Settle-Carlisle Partnership


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Geology

Introduction

The rocks that form the landscape of Settle-Carlisle Railway cover a vast period of geological time from about 480 million years ago to the present day. They tell a story of the journey of the region as it has drifted north from close to the south pole to its current position. This story includes the opening and closing of oceans, the building and erosion of mountain ranges and the formation and break-up of a vast super-continent. The rocks were laid down in warm, shallow tropical seas, huge delta systems, arid deserts and beneath thick ice sheets.

This section was created in association with the University of Leeds (Science and Tourism Project). Visit the website at www.see.leeds.ac.uk/scienceandtourism.

Dentdale – John Peter Buckley

Geological History

The oldest rocks found along the Settle to Carlisle line are Ordovician in age (about 480 million years old). They were deposited along the edge of a thin continent called Avalonia (which consisted of England and Scandinavia) that lay near the south pole. It was an archipelago similar to Indonesia today. Slowly, during the Ordovician and Silurian (about 480 to 420 million years ago), Avalonia drifted northwards. Sediments were washed from the continent into the sea along its northern margin. These sediments now form the Ordovician and Silurian rocks, outcrops of which can be seen in Ribblesdale, the Fells and the Lake District.

On the far side of the ocean from Avalonia lay the supercontinent of Laurentia (North America and Scotland). As the ocean closed, the edge of Avalonia was subducted beneath Laurentia and partly melted forming magmas. These magmas rose to the surface to erupt from a series of island volcanoes (similar to Japan today). The remains of these volcanoes are the Borrowdale Volcanic Group which form the distinctive landscape of the Lakes.

By the Mid Silurian, Avalonia was welded against Laurentia. However, movement continued along this boundary as continents to the south collided with Laurentia to form the supercontinent Pangaea (all the present day continents). Massive granites were intruded deep underground across what is now northern England. These include the granites that underlie the Askrigg and Alston blocks (see map) and the Lake District. Granite is lighter than the surrounding rocks, which is why these areas have formed high ground during much of the geological history of the region. The Pennine Fault Zone also formed at this time.

During the late Devonian to Early Carboniferous (about 380 to 340 million years ago), the region was stretched to form a low lying area known as the Pennine Basin. This was flooded by a shallow sea. Britain still lay to the south of the equator and the climate was hot and dry. The Askrigg and Alston blocks divided the basin into areas of deeper water and shallower water. Reefs and lagoons formed in the shallow water, much like the Bahamas today. Mudstones were deposited in the deeper water, together with limestone debris washed down from the coast by underwater landslides. These rocks form the Carboniferous Limestone Supergroup.

By the Late Carboniferous, rivers had developed across the mountains of the Scottish Southern Uplands and sediments began to be washed down into the Pennine Basin. A series of large deltas formed across the basin and the layers of mudstones, siltstones and sandstones deposited form the Millstone Grit. Britain had continued its drift north and now lay at the equator. The area would have been similar to the Mississippi Delta today with a hot, humid climate and rivers, swamps and lagoons surrounded and filled with lush vegetation. The thin coal seams and soils found in the Millstone Grit are the remains of this vegetation that grew in the delta.

As the Pennine Basin continued to fill, the region became a low lying landscape of swampy jungle, flood plains, lakes and river channels up to 10km wide, only very occasionally invaded by the sea (similar to the Amazon Basin). The Coal Measures have similar rock layers to the Millstone Grit except the coal seams are much thicker.

Towards the end of the Carboniferous, about 295 million years ago, the Whin Sill was formed by molten rock being injected between the layers of sediments. This then cooled and crystallized to form the Sill. It is made of dolerite, a hard wearing igneous rock which forms a ridge across much of northern Britain; Hadrian’s Wall is built along part of it.

