Category:7. Northern England district: Difference between revisions

This account provides a broad perspective of the geology of the Northern England region which extends from the Scottish border southwards to Morecambe Bay in the west and Teesside in the east, covering England’s four northernmost counties of Cumbria, Durham, Tyne & Wear and Northumberland, and also extending into some northern parts of Cleveland, Yorkshire and Lancashire. Figure 1 provides a geological sketch map of this region showing the rock types occurring in relation to the major towns and cities. This account outlines the geology to a depth of at least a kilometre and summarises the current and historical use of the geological resources in the area.

Within the region a striking diversity of rocks and geological structure are present resulting from a geological history that spans almost 500 million years. The outstanding scenery of this landscape is recognised in the large areas covered by the Lake District and Northumberland National Parks and Areas of Outstanding Natural Beauty which include the Northumberland Coast, the Solway Coast, and the North Pennines.

The geology of the region is well known at and near the surface from natural exposures of rocks along rivers, crags and coastal cliffs, together with observations in quarries and opencast pits and mine workings. Shallow boreholes are also plentiful, commonly drilled for road and other construction work. Below about 50 m most information comes from boreholes drilled for water, coal or for minerals but these are mainly confined to areas where economic resources have been exploited. There are relatively few boreholes deeper than 250 m, though one is the deepest onshore borehole in the UK which was drilled to a depth of 4170 m at Seal Sands on Teesside. Other geological information relating to the deep subsurface has been interpreted from geophysical seismic surveys that provide an understanding of the deep geological structure of the region by sending sound waves through the ground. This is particularly important for interpretation of the geology in the Solway Lowlands and eastwards into Northumberland. Other geophysical surveys, carried out on land or by low flying aircraft, reveal patterns of the Earth’s gravity and magnetic field and have been used to detect and infer the deep geological structure beneath the Lake District, the North Pennines and The Cheviot.

Solway Lowlands and the Vale of Eden

This area comprises lowlands to the south of the Solway Firth, along with the valley of the River Eden. Penrith and Carlisle lie within this area.

Sedimentary Bedrock

The gently rolling lowlands that border the Solway Firth are underlain by up to 500m of reddish brown Mercia Mudstones containing thin beds of mineral salts, which formed as extensive coastal mudflats and lake deposits. These are overlain west of Carlisle by grey mudstones laid down around 180 million years ago; which represent the youngest sedimentary bedrock in the region.

As in the southern Lake District, these mudstones overlie the Sherwood Sandstone. Together this package of rocks extends to depths of about 200 m northeast of Maryport, to nearly 800 m west of Carlisle, and just over 1300 m at Silloth. At greater depth lie older, Carboniferous sediments up to 2 km thick, including Coal Measures in a concealed extension of the coalfield of west Cumbria described above.

In the Vale of Eden two units of sandstone separated by mudstone form the uppermost part of the sedimentary bedrock. These units are inclined eastwards, with the base of the lower sandstone descending to about 650 m below the surface adjacent to the edge of the north Pennine Hills. The upper sandstone unit is the Sherwood Sandstone (Figure 6). The mudstone beneath contains layers of gypsum and other mineral salts which have been extracted in the past for use in the chemical industry and for the manufacture of glass, cement and plasterboard. Both sandstone units are locally significant aquifers in this area. Beneath the lower sandstone unit are older mudstone and sandstone containing thin beds of limestone, and these in turn overlie the Carboniferous Limestone.

Basement rocks

The basement rocks of the adjoining Lake District do not occur at the surface in this area, but are present at depth. The basement rocks descend eastwards beneath the Vale of Eden to lie at about 2 km depth adjacent to the north Pennine Hills. By contrast, the top of the basement beneath the Solway Lowlands lies much deeper at about 8 km.

North Cumbria and Northumberland

The area of North Cumbria and Northumberland adjacent to the Scottish border is predominantly rural with no major towns.

Sedimentary Bedrock

The fells of the Bewcastle and Liddesdale areas are underlain by a series of alternating layers of sandstone, limestone and mudstone of similar age to the Carboniferous Limestone, sandstones and Coal Measure already described from the Lake District (Figure 7). These were laid down in shallow seas and by rivers commencing around 360 million years ago. These layers dip south toward the Tyne Valley, where they form a sequence that is more than 4 km thick.

Inland from the Northumberland coast a series of ridges are formed by hard, resistant rocks. Most of these ridges are beds of tough sandstone or limestone within softer mudstone layers. However, the most prominent of these ridges is made by a sheet of much harder, dark rock known as dolerite, forming the Whin Sill. This rock was injected as a sheet of molten rock or magma between the sedimentary layers, rising from great depth about 300 million years ago. This sheet, which has an average thickness of about 50 m in this area, is exposed from north Northumberland westwards to the Tyne valley and it provides the imposing foundation for Hadrian’s Wall for much of its length (Figure 8). Boreholes show that the Whin Sill continues southeastwards at depth where it is encountered for example at 460 m depth at Longhorsley. At Whitley Bay it has split into two layers which occur at depths of about 800 and 1050 m.

