Carboniferous limestone
Encyclopedia
Carboniferous Limestone is a term used to describe a variety of different types of limestone
occurring widely across Great Britain
and Ireland
which were deposited during the Dinantian
epoch
of the Carboniferous
period. They were formed between 363 and 325 million years ago. Limestones are known from most geological periods though those dating from the Carboniferous, the Jurassic period and the Cretaceous period (commonly referred to as The Chalk
) are the most widespread.
The Mendip Hills
consist of Carboniferous limestone, showing notable geomorphological features, including Cheddar Gorge and the Avon Gorge
near Bristol
. Outcrops occur around the edge of the coalfields in south and north Wales
, where the Eglwyseg
Escarpment, near Llangollen
and the Great Orme
near Llandudno
are prominent features. There are a few outcrops in Shropshire
such as Titterstone Clee Hill
and at Little Wenlock
. It covers much of the area of the Derbyshire
Peak District
. However, the main outcrop in terms of area is in the Pennines
and surrounding the Lake District
.
s and crinoids are very much in evidence as components of Carboniferous limestone; indeed the rock is full of fossils.
Carboniferous limestone has horizontal layers (beds) with bedding planes, and vertical joints. These joints are weaknesses in the rock, which are exploited by agents of both denudation
and weathering
. They also lead to the most important characteristic of Carboniferous limestone - its permeability
. Water seeps through the joints in the limestone. This creates a landscape that lacks surface drainage but which has all manner of characteristic surface and subsurface features. The Carboniferous Limestone has been folded and faulted by massive Earth movements which can be seen by the fact that the rocks are now above sea-level and no longer horizontal. The rocks generally dip (slope) gently eastwards and, in some places, clear folds in the rock can be seen especially at the Great Orme and Bryn Alyn (Denbighshire).
to the village of Malham, UK via Watlowes Valley and back again via Gordale Scar
. The walk has an example of nearly every significant surface limestone feature.
Small surface depressions called shakeholes, which are 1-3m deep and 3-5m across, form as a result of the subsurface collapse of limestone. Shakeholes are very common throughout the Yorkshire Dales. Larger depressions are called dolines.
Streams flowing from higher impermeable slopes sink into the ground when they reach permeable limestone. During dry spells all water sinks very quickly on reaching the limestone, through sinkholes. In wetter conditions water flows a greater distance across the limestone as underground channels and chambers fill up. Large sinkholes are called 'swallowholes' or 'potholes'. Gaping Gill
, Alum Pot and the Buttertubs are well-known examples.
Dry valleys are valleys without streams. Watlowes Valley is an excellent example. It was formed originally by a subglacial meltwater stream which existed during the last major Ice Age
. After the ice retreated, the valley was further developed by a meltwater stream flowing across the limestone while it was frozen solid.
Watlowes Valley is a particularly good example of a dry valley because it has a textbook profile - the south-facing side is less steep than the north-facing side. This results from the weathering and mass movement processes that have operated in the post-glacial period.
A limestone pavement
is an area of almost bare, flat rock and is arguably the most fascinating feature of any area of carboniferous limestone. They develop after the rock has been exposed by the scouring action of an ice sheet
or glacier
. Existing joints are subsequently exploited by the action of chemical weathering carbonation
to form deep grykes and rounded blocks called clints. Grykes have a habitat of their own, which encourages the growth of shade-loving ferns such as hart's tongue and Dog's Mercury
.
During the last Ice Age, Malham Cove
- the most spectacular feature in the Yorkshire Dales - was a waterfall comparable in size to the Horseshoe Falls
of Niagara. At the end of the Ice Age the limestone, which had been frozen solid, once again became permeable, allowing the water to disappear through its joints. Now Malham Cove is a high cliff (83m high) - it is completely dry, and a great attraction to rock climbers.
A gorge is a steep-sided valley
, generally formed in a limestone area as the result of the collapse of a roof above a cave system. Gordale Scar
is an excellent example.
