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The rocks on Clevedon Beach and below the Pier Toll house tell a dramatic story of 140 million years of the Earth’s history involving huge rivers, tropical seas, the formation and erosion of great mountains, deserts populated with dinosaurs, and formation of a large fault which allowed hot fluids rich in metals to form exotic minerals. The visitor to the beach can see the direct evidence for this remarkable story within 100 metres by looking at the rocks on which the Toll House is built and the rocks exposed between the Pier and just beyond the jetty. Geological Time – the Earth’s history is divided into 4 eras and 12 periods (see Plate 1).
Plate 1: The Earth’s history is divided into 4 eras and 12 periods.
The geological time scale showing the most recent 541 million years of the Earth’s history known as the Phanerozoic. The vertical red arrow next to Plate 1 indicates the time interval recorded in the rocks of Clevedon beach from late Devonian to the middle Triassic. (Diagram from British Geological Survey).
The geology at Clevedon Beach represents the time from about 360 to 220 million years ago and includes four of the Periods, namely the Devonian, Carboniferous, Permian and Triassic. To get an idea of what an immense time span this is try counting 100 years which should take about a minute. To count 140 million years would take you almost 3 years!
The World was very different in those ancient times. Geologists have been able to reconstruct the arrangement of the continents and the oceans and there is very little resemblance to the present day. About 360 million years ago there were two giant continents called Laurussia and Gondwanaland as well as many smaller islands and archipelagos.
In contrast to the present day most of the land masses most of the land masses in the devonian were in the southern hemisphere with Clevedon located about 20oS. Rocks of this age at Clevedon Beach record the Earth at the end of the Devonian Period and at the beginning of the Carboniferous Period.
The location was on the south-eastern coast of Laurussia. In the Devonian the environmental setting was a huge river system draining from mountains to the north-west. The Carboniferous Period started when the coastal plain became submerged below a shallow tropical ocean.
Plate 2: The World 360 million years ago. The yellow dot shows the location of Clevedon on the southern edge of Laurussia continent.
About 220 million years ago the arrangement of the continents had changed dramatically. The continents of Laurussia and Gondwanaland had merged to form a single giant super-continent called Pangea. The collision of the giant continents during the Permian period had formed the great Pangean mountain range. Clevedon was now located on the northern margins of these mountains. The landmasses were now more evenly divided between the northern and southern hemisphere and Clevedon was now located in the continental interior far from the ocean at a latitude of about 20oN.
Watch an animation of how the continents moved here. Note that you may need to register to see the animation. See if you can match the two maps at 360 and 220 million years in the animation.
Plate 3: The world 220 miilion years ago. The yellow dot shows the location of Clevedon in the interior of the Pangea supercontinent in the northern hemisphere at a latitude similiar to the modern Sahara Desert.
Basic Geology
Rocks with different colours, surface features and structures are found on Clevedon Beach. Distributions of rocks of different age are shown on a geological map (Plate 4).
To understand the geology and get your eye in we suggest you go on to the top of the beach and look out to sea to view the rocks on which the Toll House is built as in the photographs in Plate 5. Devonian sandstones (D) are exposed in the corner of the beach below the toll house and next to the marine wall. Above these rocks making up much of the cliffs are Triassic breccias (TB) which are rocks made of fragments of older rocks.
The geological boundary between the Triassic Breccia and Devonian sandstones is an erosion surface (known as an unconformity) representing 140 million years of time. On the beach the flat cream rocks (TL) were formed in Triassic lakes.
Plate 5a and 5b – below. View out towards the Pier (left) and towards the toll house (right) showing different rock units close to the Pier: Devonian sandstones (D), Triassic breccias (TB) and Triassic lake sedimentary rocks (TL). The orange dashed line shows the geological fault and the yellow dashed line shows the unconformity.
Now take a view from the Pier to the south from just outside the Toll House (right). Here The Triassic sedimentary rocks (TL) are in the foreground while the lower Carboniferous limestones (LC) are beyond the yellow line which is also an unconformity.
