More about the Geology of Lunedale

THE MAKING OF LUNEDALE - A brief journey through half a billion years of earth history

Brian Young
Retired British Geological Survey District Geologist for Northern England

Everywhere on earth the landscape we see, whether it is the shape of the hills and valleys, the variety of plants and animals, the farming and land-use, the types of industry or even the style of individual buildings, ultimately depends on the rocks beneath the surface. And so it is in Lunedale. In order to understand and appreciate more fully the character of the valley it is worth considering the variety of the dale’s rocks and the events and processes that, over millions of years of earth history, have made this such a special place.

This chapter can give only flavour of the almost half a billion years of history recorded in the dale’s rocks and landscape. More detailed accounts of the story can be found in the books listed at the end of the chapter and in the references quoted in them.

In Lunedale, as elsewhere in the Northern Pennines, most of the rocks we see around us today were formed during the Carboniferous Period of earth history, between about 355 and 290 million years ago. However, before looking at these more closely, let us look back another 100 million years or so to the formation of the rocks that form the foundation of the Pennines.

THE MAKING OF LUNEDALE
A brief journey through half a billion years of earth history

Brian Young
Retired British Geological Survey District Geologist for Northern England

Earliest beginnings – the foundations of the dales

Between about 500 and 440 million years ago, during Ordovician Period, the part of the earth’s crust that was eventually to become Northern England formed part of a deep ocean on the northern edge of a continental plate that lay roughly 50° S of the equator. This content was moving slowly but inexorably northwards towards another continent that included the areas that we know today as Scotland and North America. Enormous stresses in the earth’s crust, due to these movements, caused a period of violent volcanic activity which lasted for around 10 million years. Eventually, about 450 million years ago, the continents collided, destroying the ocean that had separated them and crumpling the rocks to form a new range of mountains. The muds deposited beneath the deep ocean were turned to slates by these vast earth movements: the volcanic episode built up enormous thicknesses of lavas and volcanic ashes and large bodies of granite were injected into these rocks, deep beneath the surface. Today, we see these rocks most easily in the Lake District, but these same rocks provide the foundation for the Northern Pennines and thus Lunedale. Although they are buried deep beneath the surface in the dale, we catch glimpses of them nearby on the Pennine escarpment at places such as Dufton and Knock Pikes, and beneath Cronkley Scar in Upper Teesdale.

Equatorial Lunedale – Carboniferous times

By about 350 million years ago, during the Carboniferous Period, these new mountains were almost eroded away and their remnants subsided beneath the shallow waters of a wide sea that covered much of what is today northern England. At this time our area lay almost astride the equator, beneath a warm shallow sea that teemed with tropical marine life. It probably resembled the seas around the Bahamas today. Gradually, layers of lime-rich mud, formed from these sea creatures, built up on the sea floor to form the layers of limestone we know today.

At this time, rivers draining a huge landmass that lay somewhere to the north east over what is today the northern North Sea, brought mud and sand into the sea. On occasions so much mud and sand were being washed in that the limestone was overwhelmed by layers of mud that eventually hardened to form shale, and sand that was preserved as sandstone.

At times, the sand and mudbanks became colonised by tropical forests, full of huge primitive trees and ferns, the remains of which were locally preserved beneath more mud and sand to form the thin coal seams we see today in a few places.

Over time, the surface repeatedly subsided, allowing the sea to flood the area again, and so bringing about the formation of more limestone, only to be followed by more layers of mud and sand. This cycle of events was repeated many many times during Carboniferous times, building up the very distinctive series of beds of limestone, shale and sandstone that characterise Lunedale and the other northern Pennine dales. Geologists call each cycle of limestone, shale and sandstone a ‘cyclothem’. As Carboniferous time wore on, conditions changed so that less limestone and more sandstone was formed.

It is this pattern of cyclothems that gives the dale’s landscape much of its distinctive character. The limestones and sandstones are usually harder resistant rocks that form steep slopes or ledges on the hillsides. By contrast, the shales are weaker and easily worn away and typically form much gentler slopes between the ledges.

