The geology of the North Pennines and its impacts on landscape
The special character of the North Pennine landscape has its foundation in the underlying rocks and the geological processes which have shaped it over hundreds of millions of years of Earth history. Tropical seas, deltas, rainforests, molten rock, deserts and ice sheets have all played a part in creating the bare bones of the landscape. People arrived in the North Pennines about 10,000 years ago, heralding a new stage in the evolution of the area – a landscape that is continually evolving through natural processes and human activity.
The oldest rocks
The deep roots of the North Pennines are slates and volcanic rocks, which formed between 500 and 420 million years ago, in the Ordovician and Silurian periods of Earth history. These rocks were once mud and volcanic ash at the edge of a wide ocean. When the ocean closed about 420 million years ago, they were squashed and altered to form hard slaty rocks. They are mostly buried, but can be seen in one part of Upper Teesdale and form distinctive conical hills along the North Pennine escarpment.
Weardale Granite and the Alston Block
About 400 million years ago, a huge mass of molten rock rose up into the slates and volcanic rocks. It cooled and crystallized underground to form the Weardale Granite – a hidden but fundamental geological feature of the North Pennines. Earth movements and extreme weather over time led to the removal of the upper parts of the granite, slates and other rocks. Granite is less dense than most other rocks in the Earth’s crust and is relatively buoyant. Because of this, the area above the granite – much of the North Pennines – has remained higher than surrounding areas for millions of years, and is known by geologists as the ‘Alston Block’. The North Pennines is an upland area today because of the effect of the Weardale Granite.
Tropical seas and swamps
About 350 to 300 million years ago – in the Carboniferous Period of Earth history – the North Pennines was near the equator and was periodically covered by shallow tropical seas. Skeletons of sea creatures accumulated as limy ooze on the sea floor. Rivers washed mud and sand into the sea, building up vast deltas on which swampy forests grew. In time, the limy ooze became limestone, the mud and sand became shale and sandstone, and the forests turned to coal. Periodically, the sea flooded in, drowning the deltas and depositing limestone again. This cycle happened many times, building up repeating layers of limestone, shale, sandstone and thin coal seams, known as ‘cyclothems’. Limestone and sandstone are resistant to erosion, whereas the softer shales wear away easily. This contrast produces the distinctive terraced hillsides and flat hilltops of the North Pennines. Limestone also has its own special features. It dissolves gradually in rainwater creating ‘karst’ features such as sinkholes and limestone pavements. Sandstone and limestone have been quarried in the North Pennines for centuries, and the use of local sandstone gives distinctive character to the area’s settlements and drystone walls.
The Whin Sill
Stretching of the Earth’s crust 295 million years ago caused molten rock at over 1000ºC to rise up and be injected between layers of sandstone, limestone and shale. The molten rock cooled and solidified underground to form a roughly flat-lying sheet of rock, known as a ‘sill’. This is made of hard black dolerite or, as it is known locally, whinstone. While molten, its great heat baked and altered surrounding rocks, creating the unique ‘Sugar Limestone’ of Upper Teesdale. As the sill cooled it contracted, producing vertical cracks along which the dolerite breaks into rough columns. These columns can be seen in Whin Sill cliffs and quarry faces. After millions of years of erosion, the Whin Sill is now exposed at the surface where its cliffs form dramatic landscape features in Upper Teesdale and along the North Pennine escarpment.
Mineral riches
The North Pennines is world-famous for its remarkable mineral veins and deposits, known collectively as the Northern Pennine Orefield. The veins of lead ore and other minerals formed about 290 million years ago when mineral-rich waters flowed through cracks and fractures deep underground. As the fluids cooled, their dissolved minerals crystallized within the fractures, forming mineral veins. Sometimes the fluids reacted with limestone on the sides of the fractures, altering the rock and forming mineral deposits known as ‘flats’. Mining for lead ore in the North Pennines probably goes back at least to Roman times, but it had its heyday in the 18th and 19th centuries when the area’s lead mines were of world importance. Other commercially mined minerals include sphalerite (zinc ore), iron ores, fluorite (also known as fluorspar), and barium minerals such as baryte and witherite. Mining for these minerals has had a profound effect on the landscape. Although the mines have all closed, the landscape is imprinted with the legacy of the area’s mining past – from shafts, hushes, spoil heaps and chimneys to the patterns of settlement and ‘miner-farmer’ landscapes.
Deserts and floods
The Eden Valley is underlain by red sandstones which give the villages at the foot of the North Pennine escarpment their distinctive character. These rocks formed between 290 and 210 million years ago, in the Permian and Triassic periods of Earth history, when the North Pennines baked in a hot desert environment just north of the equator. Sands from desert dunes, flash floods and rivers hardened into red sandstones. These rocks lie next to the much older slates and volcanic rocks of the distinctive ‘pikes’ and are separated from them by faults – cracks in the Earth’s crust along which there has been movement.
Ice age
From over 200 million years ago, there is little tangible evidence for what was happening in the North Pennines. It is known that Britain drifted north to its present position, and that about two million years ago world climate cooled dramatically, heralding the start of a series of ice ages. The landscape of the North Pennines owes much of its character to the action of ice and meltwater. About 20,000 years ago northern Britain lay frozen under a huge blanket of ice. A kilometre-thick ice sheet covered the North Pennines and streamed over the landscape, smoothing and scouring the hills and valleys. It dumped a mixture of clay, gravel and boulders known as ‘till’ and created streamlined mounds of glacial debris called drumlins. Some of the highest land in the North Pennines may have poked above the ice at times during the ice age. These hilltops would have been frozen wastes of frost-shattered rock.
After the ice
About 15,000 years ago the arctic conditions started to give way to a milder, wetter climate. The ice began to melt, leaving a landscape of bare rock, unstable slopes and piles of glacial debris. Torrential meltwaters carved drainage channels and deposited sand and gravel in the valleys. Amidst this rapidly changing landscape, arctic plants, grasses and dwarf shrubs began to colonise the bare land. These were eventually replaced by woodland – part of the great wildwood which once covered much of Britain. Sparse birch and Scots pine dominated the higher parts of the North Pennines. About 7,500 years ago, rainfall increased and blanket bog began to form on the waterlogged uplands. In these areas woodland cover decreased, leaving tree stumps buried and preserved in peat.
People and the landscape
Ever since people first came to the North Pennines, perhaps 10,000 years ago, human activity has profoundly influenced the landscape. The first settlers arrived in a landscape of wooded valleys, very different from today’s meadows and grassland. Woodland flourished in the valleys until about 5,000 years ago when early farmers began to fell the trees. Through the following millennia, many different peoples – Celts, Romans, Saxons, Vikings, Normans – left their mark in settlements, fortifications, field systems, graves and mines. But it is in the last few hundred years that people have had the greatest impact on the North Pennine landscape. Centuries of exploitation of the area’s rich mineral resources have not only left a rich heritage of mining remains, but have influenced the pattern of settlement and agriculture and even the shape of the fells and dales themselves.
Today’s landscape
Today’s North Pennine landscape is the product of millions of years of geological processes and just a few thousand years of human activity. All these have lent a hand in creating both the shape of the countryside and the intricate ‘quilt’ of land use and settlement draped over it. Most people live in the dales which cut through the wild moorland landscape. Villages, farms and dry stone walls built of local stone reflect the underlying geology of the area. The imprint of lead and other mining activity is still strong, with chimneys, hushes, adits, mineshops and other features providing a reminder of our industrial past.