We now make a tour around the beach visiting the rocks in order of age to tell the incredible story of the Earth’s history to be found in less than 100 metres of beach. The tour is referenced to the numbered spots on the geological map. We will describe and discuss each rock type. The landscapes are illustrated with spectacular illustrations by palaeo-artist Bob Nicholls.
Plate 4: Geological map of Clevedon beach with numbered observation sites discussed in the text.
Plate 5a: View out towards the Pier.
Plate 5b. View towards the Toll House
Plate 6. View of Clevedon Beach from outside the Toll House.
The Devonian Rivers and flood plains
There is a small area of Devonian sandstones in the corner of the beach site 1 and shown as D in Plate 5. These rocks were formed in fast-flowing rivers on the coastal plains of Laurussia and the evidence for this is found in distinctive cross-bedding. Rivers carry immense amounts of sand which they deposit in channels. The river flow waxes and wanes through the seasons depending on rainfall in the mountains and pulses of sand are deposited to form succession of layers which geologists call beds.
Often the sand is dragged along the river bottom by fast flows to form large dunes which make the orientation of sand beds vary as shown in Plate 7. You can see a similar effect in the Severn Estuary today at low tide on banks of silt and fine sand (Plate 8) although flows here are tidal in origin. Devonian sandstones at Clevedon contain no fossils because fast-flowing and sand-laden rivers are not good environments for fossil preservation.
The name Devonian originates from Devon where rocks of this age were first recognised. Devonian rocks occur in many places in the UK and indeed make up much of the Shetland islands and areas of southern Scotland for example. They are sometimes referred to as the Old Red Sandstone. Geologists observe variations in the rocks that enable the ancient landscape to be reconstructed (Plate 9). To the south of Clevedon most Devonian rocks formed in an ancient sea while to the north the rocks indicate rivers and lakes. Devonian rocks on the coastal path from Clevedon to Portishead includes rocks with plant and fresh water fish fossils. Further north in what is now Scotland there were large mountains in the interior of the Laurussia continent (Plate 2). We can imagine Clevedon being in a quite similar location to the great river plains of Asia such as the Brahmaputra, Indus and Ganges draining away from their sources in the Himalayas into the Indian Ocean across great flood plains (Plate 9).
Plate 7: Bedded Devonian sandstone just below the Toll House. The sandstone beds were originally near horizontal but were tilted to a dip angle of about 30o by tectonic forces that formed the Pangaean Mountains in the Permian Period. Cross-bedding in which some layers truncate other layers with slightly different dip are fossil dunes formed by fast-flowing rivers (example marked by arrow).
Plate 8: Sand bank with dunes in the Severn Estuary formed by fast tidal currents. Such structures are preserved in rocks as cross-bedding as seen in Plate 7.
Plate 9a: Devonian landscape around Clevedon. Fast flowing anastomosing rivers are sourced in mountains to the north. The first land plants on Earth evolved in the Devonian: typical vegetation is shown including large clubmoss trees. A group of amphibians (tetrapods) bask among dunes on a sand bank. The Devonian period saw the first appearance of amphibians, spiders and flightless insects.
Plate 9b: Identification of Species in Devonian landscape.
Tropical seas of the Carboniferous
In the Bristol and North Somerset region the Devonian sandstones transition into shallow marine sedimentary rocks marking the sinking of the land below the sea and the start of the Carboniferous period. The Carboniferous rocks can be seen south of site 2 (see Plate 6). These rocks are only 1 or 2 million years younger than the Devonian sandstones but are very different in appearance with structures and fossils that indicate formation in marine environments.
The rocks between site 2 and the jetty show well defined thin bedding (Plate 10 – below left) and wavy structures (Plate 11 – below middle) which are sand ripples likely related to wave action and tides. Similar sedimentary structures can be observed in the Bristol Channel at low tide (Plate 12 – below right). The rocks contain sand, silt and mud and so are likely to have been formed when rivers discharged in a shallow sea. There are few fossils in these rocks between but when found they are marine organisms.