There are several limestones in Lunedale, but most conspicuous and most easily seen is the Great Limestone, so called from it generally being thicker than most of the others. This is the rock worked until recently at Selsett Quarry and that forms the grey limestone country at the dale’s head near the Cumbrian border. Evidence of its marine origin can be seen within it as fossilised shells, corals and as the distinctive small ring-shaped fragments of the extinct creatures related to modern sea urchins and star fish, known as crinoids or ‘sea lillies’. Other local limestones include the Four Fathom, Three Yard and Five Yard limestones, named centuries ago by quarrymen and miners who recognised their characteristic thicknesses. The Scar Limestone, that crops out on the hillside near Kirkcarrion is so-called from its tendency to form scars on the hillside: the Botany Limestone, seen on Mickleton Moor takes its name from Botany Farm. Most of the limestones are easily spotted in the landscape where they form light grey scars, or low crags, and where they typically support richer, brighter green, grass than the surrounding rocks. Many of the limestone outcrops were quarried to burn in small nearby kilns to make quicklime for lime mortar or slaked lime for soil improvement.

Like the limestones, the sandstones are usually rather hard rocks and commonly form well-marked ridges or benches in the landscape. They too have been widely quarried and have provided much of the stone from which local farms, churches and the many miles of drystone walls have been built. Some of the sandstones split readily into very thin slabs and have been used as roofing for both houses and farm buildings. The shales are seen only in a few places, mainly in the steep and rapidly eroding banks of streams.

Ancient stresses – bending and breaking the rocks

Towards the end of the Carboniferous Period, enormous forces in the earth’s crust bent and stretched the rocks. In Lunedale, rather spectacular folded rocks may be seen in the old opencast workings at Close House Mine. Elsewhere in the dale, one of the effects of such movements was to tilt the rocks southwards. This tilting is readily appreciated in the landscape from the Brough to Middleton road. Driving or walking eastwards from the neighbourhood of Wemmergill, erosion has picked out the conspicuous southerly dip of the originally horizontal beds of limestone and sandstone. Bending and titling of the rocks was accompanied in places by severe fracturing. Geologists call such fractures, along which the rocks have been displaced, faults. Being, in effect, huge cracks, they are usually lines of weakness and are easily eroded into valleys. Lunedale more or less follows the line of one of the largest faults in this part of northern England. Known as the Lunedale Fault, this great series of fractures can be traced from the neighbourhood of Brough in Cumbria, along the northern side of the dale, through Close House Mine and Greengates Quarry to just south of Mickleton, beyond which it can be traced through eastern County Durham. Along its course the rocks on its southern side have been displaced downwards by many metres relative to those on its northern side.

Molten and baking rock – the Whin Sill

These earth movements allowed molten rock or ‘magma’, at temperatures of around 1100°C, deep within the earth, to rise up, or intrude, along these lines of weakness. This magma did not reach the surface in northern England, but spread out as layers between the existing rocks, rather like the jam in a sandwich. As it did so it cooled and crystallised, forming the hard dense black rock called dolerite. This is the rock we see today forming the sombre dark crags along the southern side of Teesdale, at High Force, Cauldron Snout, and in places on the northern slopes of Lunedale above Grassholme Reservoir. Dolerite makes an excellent roadstone and has been much quarried for this purpose around Middleton: it is still worked at a huge quarry near High Force. A very spectacular exposure of this rock can be seen in Greengates Quarry where a vertical sheet, or dyke, of dolerite has been quarried for roadstone where it cuts beds of sandstone and limestone. The immense heat of the molten dolerite here altered the limestone to a complex rock with conspicuous crystals of the minerals garnet and vesuvianite. Elsewhere in Teesdale, other limestones were baked into white marble, known locally as ‘Sugar Limestone’. The term ‘Whin Sill’ is applied to this group of so-called intrusive rocks. Although now a formal scientific term used by geologists, the name Whin Sill is actually derived from local quarrymen’s terms. ‘Sill’ was a quarrymans’ term for any more or less horizontal layer of rock: ‘Whin’ was a name given to a hard black rock that was difficult to break. It is said that it comes from the ‘Whinnn….’ noise made when pieces are broken off. Modern research techniques enable us to date the formation of the Whin Sill at about 295 million years ago.

