Overview of environment - a warm climate with low sea levels and a high amount of oxygen in the atmosphere (35% compared to 21% today) leading to the development of extremely large plants and insects. The earth's climate shifted constantly between glacial and interglacial climates, similar to today.
Significant events - the continents collided to form Pangea, leading to many orogenies (the crust folding to form mountains), at the end of this period the Carboniferous rainforest collapsed due to climate change.
Period 2 Silurian
Silurian Period - 443-416 million years ago
Cartoon image of a Silurian sea
Overview of environment - a warm and stable climate with the optimum amount of oxygen to create ozone and high sea levels, many of the continents were flooded, leading to shallow seas and mound like reefs with a vast expanse of fish.
Significant events - the evolution and diversification of fish (a rapid spread of jawless fish, the first appearance of freshwater fish and fish with jaws)
100 mya gap - here is a gap of 100 mya where we have no geological information, we believe that all of the rocks over this period were deposited and then weathered and eroded away, leaving no record for geologists today. We call this an unconformity.
Strata 10 Silurian Gornal Grit
Silurian Gornal Grit
Ruiton Quarry in Gornal Grit, July 1921. Working face roughly 3m high. British Geological Survey GeoScenic image P201920.
A type of sandstone laid down in a river delta 420 million years ago, it is one of the hardest and most durable rock types found anywhere in the Black Country. As a result, it was widely employed as a building stone in the local area – and can still be seen in numerous 19th century walls and churches.
The black-and-white photograph shows an oblique view of Ruiton Quarry in Upper Gornal in the summer of 1921, where the Gornal Grit was being worked for building stone. A pile of roughly dressed stone blocks is visible in the lower left.
Strata 11 Silurian mudstone
Silurian mudstone
Hand specimen of a grey Silurian mudstone. Sample is about 7 cm across.
Calcareous mudstones of the shelf contain a rich fossil fauna that includes bivalves, brachiopods and trilobites. Reefs, built up by corals, crinoids, stromatoporoids and bryozoans.
In this photograph of a small hand specimen of Silurian calcareous mudstone, thin laminations (fine layers) can be seen flaking off the surface, and the ribbed shells of several fossil brachiopods are visible, peeking out of the rock.
Close-up view of a hand specimen of Silurian limestone crammed with fossils. Sample is roughly 6 cm across.
Silurian limestones or dolomites (the latter altered from limestone by partial secondary substitution of magnesium for calcium in the calcium carbonate) are widely quarried for crushed rock.
This limestone contains closely spaced nodules in a grey matrix.
Several fossils are visible in the photograph of a small hand specimen of Silurian limestone from Wren's Nest, a locality famous for fossils. You might be able to spot ribbed, curved brachiopod shells, cylindrical crinoid stems, and both 'stick' and 'net' bryozoans.
Hand specimen of a grey Silurian mudstone. Sample is about 7 cm across.
Calcareous mudstones of the shelf contain a rich fossil fauna that includes bivalves, brachiopods and trilobites. Reefs, built up by corals, crinoids, stromatoporoids and bryozoans.
In this photograph of a small hand specimen of Silurian calcareous mudstone, thin laminations (fine layers) can be seen flaking off the surface, and the ribbed shells of several fossil brachiopods are visible, peeking out of the rock.
View of the 'Ripple Bed' locality at Wren's Nest. The rock face is about 15 m high.
Silurian limestones or dolomites (the later altered from limestone by partial secondary substitution of magnesium for calcium) are widely quarried for crushed rock.
The rippled limestone slabs at the top of the grassy slope in this photo represent the top of the Nodule Limestone at Wren's Nest Nature Reserve. The Upper Quarried Limestone used to occupy the trench in the foreground - as the name suggests, it has now been removed by quarrying and the trench partly infilled.
Silurian Nodular limestone at Wren's Nest. The cliff is about 12 m high.
Silurian limestones or dolomites (the latter altered from limestone by partial secondary substitution of magnesium for calcium in the calcium carbonate) are widely quarried for crushed rock.