, there are numerous caves, three of which - Ingleborough Cave
, White Scar Caves
and Stump Cross Caverns
- are now show caves for the public.
The caves themselves and their associated formations vary greatly in size, but they all depend on the process of carbonation
for their creation. Carbonation is a reversible process: it results in the redeposition of calcite, and the development of stalagmite
s, stalactites, straw stalactites, helictites, pillars
and flowstone
. the rate of growth of these formations is about 1cm per 200 years - it is therefore an extremely slow process. Some stalactites may be millions of years old.
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
occurring widely across Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
and Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
which were deposited during the Dinantian
Dinantian
Dinantian is the name of a series or epoch from the Lower Carboniferous system in Europe. It can stand for a series of rocks in Europe or the time span in which they were deposited....
epoch
Epoch (geology)
An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...
of the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
period. They were formed between 363 and 325 million years ago. Limestones are known from most geological periods though those dating from the Carboniferous, the Jurassic period and the Cretaceous period (commonly referred to as The Chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
) are the most widespread.
Outcrops
Mendip Hills
The Mendip Hills is a range of limestone hills to the south of Bristol and Bath in Somerset, England. Running east to west between Weston-super-Mare and Frome, the hills overlook the Somerset Levels to the south and the Avon Valley to the north...
consist of Carboniferous limestone, showing notable geomorphological features, including Cheddar Gorge and the Avon Gorge
Avon Gorge
The Avon Gorge is a 1.5-mile long gorge on the River Avon in Bristol, England. The gorge runs south to north through a limestone ridge west of Bristol city centre, and about 3 miles from the mouth of the river at Avonmouth. The gorge forms the boundary between the unitary authorities of...
near Bristol
Bristol
Bristol is a city, unitary authority area and ceremonial county in South West England, with an estimated population of 433,100 for the unitary authority in 2009, and a surrounding Larger Urban Zone with an estimated 1,070,000 residents in 2007...
. Outcrops occur around the edge of the coalfields in south and north Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
, where the Eglwyseg
Eglwyseg
The Eglwyseg valley is an area to the north east of Llangollen in Denbighshire, Wales; it is within the boundaries of Llantysilio Community. The name also refers to a widely-scattered hamlet in the valley....
Escarpment, near Llangollen
Llangollen
Llangollen is a small town and community in Denbighshire, north-east Wales, situated on the River Dee and on the edge of the Berwyn mountains. It has a population of 3,412.-History:...
and the Great Orme
Great Orme
The Great Orme is a prominent limestone headland on the north coast of Wales situated in Llandudno. It is referred to as Cyngreawdr Fynydd in a poem by the 12th century poet Gwalchmai ap Meilyr...
near Llandudno
Llandudno
Llandudno is a seaside resort and town in Conwy County Borough, Wales. In the 2001 UK census it had a population of 20,090 including that of Penrhyn Bay and Penrhynside, which are within the Llandudno Community...
are prominent features. There are a few outcrops in Shropshire
Shropshire
Shropshire is a county in the West Midlands region of England. For Eurostat purposes, the county is a NUTS 3 region and is one of four counties or unitary districts that comprise the "Shropshire and Staffordshire" NUTS 2 region. It borders Wales to the west...
such as Titterstone Clee Hill
Titterstone Clee Hill
Titterstone Clee Hill, sometimes referred to as Clee Hill or Titterstone Clee, is a hill in the rural English county of Shropshire, rising at the summit to 533 m above sea level....
and at Little Wenlock
Little Wenlock
Little Wenlock is a small village and civil parish in Shropshire, England. It was mentioned in the Domesday Book, when it belonged to Wenlock Priory...
. It covers much of the area of the Derbyshire
Derbyshire
Derbyshire is a county in the East Midlands of England. A substantial portion of the Peak District National Park lies within Derbyshire. The northern part of Derbyshire overlaps with the Pennines, a famous chain of hills and mountains. The county contains within its boundary of approx...
