Carboniferous Limestone

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.

Outcrops
Cross-section of a Carboniferous limestone bored by Jurassic organisms; borings include Gastrochaenolites (some with boring bivalves in place) and Trypanites; Mendip Hills, England; scale bar = 1 cm.

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 Ormes Head are prominent features. There are a few outcrops in Shropshire such as Titterstone Clee hill and 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.

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, brachiopods 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).

Surface Features

The 'classic limestone walk' is a circular 10 km route from the field centre on the north side of Malham Tarn 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.

Sub-Surface Features

The most common examples of subsurface features in a limestone landscape are caves. In the Yorkshire Dales, 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 stalagmites, 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.

Economics

Carbonifereous limestone has not been much used as a building stone, because it is brittle, but it is extensively quarried for other purposes:

* It is crushed for roadstone and aggregate wherever it outcrops, particularly in the Mendips and north Wales.
* It is burned for lime in many places. In certain places (e.g. Tunstead in the Peak District, and Horton in Ribblesdale in the Pennines), it is sufficiently pure for production of chemical-grade lime.
* It is used in cement manufacture at plants in England (4), Wales (2), Scotland (1) and Ireland (4).
* In ground form, it is used for power industry flue gas desulfurization.
* In many places it is metalliferous, and has yielded lead (in the Peak District and Weardale, and copper (in North Wales, where important Bronze Age mines are to be found).
* It was important in the early Industrial Revolution when, following the inventions of Abraham Darby, it was used in combination with nearby coal and ironstone from the Coal Measures in the iron industry.