Hot waters – the formation of mineral veins

Shortly after the intrusion of the Whin Sill, residual heat from the granite buried beneath the Pennines, caused hot mineral-rich waters to circulate through cracks and faults in the rocks deep beneath the surface, in a manner similar to the circulation of water in a domestic central heating system. As they cooled, these solutions deposited their dissolved minerals on the walls of the fractures, rather like the furring up of central heating pipes, building up mineral veins.

In common with the countless other veins of the Northern Pennines, the veins in Lunedale contain galena(PbS), the main ore of lead. This attracted miners centuries ago who developed mines at Lunehead and near Closehouse. The galena is accompanied by much larger amounts of other minerals, the most abundant of which, here in Lunedale is baryte, or barytes (BaSO₄). For centuries this was regarded as worthless by the lead miners and either left in the ground or thrown onto the dumps. However, late in the 19th century it became a valuable raw material for making paint and other industrial uses. The rising value of barytes to some extent offset the disastrous effects of the worldwide collapse of lead prices in the late 19th century. Lunehead mines were re-worked for barytes until 1937, though unsuccessful exploration for further reserves continued until the 1970s. The Closehouse deposits, which had always been poor in lead, were opened up as one of the Northern Pennines’ major sources of barytes at the end of World War II and continued in production into the 1990s. During this time a range of interesting and rare minerals, including fine examples of anglesite (PbSO₄), pyromorphite(Pb₅(PO₄)₃Cl), rosasite (Cu,Zn)₂CO₃(OH)₂) and leadhillite (Pb₄SO₄(CO3)₂(OH)₂), were found at Closehouse. See images.

In Hunter’s Vein at Lunehead, the barium carbonate minerals witherite (BaCO₃) and barytocalcite (BaCa(CO₃)₂), occur in some abundance. Elsewhere in the world these are very rare minerals indeed: here in the Northern Pennines, for reasons that are yet to be understood, they are rather common. Extremely fine examples of barytocalcite from Lunehead are to be found in major museums including the Natural History Museum in London. Recent work by English Nature (now Natural England) has re-exposed the vein in the old opencast trenches, creating one of very few, perhaps the only, place in the world where this mineral may be seen exposed in situ at the surface.

Geological research at the Lunedale mines has added much to our understanding of the nature and origins both of the North Pennine mineralization and of similar deposits around the world.

Arctic Lunedale – the ice ages and after

From about 290 million years ago, Lunedale’s story, as revealed by its rocks, falls largely silent until about 2 million years ago, by which time northern England had come to occupy its present position on the earth’s surface. At this time, an episode of global cooling caused the polar ice caps to extend southwards to cover much of what is now Great Britain and northern Europe. On several occasions ice sheets, up to around 1 km thick covered our area. Between these periods the climate warmed to become at least as mild as it is today. The last major ice sheets melted here as recently as about 11,500 years ago. The ice sheets scoured the valleys and left in their wake great spreads of glacial debris, mainly in the form of boulder clay dumped beneath and from within the ice. Thick deposits of this stony clay mantle the floor of much of Lunedale and extend well up its sides, concealing the underlying solid rocks, especially in the neighbourhood of Selset Reservoir.

As the ice finally melted, vast quantities of melt water carved steep-sided gullies, many of which form conspicuous features on the north side of the dale between Harter Fell and Grassholme Reservoir. Continuing climatic variations in the succeeding few thousand years resulted in the extensive spreads of peat that cover much of the higher fell country.

The dale as we see today is just the latest stage in the continuing evolution of Northern England. Much has changed over the last half billion years, and will continue to change as these complex earth processes continue inexorably to shape and re-shape the landscape.

Geological Maps

Lunedale’s geology is shown in detail on British Geological Survey 1:50 000 scale Sheet 31 (Brough-under-Stainmore). This is available in two editions: the SOLID edition which depicts bedrock, or solid, geology only and the DRIFT edition which shows superficial deposits, including boulder clay, in addition to the solid rocks.
These maps can be obtained directly from the British Geological Survey Sales Desk, Keyworth, Nottingham NG12 5GG (e-mail: sales@bgs.ac.uk), or through booksellers.