This limestone contains closely spaced nodules in a grey matrix.
Silurian Nodular limestone forms the top half of the cliff in this photograph taken at Wren's Nest. This exposure shows the typical knobbly texture of this unit. The well-bedded grey strata below are the older Lower Quarried Limestone.
The Lower Quarried Limestone exposed in cliffs at Wren's Nest. The cliff is about 15 m high.
Silurian limestones or dolomites (the latter altered from limestone by partial secondary substitution of magnesium for calcium in the calcium carbonate) are widely quarried for crushed rock.
The well-bedded grey strata forming the lower half of this cliff at Wren's Nest belong to the older Lower Quarried Limestone. The knobbly rock in the upper part of the cliff is the Silurian Nodular limestone.
Strata 17 Silurian Shale
Silurian Shale
A steeply-dipping exposure of Silurian Coalbrookdale Formation at Wren's Nest. The exposure is about 6 m high. Photograph courtesy of Mike Brooks.
Mudstone layers including the Coalbrookdale Formation, which contains numerous fossils beautifully preserved in calcareous nodules contained within a volcanic ash layer.
This oldest unit at Wren's Nest is not well-exposed, as the mudstones are more easily eroded than the limestones. The exposure in the photograph shows typical slabby shales, dipping steeply towards the observer.
Strata 2 Carboniferous mudstone
Carboniferous mudstone
Cliff of interbedded Carboniferous mudstones and sandstones. BGS GeoScenic image P209907
A sedimentary rock made of very small particles of mud and clay – it is “fine-grained” and often called shale.
The image here shows a cliff exposure in a clay pit of dark grey, upper Carboniferous mudstones (shales) interbedded with paler sandstones. The tougher sandstones weather proud as resistant bands between the softer shales, which form gentler, sloping shelves. The cliff is about 25m high overall. Rusty patches at the base of the cliff are seeps of iron-stained groundwater.
Strata 3 Carboniferous coarse sandstone
Carboniferous coarse sandstone
Hand specimen of cream-coloured Carboniferous sandstone. Sample is about 5 cm across.
A type of sandstone that is characterised by its pale yellow to pink colour and composed of fine to medium-grained sand.
The creamy-yellow Carboniferous sandstone sample in the image shows a well-sorted texture, with a narrow range of grain sizes. The darker tan line along the top left edge is alteration due to weathering; the irregular top surface is fresh rock. The smooth lower left face has been cut using a diamond-tipped rock saw, to produce a thick slice of rock from which the thin section was created for scanning.
Cliff of interbedded Carboniferous mudstones and sandstones. BGS GeoScenic image P209907
A sedimentary rock made of very small particles of mud and clay – it is “fine-grained” and often called shale.
The image here shows a cliff exposure in a clay pit of dark grey, upper Carboniferous mudstones (shales) interbedded with paler sandstones. The tougher sandstones weather proud as resistant bands between the softer shales, which form gentler, sloping shelves. The cliff is about 25m high overall. Rusty patches at the base of the cliff are seeps of iron-stained groundwater.
Strata 5 Carboniferous river channel sandstone
Carboniferous river channel sandstone
Stourbridge students examining a coarse, gritty sandstone exposure on the floor of Doulton's Claypit.
This rock was formed in a massive river delta, like the Amazon or Nile today. This river was busy dumping millions of tons of grit and sand on Yorkshire and the Midlands, which was eventually turned into this rock.
In the field photograph the fresh rock is pale grey, similar to the weathered surfaces on the right of the image, but viewing with a hand lens reveals numerous brightly-coloured sand-grains and granules. The rusty colours on the left of the exposure are due to iron oxides. Just below the centre of the image is a flat, rust-streaked plane inclined towards the observer: this is a small fault plane known as a slickenside - effectively, a fossil earthquake!
Cliff of interbedded Carboniferous mudstones and sandstones. BGS GeoScenic image P209907
A sedimentary rock made of very small particles of mud and clay – it is “fine-grained” and often called shale.