Water supply across this area comes mainly from surface reservoirs. Of these, Kielder Water in the uppermost reaches of the River North Tyne, is the largest and supplies Tyneside, Wearside and Teesside by transfer of water from the Tyne into the Wear and Tees. In addition, many of the thicker sandstones in the area have been developed as aquifers giving good yields above depths of about 250m with the flow occurring along fractures.

Basement rocks

Basement rocks, comprising hard mudstone, lavas and an intrusion of granite about 395 million years old form the Cheviot Hills in the northeast of this area. This granite is of comparable age to the younger granites present in the Lake District, e.g. at Shap. The basement is present at depth farther south beneath the sedimentary bedrock, but the depth to this basement as indicated by seismic surveys reaches 4-5 km in the Tyne valley and here the character of the basement rocks has not been investigated with boreholes.

North Pennines and the Northumberland-Durham Coalfield

The central and eastern parts of the region comprise the North Pennines and the lower lying and coastal areas of the Northumberland and Durham Coalfield. The Pennines are predominantly rural but the coalfield area is densely populated with Newcastle-upon-Tyne, Sunderland, Durham, Stockton-on-Tees and Darlington as the main urban centres.

Sedimentary Bedrock

South of Hadrian’s Wall lie the high, flat moorlands of the North Pennines. The moors rise gently westward to the spectacular west-facing Pennine edge at Cross Fell that overlooks the Vale of Eden. A distinctive feature of the North Pennine landscape is the marked terraces on the hillsides (Figure 9) caused by erosion of the soft mudstone and alternating hard sandstone and limestone layers, all deposited between 340 and 300 million years ago.

The Whin Sill is also present in this area, particularly in upper Teesdale, and farther eastwards it has been encountered within many old mines and in boreholes at depths increasing to 880 m on the North Sea coast. This sheet-like intrusion, described in more detail above, thickens from about 80 m north of upper Teesdale to about 100 m beneath Tynemouth where it splits into several layers. The rocks in this area, in particular the Whin Sill and harder limestone and sandstone beds, host a large number of veins of lead and zinc ore that formed the basis of the historic mining industry of this area.

Layers of mudstone, sandstone and coal belonging to the Coal Measures are more than 850 m thick in the Northumberland and Durham Coalfield (Figure 10). These rocks were formed in a tropical, humid and forested lowland swamp crossed by rivers and subjected to short-lived incursions by the sea. Most of the valuable coal seams are present in the middle part of the sequence, with the thickest of the individual seams reaching 2.5 m. This coalfield is the oldest area of commercial coal mining in Great Britain, with records of workings dating from the 12th century, and it provided the basis for the development of the steel and shipbuilding industries of Tyneside and Wearside. Mining extended offshore for up to 7.5 km and in coastal County Durham seams were mined at depths up to 500 m. Though only a handful of coal mines remain in operation today, and all base metal mining has now ceased in the region, the effects of mining remains a major influence on the landscape.

East from the city of Durham, the rocks of the Coalfield descend beneath successively younger sediment layers, absent in the northwest, which are intermediate in age between the Coal Measures and red sandstone of the Whitehaven coast. First is a layer of younger limestone, traditionally called the Magnesian Limestone, that is only present in this southeastern part of the region and was laid down about 270 to 250 million years ago. It is marked by a distinctive belt of arable country terminated by the limestone cliffs of the Durham coast. The Magnesian Limestone (Figure 11) forms a good quality aquifer though the waters are very hard, giving problems with limescale, and there is a high vulnerability to surface nitrate pollution from fertilisers used in the arable farming. Water yields depend largely on the density of fractures present and the extent of connection between them. At greater depths, water quality is poorer due to an increased concentration of dissolved mineral salts from the overlying layers.

Southwards, the Magnesian Limestone is concealed by younger mudstone and thick beds of mineral salts, and then by the Sherwood Sandstone and brownish Mercia Mudstone. The mineral salts in these layers formerly supplied the chemical and other industries on Teesside.

In the Teesside area, older sandstone and mudstone with variable amounts of limestone have also been proved below the Magnesian Limestone to extend to depths exceeding 4 km.

Basement rocks

In this area basement rocks are only found at the surface along the western edge of the Pennine Hills. These hard mudstones are of similar character to those seen in the northern Lake District and underlie the sedimentary bedrock that forms most of the moors. These older mudstones have also been found at depth in a few boreholes beneath the North Pennines. By contrast, in the Weardale area a large intrusion of granite, about 400 million years old, has been encountered in boreholes at 275 to 380 m beneath the surface. These basement rocks extend to depths exceeding 3 km underneath the whole area.