By the early Permian, Britain was at the centre of the Pangaea super-continent. It was now north of the equator and lay within a desert the size of the Sahara. It was a desert of mountains, rocks, huge dune fields blown by trade winds, irregular rainfall, flash floods and extreme temperature changes between night and day. Today, the remains of this ancient desert are found within the Vale of Eden. They were deposited in a basin that formed along the Pennine Fault Zone. The line of this fault is still visible; it forms the escarpment between the Vale of Eden and the Pennines. The earliest desert rocks are the Brockram, which are broken fragments of Carboniferous limestones and sandstones washed down into the growing basin by flash floods. Above these are the brick-red desert Penrith Sandstones. These desert conditions lasted some 30 – 40 million years, before giving way to a hot, arid plain of seasonal rivers, salt flats and lagoons, similar to the Persian Gulf today, in which the Eden Shales were deposited. The Boreal Ocean lay just to the north and occasionally flooded the region depositing limestone layers within the Eden Shales.

The Triassic saw the beginning of the break up of Pangaea and the opening of the Atlantic. The red/yellow St Bees Sandstone was deposited from a river flowing NNW through the region of low lying desert plains. In the Late Triassic the sea repeatedly invaded leaving behind desert lakes in which the shales, gypsum, anhydrite and salt of the Mercia Mudstones were deposited.

There is now a gap of over 140 million years in the age of the rocks along the railway from the Mercia Mudstones (200 million years old) to the Armathwaite Dyke (57 million years old). During which time the region drifted from about 16° north to 40° north (roughly the latitude of the present day Mediterranean). From elsewhere it is known that with the opening of the Atlantic the region was flooded during the Jurassic and Cretaceous when chalk was deposited across much of Britain. These sediments buried the earlier Ordovician to Permian sediments, but at the end of the Cretaceous (about 65 million years ago) northern England was uplifted and the rocks deposited after the Mercia Mudstones were eroded away. The Armathwaite dyke formed during the volcanic activity associated with the opening of the Atlantic. It is part of a swarm of dykes (thin, vertical intrusions of igneous rock) that radiate out from the Isle of Mull.

Britain saw at least three advances and retreats of ice sheets during the last ice age. Evidence for a warmer period before the last advance of the ice sheets was found in the Victoria Cave near Settle where the bones of hippopotamus, straight-tusked elephant, rhinoceros, giant deer, bison and lion were discovered.

As glaciers move, they grind and pluck up rocks and sediment. They carry these, sometimes for great distances, before they lay them down again. These deposits are known as till (a mix of boulders, rock fragments and gravel held together by sandy clay) and are found across much of the region. In places such as the vale of Eden and Ribblesdale, movement of ice over the till has moulded it into drumlins – mounds up to 30m high, which taper in the direction of movement. Movement of the glaciers also scoured the landscape creating open U-shaped valleys and limestone pavements.

Ice is not the only mechanism that has created today’s landscape. Limestones consist of calcite and as rain water, which is naturally slightly acidic, percolates down through joints, along faults and between rock layers it dissolves this calcite forming caves and underground passages. Where caves and passages are near the surface, they can collapse into sink holes.

In the cool period following the ice age, hill peat was laid down. It is no longer being deposited as the climate is now too warm. In the last 10,000 years, since the ice age, rivers and streams have created flood plains from the sediments they carry.

Economic Geology

The rocks along the Settle to Carlisle railway have been mined and quarried for centuries. Several quarries are still active today: the quarries at Horton produce crushed rock for the building industry; Baronwood pit near Armathwaite is a source of sand; and gypsum and anhydrite are mined at Birkshead for the British Gypsum plaster mill at Kirkby Thore. Within the Carboniferous Limestones are veins of lead, zinc, fluorspar and barytes, all of which have been mined in the past. Lead mining is known in the area from Roman times. The limestone itself has been quarried to use in the chemical industry, glass manufacture and as agricultural fertilizer. The mudstones and shales of the Millstone Grit have been worked for bricks and tiles and the sandstones for building stone, and of course, as millstones. The coal seams have also been exploited, but are of far less importance than those of the main Coal Measures further south.

In the past the line has been blocked for periods of up to one month by snow

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