Carboniferous

This period of geological time is informally divided into the Lower Carboniferous and the Upper Carboniferous. In South Gloucestershire the following series are represented, the youngest at the top:

The Carboniferous Limestones (Dinantian)

The succession of the different limestones may be seen in the large quarries at Slickstones ST713905, Chipping Sodbury ST724831, Codrington ST724783 and Wick ST710732.

There is no public access to these quarries. Study groups may be granted permission to enter on application to the quarry owners.

The Black Rock Limestone is extensively quarried. It is very dark in colour and contains very many fossils. Another well-known limestone is the Clifton Down Limestone, which contains good examples of a fossil coral Lithostrotian spp. I will send photo There are many other beds of limestones and shales representing slightly different environmental conditions at the time of their deposition, but all with a dominance of calcium in the waters at the time of formation.

The beds of limestone were initially soft layers of lime-rich, muddy, sea-floor sediments. The material from which they were formed may be almost entirely shell debris or material derived from the seawater or both. Commonly found in the limestone are fossils including shells of Productids and Spiriferids and the corals of Zaphrentis, Caninia and Lithostrotion.

These sediments were laid down in tropical seas when the region was just south of the equator. Each layer of sediment was covered by others until a great thickness was reached and the beds were eventually converted into hard rock. The abundance of fossils in some of the beds testified to the success of a great variety of marine life forms existing at the time. It is calculated that the fossil content of the rock represents only one-millionth part of the life forms of the times.

Whilst the majority beds in this sequence are limestones, there are some clay-rich bands and some sandstones. The few clay-rich horizons denote shallow water of low energy, possibly drying out occasionally.

The sandstones also give evidence of being formed in shallow water. Their top surface is rippled. These form marker horizons known locally as the Middle and Upper Cromhall sandstones.

After the rocks were formed, due to the collision between two large continental landmasses, the hard, flat-lying rocks were folded into mountains. The mountain chain stretched from America to Czechoslovakia.

The folding was on a grand scale, Cromhall, Wickwar, Chipping Sodbury, Codrington and Wick have quarries producing the same materials at the eastern edge of the basin, while Olveston, Tytherington and others worked the same beds on the western rim.

The mountain building process and the uneven loading of the crust have produced thrust and fault planes. The thrusts have allowed some older rocks to sit on younger rocks. Faults have cut across sets of beds, allowing those on one side of the fault to be higher or lower than those on the other side. These separated units can be correlated using the fossils.

All the quarries show the steeply dipping bedding planes at an angle of about 40 degrees.

The Carboniferous limestones are used for hardcore today, but in the past they were used extensively for vernacular buildings and this is well illustrated in the former medieval town of Chipping Sodbury where there is an interpretation board in the car park near the church. ST 728824.

Towards the end of the Dinantian, environmental conditions changed. Sequences of black mudstones, limestones, seat earth (a fossil soil) and sandstones, typical of rocks formed in a delta, are found.

Deltas occur where big river systems sub-divide into many distributing channels and islands at the point where they reach the sea. The Mississippi delta is a modern example. The delta front builds out into the sea, depositing clays, silts sands and gravels. From time to time the pattern of channels shifts, and so the sequence of deposits changes.

In South Gloucestershire the rocks formed in these conditions are the Upper Cromhall Sandstones which outcrop around Cromhall, where there is a big quarry producing roadstone. It is not possible to see the quarry, however, samples of rock from the quarry are displayed on the green at Cromhall where there is also an interpretation board.

A repeating succession of rocks is termed cyclic sedimentation and is typical of much of the Carboniferous Period, but the nature of the cycles changed according to the availability of different sediments and the environmental conditions, which prevailed at the time.

The Namurian

In South Gloucestershire this age is represented by the Quartzitic Sandstone Formation which is present in the area around Cromhall, where it attains its maximum thickness of about 300 meters. These rocks are also formed in a deltaic environment. The rocks are mudstones, seat earth and sandstone, with the occasional thin coal seam, a forerunner of the next development in the Carboniferous.

Coal Measures Westphalian

The rocks formed towards the end of the Carboniferous period are mudstones and sandstones which contain many coal seams, and which are together called the Coal Measures. They are divided up into three main units (with approximate thickness), youngest at the top:

When the original muds and sands were deposited beneath lakes and rivers some 300 million years ago, the area must have been on the equator. Fossils in the rocks help us to imagine the former tropical rain forest that covered the land, the remains of which are preserved in the coal seams.

