Category:7. Northern England district

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.

Geologically recent surface deposits

Geological deposits of relatively recent origin, known as superficial deposits, formed over the past 2 to 3 million years and span the Ice Ages and Interglacial periods. The deposits occur in areas such as the Solway Lowlands and south Cumbria and are locally up to 60m thick, but elsewhere these deposits are patchy and typically less than 20 m or so thick. These deposits include layers of boulder clay, sand and gravel deposited from ice sheets and typically less than 60 m thick, which are widespread within the region but are mainly confined to the bottoms of the deep valleys cut out by glaciers in the Lake District. In lowland areas there are spectacular spreads of small whale-backed hills called drumlins that are formed of glacial materials, while clay deposits can give rise to shallow lakes and wetlands such as the Solway mosses. Spreads of silt, sand and gravel are extensive and up to 50 m thick along the major river valleys especially those of the Tyne and Tees. Upland areas and the lowlands around the Solway Firth support bogs with the accumulation of peat. Peat thicknesses are usually less than 3 m, though about 9m are present in the extensive deposit east of Cross Fell in the North Pennines. Most of the superficial deposits are soft and easily eroded, as they have not been deeply buried and consolidated to form strong rocks.

Geology at depth

Below the superficial deposits, or with just a cover of soil where such deposits are absent, are older rocks which geologists broadly split into two distinct types:

• The sedimentary bedrock geology is composed of quite hard rocks formed from a few hundreds to a few tens of millions of years ago as layers of sediments which were deposited in shallow seas, deserts and swamps in times when Britain lay closer to the Equator and the climate and landscape were very different from those of today.
• The basement geology, which underlies the bedrock, is over 360 million years old and mainly comprises harder and denser rocks which have been strongly consolidated and folded. They include both rocks originally deposited as sediments and others that are products of volcanic activity or formed from the solidification of molten rock below ancient volcanoes.

This region has been affected by several phases of earth movements that caused uplift and allowed some of the sedimentary bedrock to be removed by erosion (in some parts of the region up to several thousand metres of rocks are thought to have been removed). These processes have brought older rocks that were once deeply buried close to the surface. Figures 2, 3 and 4 provide schematic vertical sections through the geology, referred to as geological cross-sections, which illustrate the variations in geology across the region.

Individual areas

For the purposes of more detailed description of the bedrock and basement geology the region can be divided into four areas, namely: Lake District and surrounding areas, bordered to the north and northeast respectively by the Solway Lowlands and the Vale of Eden, North Cumbria and Northumberland, and the North Pennines and Northumberland-Durham Coalfield.

Lake District and surrounding areas

This area includes the upland area of the Lake District and the surrounding coastal lowlands to the west and south where the main settlements include Workington, Whitehaven, Barrow-in-Furness and Kendal.

Sedimentary Bedrock

On the flanks of the Lake District layered sedimentary rocks occur at or near the surface across much of west and south Cumbria.

The youngest bedrock in this area comprises red sandstones and mudstones, around 250 million years old and forming undulating lowlands to the west of the Lake District and south of Whitehaven. For example, the spectacular cliffs at St Bees Head are formed of reddish sandstone laid down by a massive, northward flowing river system. These rocks are referred to as the Sherwood Sandstone and provide substantial quantities of groundwater from both the water-filled pores between the sand grains and from fractures cutting the rock. Deposits like this are known as aquifers and the Sherwood Sandstone is the second most important aquifer in England. The water quality has been affected by the invasion of saline water in areas of heavy extraction near Barrow-in-Furness and this probably occurs all along the Cumbrian coast. In south Cumbria, the sandstones are overlain by red mudstones, known as the Mercia Mudstone. Locally these contain layers of rock salt and these have been worked in the past by brine pumping on Walney Island.

Beneath the Sherwood Sandstone are older sediments, comprising Coal Measures and the underlying Carboniferous Limestone. The most widespread of these layers is the Carboniferous Limestone, deposited in warm, clear seas and occurring from Caldbeck west almost to Maryport where it is up to 270 m thick. To the south, bordering Morecambe Bay, the limestone is widespread at or near the surface and is encountered at depths to around 500 m. Locally, it contains substantial deposits of iron ore which have been mined extensively in the past. In west Cumbria, between Maryport and Whitehaven, the limestone lies at depths of 70 to 640 m and is overlain by alternating beds of mudstone and sandstone, and in higher layers there are many seams of coal present in the Coal Measures. The limestones formed in relatively shallow seas, with vast quantities of sand and mud gradually building up to form large river deltas. When the tops of these deltas were exposed, massive swampy forests grew up and the vegetation from these forests was later buried and compressed to produce layers of coal. Mining of coal in West Cumbria dates from the 16th century. The mines reached depths up to 400 m and extended out beneath the Irish Sea for up to 6.5 km. Together, the resources of iron ore and coal supplied the former heavy industries of Workington, Whitehaven and Barrow-in-Furness.

Basement rocks

Carboniferous Limestone rests on the basement rocks of the Cumbrian fells, the oldest rocks in Northern England. At the heart of the Lake District lies Scafell Pike which, at 977 metres above sea level, is England’s highest mountain. The craggy mountainous terrain surrounding Scafell (Figure 5) is formed by rocks that were originally ash deposits and lavas erupted from volcanoes about 450 million years ago (Borrowdale Volcanic Group). This part of the Lake District is associated with extensive base metal mineralisation and was once one of the world’s principal copper producing areas.

To the north, the smoother, more rounded mountains around Skiddaw are formed of dark slates derived from older sandstone and dark grey mudstone (Skiddaw Group), while the southern Lake District is characterised by undulating hills and rocky crags made up of layers of limestone, mudstone and sandstone of the Windermere Supergroup and Reston Group, that are younger than the volcanic rocks. Each of these basement rock units is known to extend to several kilometres below the surface. As well as the deformed sediments and volcanic rocks, the basement includes a number of intrusions of granite, originally emplaced as molten rock or magma. The main examples are at Ennerdale, Eskdale and Shap.

Generally, the basement rocks are impermeable, meaning that they do not let water pass through them and therefore they do not supply large amounts of groundwater. However, the Lake District’s hills form an important gathering ground for rainwater and some of the large lakes on the valley floors, such as Thirlmere, have been dammed to form reservoirs that supply water to nearby towns and cities such as Manchester.

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.