Peak District
Peak District
The Peak District is an upland area in central and northern England, lying mainly in northern Derbyshire, but also covering parts of Cheshire, Greater Manchester, Staffordshire, and South and West Yorkshire....
. However, the main outcrop in terms of area is in the Pennines
Pennines
The Pennines are a low-rising mountain range, separating the North West of England from Yorkshire and the North East.Often described as the "backbone of England", they form a more-or-less continuous range stretching from the Peak District in Derbyshire, around the northern and eastern edges of...
and surrounding the Lake District
Lake District
The Lake District, also commonly known as The Lakes or Lakeland, is a mountainous region in North West England. A popular holiday destination, it is famous not only for its lakes and its mountains but also for its associations with the early 19th century poetry and writings of William Wordsworth...
.
Characteristics
Carboniferous limestone is a sedimentary rock made of calcium carbonate. It is generally light-grey in colour, and is hard. It was formed in warm, shallow tropical seas teeming with life. The rock is made up of the shells and hard parts of millions of sea creatures, some up to 30 cm in length, encased in carbonate mud. Fossil corals, brachiopodBrachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s and crinoids are very much in evidence as components of Carboniferous limestone; indeed the rock is full of fossils.
Carboniferous limestone has horizontal layers (beds) with bedding planes, and vertical joints. These joints are weaknesses in the rock, which are exploited by agents of both denudation
Denudation
In geology, denudation is the long-term sum of processes that cause the wearing away of the earth’s surface leading to a reduction in elevation and relief of landforms and landscapes...
and weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
. They also lead to the most important characteristic of Carboniferous limestone - its permeability
Permeability (fluid)
Permeability in fluid mechanics and the earth sciences is a measure of the ability of a porous material to allow fluids to pass through it.- Units :...
. Water seeps through the joints in the limestone. This creates a landscape that lacks surface drainage but which has all manner of characteristic surface and subsurface features. The Carboniferous Limestone has been folded and faulted by massive Earth movements which can be seen by the fact that the rocks are now above sea-level and no longer horizontal. The rocks generally dip (slope) gently eastwards and, in some places, clear folds in the rock can be seen especially at the Great Orme and Bryn Alyn (Denbighshire).
Surface Features
The 'classic limestone walk' is a circular 10 km route from the field centre on the north side of Malham TarnMalham Tarn
-External links:****...
to the village of Malham, UK via Watlowes Valley and back again via Gordale Scar
Gordale Scar
Gordale Scar is a dramatic limestone ravine 1 mile or 1.5 km NE of Malham, North Yorkshire, England. It contains two waterfalls and has overhanging limestone cliffs over 100 metres high. The gorge was formed by water from melting glaciers...
. The walk has an example of nearly every significant surface limestone feature.
Small surface depressions called shakeholes, which are 1-3m deep and 3-5m across, form as a result of the subsurface collapse of limestone. Shakeholes are very common throughout the Yorkshire Dales. Larger depressions are called dolines.
Streams flowing from higher impermeable slopes sink into the ground when they reach permeable limestone. During dry spells all water sinks very quickly on reaching the limestone, through sinkholes. In wetter conditions water flows a greater distance across the limestone as underground channels and chambers fill up. Large sinkholes are called 'swallowholes' or 'potholes'. Gaping Gill
Gaping Gill
Gaping Gill is a natural cave in North Yorkshire, England. It is one of the unmistakable landmarks on the southern slopes of Ingleborough – a deep pothole with the stream Fell Beck flowing into it...
, Alum Pot and the Buttertubs are well-known examples.
Dry valleys are valleys without streams. Watlowes Valley is an excellent example. It was formed originally by a subglacial meltwater stream which existed during the last major Ice Age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
. After the ice retreated, the valley was further developed by a meltwater stream flowing across the limestone while it was frozen solid.