Plate 10 (above left). Lower Carboniferous beds of fine Sandstone, siltstone and mudstone. Plate 11 (above middle). Wavy beds of sand and silt formed by wave action in a shallow sea. Plate 12 (above right). Sand, silt and mud flats in the Severn Estuary near Clevedon at low tide showing rippled surfaces due to wave action.
At site 3 just beyond the jetty fossils become much more abundant and the rock is here limestone with much less sand and silt. A very prominent bed which occurs 2 metres from the jetty is full of plates of fish bone likely from their armoured heads providing protection from predators, including ancient sharks (Plate 13). Beds full of shells (Plate 14) likely represent a major storm that ripped up a shell colony. Anastomosing structures in some rock surfaces were formed by soft-bodied organisms, probably marine worms (Plates 15 and 16). We don’t know what these creatures looked like. At site 4 there are beds almost entirely made of crinoid ossicles, which are hollow discs looking like Polo mints, and still intact crinoid stems (Plates 17 and 18). They originate from stems of a creatures related to modern echinoderms sometimes called sea lilies (Plate 19). Visitors may be able to find occasional solitary and colonial coral fossils and conical shells of creatures called cephalopods (Plate 18 – scroll down).
Plate 13: Dark grey pieces of fish bone at site 3 likely from around the head. Image about 10 cm across.
Plate 14: Shelly limestone at site 3. The shells are brachiopods.
Plate 15: Trace fossil of soft-bodied creature probably marine worms in muddy limestone.
Plate 16: Trace fossil of bottom feeding soft-bodied creature on rock surface.
Plate 17: Crinoid ossicles and stems at site 4
Plate 18: Orthocones with crinoid ossicles (photo by Maurice Tucker)
Like much of the Mendips the limestones indicate huge shallow tropical sea now well away from the influence of rivers and a great biodiversity like modern coral reefs (Plate 19).
Plate 19a: Early Carboniferous Seascape indicated by fossils and rock types on Clevedon Beach. A forest of crinoids with spectacular flower-like crowns of pinnules that allow filter feeding. On death crinoids collapse scattering ossicles across the sea floor. Bright coloured solitary corals grow in clusters and various shell fish graze on nutrients in the sediment. A nautiloid cephalopod hovers in the foreground with a long thin shell forming orthocones (Plate 18). Shoals of fish are wary of a shark in the background.
Plate 19b: Early Carboniferous landscape annotated with the main species depicted.
Limestones are almost entirely composed of a mixture of fossil debris and calcium carbonate mud formed by formation of minute crystals of calcium carbonate in the cells of bacteria. When you pick up a hand-sized piece of dull grey featureless Mendip limestone with no visible fossils you are in fact picking up a rock formed by trillions of bacteria.
Plate 20. Folds of the early Carboniferous beds are found between sites 3 and 4.
Building the Pangean Mountains – The Permian Period between 299 and 252 million years is not represented by any rocks at Clevedon Beach. Why are these rocks missing? Huge mountains are formed when continents collide, a modern example being the Himalayas caused by India colliding with Asia. The sediments or rocks in between the continents are pushed together, squashed and heaved up, a process called tectonics by geologists. The Permian was the period when Gondawana and Laurussia collided to form the Pangaea supercontinent. All the rocks in between the continents were affected by the collision and Clevedon was on the northern margins of a huge mountain range. While mountains are growing they are also eroding so no rocks are formed.
We can see direct evidence of these past mountains at Clevedon beach. The unconformities represent the ancient erosion surfaces (Plates 5 and 6) and the originally horizontals beds of the Devonian and Carboniferous rocks have been tilted (Plates 7 and 10). Between sites 3 and 4 you can also observe distorted beds known as folds (Plate 20) which are formed when the rocks are pushed together; you can create folds by pushing a rug from the side. These folds in strong rocks demonstrate the huge forces created when the continents of Laurussia and Gondwana collided.