The image here shows a cliff exposure in a clay pit of dark grey, upper Carboniferous mudstones (shales) interbedded with paler sandstones. The tougher sandstones weather proud as resistant bands between the softer shales, which form gentler, sloping shelves. The cliff is about 25m high overall. Rusty patches at the base of the cliff are seeps of iron-stained groundwater.
Strata 7 Carboniferous coal
Carboniferous coal
Etching depicting coal mining in on of the Earl of Dudley's coal pits. From: Griffiths' Guide to the Iron Trade of Great Britain (1873), via Wikimedia Commons.
Formed in dense and swampy forests, which gave rise to large deposits of peat. Over the eons the peat transformed into rich coal stores in Western Europe and North America.
The image shows a scene of underground coal mining in the nineteenth century, at a coal pit owned by the Earl of Dudley. Note the pillar of rock behind the pit pony, part of the 'pillar and stall' system of mining the coal seams. Coal fuelled the industrial revolution in Britain and the Black Country was at the heart of this surge of industry.
Strata 8 Carboniferous conglomerate
Carboniferous conglomerate
Exposure of Carboniferous conglomerate and interbedded sandstones at Brewin's Cutting.
A pebbly rock laid down on the eroded surface of Silurian rocks during the Carboniferous. Streaks of coal in this rock layer represent flattened fossil trees buried in the river channel.
In this view of Brewin's Cutting, Carboniferous strata occupy the upper left hand side of the rock face, above the unconformity marked by the oblique, vegetated step running from upper right down to Jonathan Larwood's feet. Below this surface are Silurian strata.
Exposure of fine, micaceous Silurian sandstone and interbedded siltstone at Brewin's Cutting.
They have a characteristic purplish red-brown colour because of the iron oxide content and are known as the Old Red Sandstone (ORS) facies.
In this view of Brewin's Cutting, the Silurian strata occupy the lower right hand side of the image, beneath the unconformity marked by the oblique, vegetated step running from upper right down to Jonathan Larwood's feet. Above this unconformity surface are Carboniferous strata.
Etching depicting coal mining in one of the Earl of Dudley's coal pits. From: Griffiths' Guide to the Iron Trade of Great Britain (1873), via Wikimedia Commons.
Formed in dense and swampy forests, which gave rise to large deposits of peat. Over the eons the peat transformed into rich coal stores in Western Europe and North America.
The atmosphere of the Carboniferous was very rich in oxygen (35% compared to today's 21%). This resulted in enormous plant and animal life.
The Carboniferous is famous for coal; with the high oxygen content plants grew at an astonishing rate and to gargantuan sizes.
The Carboniferous was also dominated by large swamps and bogs, so when a plant died and fell into the swamp it wouldn’t decompose (due to the anoxic nature of the swamp). More plants would fall on top forming layers upon layers of material the would be subjected to heat and pressure forming coal deposits.
The image shows a scene of underground coal mining in the nineteenth century, at a coal pit owned by the Earl of Dudley. Note the pillar of rock behind the pit pony, part of the 'pillar and stall' system of mining the coal seams. Coal fuelled the industrial revolution in Britain and the Black Country was at the heart of this surge of industry.
This graphic, designed by a group of students at King Edward VI College, Stourbridge, tells the stories of the different layers of rock ('strata') you can find at the Saltwells and Wrens's Nest NNRs in Dudley.
Click on each rock layer or arrow to find out more about its geological and social history.
Carboniferous
Silurian
This graphic is an example of a stratigraphic column. These diagrams visually represent the order of the rock layers below the Earth's surface at a particular location, as if a borehole has been drilled vertically downwards. Typically, the oldest strata lie at the bottom of the column, overlain by successively younger rock layers, with the youngest at the top. This follows the Law of Superposition, first stated in 1669 by the Danish scientist Nicolas Steno and later adopted by William Smith when he created the first geological map of Britain.