South Gloucestershire is part of a huge line of Coal Measure rocks which stretch across much of Europe and North America. These mark the position of the original forests and very often are the sites of the industry, which was based on the fossil fuel the coal provided.

The Lower Coal Series is mostly made of grey coloured mudstones and these contain the coal seams that were worked in the Kingswood area. The rocks here are folded up into a complex arch-shaped anticline, with many faults. Despite the difficult structure, there was a long history of coal mining. Up to 20 seams were worked across the area, although none was more than about 2.0m. (6ft 6in) thick.

After the deposition of the Lower Coal Measures. marine incursions ceased for a time and the area became part of a large delta with many rivers depositing their load of materials eroded from older harder rocks. These were mostly grey in colour and containing the minerals quartz, mica and felspar. Grains cemented to form the rock varied in size from coarse sand to fine silt and clay, but the sand dominated. The micas tend to be in suspension longer in sediments, so occur on the top of the beds. They can be distinguished as they glint in sunlight. The currents in the rivers varied in direction forming ripples which were partly eroded by later currents giving rise to a characteristic known as cross bedding.

The Pennant Sandstone contains few coal seams but was well known as a good building stone. Many quarries worked the stone and it had many uses. Hundreds of local buildings have Pennant Sandstone walls, but, if the stone could be split into flat sheets, it was also used for paving stones, gravestones and even for roofing. Its usual colour is greenish grey, but it is sometimes stained red or purple by iron minerals. In places such as Iron Acton, the Pennant was mined for iron ore.

The red colour makes the Pennant a very attractive stone, which has been widely used as a building stone in the villages of the southern part of the area, such as Iron Acton, Frampton Cotterell Winterbourne and Hambrook. The stone is not a freestone, i.e. one that cuts easily into blocks, so the buildings have a rough-hewn appearance. Iron ActonWall index jpg Because of this it was often used as a rubble stone and rendered. Nearer to Bristol the rock tends to be greyish green in colour. It was used extensively as a building stone for Victorian houses particularly, in Bristol.

The quarries were excavated along the river valleys of the Avon near Hanham and the Frome near Winterbourne. Harcome Quarry and Damson Bridge in Winterbourne are good sites to study the Pennant. It will be noted that there is a great deal of jointing and faulting, which resulted from later earth movements.

Harcombe Quarry ST 644796 was opened specifically to provide stone for the inside of the pillars of the railway viaduct on the main South Wales to London railway line. It is now a Local Nature Reserve and an interpretation board tells the story of the rocks and the quarry.

The Upper Coal Series are mainly red and grey mudstones, which contain a few coal seams. These were worked in the Coalpit Heath area, where the rocks are folded down into a syncline. Again none of the seams was more than 2m. (6ft 6in) thick, but there were fewer faults and the coal was easier to work.

The best-preserved coalmine in South Gloucestershire is the Brandy Bottom Colliery at Shortwood. Its distinctive engine house chimney still stands, and a huge winding wheel lies beside the cycle path. The mine probably started late in the 1700's and finally closed in 1936.

Mine shafts took the miners nearly 210 m (700 ft) underground to work up to four coal seams in the Upper Coal Series, none of which was more than 75cm (2ft 6in) thick. Coal was taken from the pit along the horse-drawn railway (known locally as the "Dramway" and on the Midland Railway (both now footpaths).

Shortwood Brick Works was near the Brandy Bottom Colliery and it used good quality red clay found in the Upper Coal Series and millions of bricks were fired for local buildings. It was well known for the strength and attractive appearance of its bricks. The works started in the late 1800šs and finally closed in 1969. The same clay is still quarried nearby but is now taken to the Ibstock Brick Works at Cattybrook.

Other good exposures of Coal Measure rocks can be seen along the Cycleway between Bristol and Bath, including Staple Hill Tunnel (ST 651 756) and Cherry Gardens Cutting (ST 669 708). Willsbridge Mill, owned by the Avon Wildlife Trust has a geology trail around the rocks of the Pennant Sandstone on their site at ST 665708. To obtain the trail send SAE to Ruth Worsley AWLT Willsbridge Mill, Willsbridge Hill, South Gloucestershire BS30 6EX.

During the later part of the Coal Measure times and into the next geological period, the Permian, there were massive earth movements caused by the meeting of plate margins. The event is termed the Variscan period of mountain building and it affected the Carboniferous rocks, which today extend across Europe as well as the British Isles. Massive folds developed, and where the stresses were too great the rocks became faulted and some places beds were thrust over other beds.

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