Watlowes Valley is a particularly good example of a dry valley because it has a textbook profile - the south-facing side is less steep than the north-facing side. This results from the weathering and mass movement processes that have operated in the post-glacial period.
A limestone pavement
Limestone pavement
A limestone pavement is a natural karst landform consisting of a flat, incised surface of exposed limestone that resembles an artificial pavement. The term is mainly used in the UK where many of these landforms have developed distinctive surface patterning resembling block of paving...
is an area of almost bare, flat rock and is arguably the most fascinating feature of any area of carboniferous limestone. They develop after the rock has been exposed by the scouring action of an ice sheet
Ice sheet
An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier...
or glacier
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
. Existing joints are subsequently exploited by the action of chemical weathering carbonation
Carbonation
Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...
to form deep grykes and rounded blocks called clints. Grykes have a habitat of their own, which encourages the growth of shade-loving ferns such as hart's tongue and Dog's Mercury
Dog's Mercury
Mercurialis perennis, commonly known as dog's mercury, is a woodland plant found in much of Europe, but almost absent from Ireland, Orkney and Shetland. A member of the spurge family , it is a herbaceous, downy perennial with erect stems bearing simple, serrate leaves. The dioecious inflorescences...
.
During the last Ice Age, Malham Cove
Malham Cove
Malham Cove is a natural limestone formation 1 km north of the village of Malham, North Yorkshire, England. A well-known beauty spot, it is a large, curved limestone cliff at the head of a valley, with a fine area of limestone pavement at the top....
- the most spectacular feature in the Yorkshire Dales - was a waterfall comparable in size to the Horseshoe Falls
Horseshoe Falls
The Horseshoe Falls, also known as the Canadian Falls, is part of Niagara Falls, on the Niagara River. Approximately 90% of the Niagara River, after diversions for hydropower generation, flows over Horseshoe Falls. The remaining 10% flows over the American Falls...
of Niagara. At the end of the Ice Age the limestone, which had been frozen solid, once again became permeable, allowing the water to disappear through its joints. Now Malham Cove is a high cliff (83m high) - it is completely dry, and a great attraction to rock climbers.
A gorge is a steep-sided valley
Valley
In geology, a valley or dale is a depression with predominant extent in one direction. A very deep river valley may be called a canyon or gorge.The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys...
, generally formed in a limestone area as the result of the collapse of a roof above a cave system. Gordale Scar
Gordale Scar
Gordale Scar is a dramatic limestone ravine 1 mile or 1.5 km NE of Malham, North Yorkshire, England. It contains two waterfalls and has overhanging limestone cliffs over 100 metres high. The gorge was formed by water from melting glaciers...
is an excellent example.
Sub-Surface Features
The most common examples of subsurface features in a limestone landscape are caves. In the Yorkshire DalesYorkshire Dales
The Yorkshire Dales is the name given to an upland area in Northern England.The area lies within the historic county boundaries of Yorkshire, though it spans the ceremonial counties of North Yorkshire, West Yorkshire and Cumbria...
, there are numerous caves, three of which - Ingleborough Cave
Ingleborough Cave
Ingleborough Cave is a show cave close to the village of Clapham, North Yorkshire, England adjacent to where the water from Gaping Gill resurges....
, White Scar Caves
White Scar Caves
White Scar Caves is a network of caves under Ingleborough in the Chapel-le-Dale valley of the Yorkshire Dales National Park in the North of England.Discovered in August 1923 by two amateur geologists, Christopher Long and J.H...
and Stump Cross Caverns
Stump Cross Caverns
Stump Cross Caverns is a limestone cave system between Wharfedale and Nidderdale in North Yorkshire, England.-Geography:The caverns at Stump Cross are located beneath Greenhow hill, 1275 metres above sea level. Their name was taken from Stump Cross, which in ancient times marked the limit of...
