and ‘sill’, meaning any more or less horizontal or flat-lying body of rock.
Recognition of an intrusive igneous origin for the Whin Sill during the 19th century was based largely on studies within the present district, notably on sections exposed at Ward’s Hill Quarry. Consequently, the term ‘sill’ became adopted by geological science for all near-horizontal and, within stratified sequences, broadly concordant, intrusive igneous bodies.
Since then, many studies of the Whin Sill-swarm and its associated dykes have drawn upon evidence gathered from its exposures in Northumberland and much of the large volume of earth science literature derived from these studies has significance well beyond the county.
In addition to its geological importance, the striking geomorphological expression of the Whin Sill, and its exploitation by the Romans, has produced an internationally recognised landscape.
Carboniferous and Permian Igneous rocks:
- Cottonshope Head Quarry [NT 803 058]
- Steel Rigg to Sewingshields Crags [NY 751 676 to NY 813 704]
- Wydon [NY 695 629]
Allerhope Burn, Barrasford Quarry, Toddle, Reaver, Blindburn, Canker Cleugh, Carshope, Cawfield Crags, Divethill and Claywalls, Earlehill Quarry, Flodden Quarry, Fredden , Preston Yeavering Bell, Haltwhistle Burn, Harelaw etc. Burns, Horsdon Channel, Kyloe Hills, Raker Crag, Shiellow Crags, Shillhope Cleugh, Upper Breamish and Bloodybush Edge, Usway Burn, Walltown Quarry and Crags, Windyhaugh.