Triassic desert and early dinosaurs – The present-day landscape of North Somerset and the Mendips is not very different to the landscape in the Triassic Period (252 to 201 million years). However, the environment was very different as at that time Clevedon was in the interior of a huge continent called Pangea (Plate 3). Lets first discuss what the rocks tell us about the conditions at that time.
Triassic rocks in the UK are dominated by sedimentary rocks formed within desert environments and this fits in with the reconstruction of the location at similar latitudes to the great deserts of the modern world like the Sahara. Bright red sandstone is a very common rock type in the Triassic strata of the UK and sometimes rocks of this age are known as New Red Sandstone.
However, there are many different rock types reflecting different environments within a desert. Triassic rocks at Clevedon Beach are typical of the shoreline of an inland basin at the edge of mountains. The Triassic breccias are formed of debris from the mountains involving landslides and sudden flows of rock debris during intense storms. In specialist geology books and articles the Triassic breccia is given the formal name Dolomitic Conglomerate. Note too that to north of the Pier Triassic rocks display bedding and are composed of conglomerates (pebble and boulder beds) and coarse sandstones indicative of episodic floods.
A prominent feature of the beach is a natural pavement of a cream to grey bedded rock (Plates 5 and 6). These rocks are mostly sandstones and siltstones that are typical of those formed in lakes at the edge of the Basin. These rocks are also a kind of limestone containing abundant small spheres of calcium carbonate known as ooids that formed in the lake. Indeed ephemeral lakes are common at the margins of many mountainous modern deserts.
There is an intriguing area just below high tide line with a distinctive red layer with strange undulations and hollows on the surface. In one place the shape of the rock surface (Plate 22) is similar to locations of the same age rocks in South Wales where dinosaur footprints have been found. Dinosaurs had three toes on their feet and so form branching structures.
The surface in Plate 22 could be a dinosaur footprint but alternatively could have been formed by some kind of disturbance of wet lake sediment, such as shaking of the ground by earthquakes.
Plate 21: Large boulders of Triassic Breccia can be found on the beach by the Toll House (site 1) and have broken off from the cliffs. The breccia consists of large fragments of different rock types formed by landslides and flows of debris sourced from Pangaean mountains.
Plate 22: Possible dinosaur footprint in Triassic rocks near Clevedon. The cream rock is part of the lake sedimentary rocks near site 2. The pink to red rock is a muddy layer that’s shows many distortions.
Plate 23a: Middle Triassic landscape looking to the west from the shoreline environment indicated by rocks at Clevedon Pier. Sparse cycloid vegetation and dust storms indicate a desert environment. The landscape is not too different to today with Wales in the background with what is now the Bristol Channel occupied by salt flats. A small ephemeral lake is present at the basin margins. Bristol dinosaurs are on patrol.
Plate 23b – (second below). Middle Triassic landscape with species identification.
Faulting and Mineralization
There is a major fault at Clevedon Beach which separates the Triassic Breccia and Devonian rocks from the Triassic lake sedimentary and Carboniferous rocks from one another (Plate 24). The fault can be found at the base of the cliff on which the toll House is built. The fault is mineralized. The minerals are rich in the metals copper, lead and zinc. You can read more about the fault and the minerals here.
Plate 24: Block diagram showing the major mineralised fault.
Further links
Stagg K., Stonebridge E., Hutchinson D., Corner T. and Barnett. Geological sites of the Bristol Region Bristol Books 2018, pp300. ISBN: 9780954523534
A synopsis of rocks of the Mendips by the British Geological Survey including those at Clevedon can be found at:
https://www2.bgs.ac.uk/mendips/rocks/dev_rocks.htm
Clevedon Pier
The Toll House
The Beach
Clevedon
North Somerset
BS21 7QU
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