- are now show caves for the public.
The caves themselves and their associated formations vary greatly in size, but they all depend on the process of carbonation
Carbonation
Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...
for their creation. Carbonation is a reversible process: it results in the redeposition of calcite, and the development of stalagmite
Stalagmite
A stalagmite is a type of speleothem that rises from the floor of a limestone cave due to the dripping of mineralized solutions and the deposition of calcium carbonate. This stalagmite formation occurs only under certain pH conditions within the underground cavern. The corresponding formation on...
s, stalactites, straw stalactites, helictites, pillars
Column
A column or pillar in architecture and structural engineering is a vertical structural element that transmits, through compression, the weight of the structure above to other structural elements below. For the purpose of wind or earthquake engineering, columns may be designed to resist lateral forces...
and flowstone
Flowstone
Flowstones are composed of sheetlike deposits of calcite formed where water flows down the walls or along the floors of a cave. They are typically found in "solution", or limestone caves, where they are the most common speleothem. However, they may form in any type of cave where water enters that...
. the rate of growth of these formations is about 1cm per 200 years - it is therefore an extremely slow process. Some stalactites may be millions of years old.
Economics
Because it is brittle, the use of Carboniferous Limestone for building stone tends to be limited to those areas where it is the most abundantly available rock. However it is extensively quarried for other purposes:- It is crushed for roadstone and aggregateConstruction AggregateConstruction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world...
wherever it outcrops, particularly in the Mendips and north Wales. - It is burned for limeLime (mineral)Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
in many places. In certain places (e.g. TunsteadTunstead, DerbyshireTunstead is a village in Derbyshire, England, situated above Great Rocks Dale north of Buxton.It should not be confused with Tunstead Milton, which is roughly five miles to the north west....
in the Peak District, and Horton in RibblesdaleHorton in RibblesdaleHorton in Ribblesdale is a small village and civil parish in the Craven district of North Yorkshire, England. It is situated in Ribblesdale on the Settle–Carlisle Railway to the west of Pen-y-ghent....
in the Pennines), it is sufficiently pure for production of chemical-grade lime. - It is used in cementPortland cementPortland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...
manufacture at plants in England (4), Wales (2), Scotland (1) and Ireland (4). - In ground form, it is used for power industry flue gas desulfurizationFlue gas desulfurizationSulfur dioxide is one of the elements forming acid rain. Tall flue-gas stacks disperse emissions by diluting the pollutants in ambient air and transporting them to other regions....
. - In many places it is metalliferous, and has yielded leadLeadLead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
(in the Peak District and WeardaleWeardaleWeardale is a dale, or valley, of the east side of the Pennines in County Durham, in England. Large parts of Weardale fall within the North Pennines Area of Outstanding Natural Beauty - the second largest AONB in England and Wales. The upper valley is surrounded by high fells and heather grouse...
, and copperCopperCopper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
(in North Wales, where important Bronze AgeBronze AgeThe Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...
mines are to be found). - It was important in the early Industrial RevolutionIndustrial RevolutionThe Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, transportation, and technology had a profound effect on the social, economic and cultural conditions of the times...
when, following the inventions of Abraham DarbyAbraham DarbyAbraham Darby may refer to:*Abraham Darby I *Abraham Darby II *Abraham Darby III *Abraham Darby IV , High Sheriff of BuckinghamshireAbraham Darby may also refer to:...
, it was used in combination with nearby coalCoalCoal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
and ironstoneIronstoneIronstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical repacement, that contains a substantial proportion of an iron compound from which iron either can be or once was smelted commercially. This term is customarily restricted to hard coarsely...
from the Coal MeasuresCoal MeasuresThe Coal Measures is a lithostratigraphical term for the coal-bearing part of the Upper Carboniferous System. It represents the remains of fluvio-deltaic sediment, and consists mainly of clastic rocks interstratified with the beds of coal...
in the ironIronIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
industry.