Journey 400 repaired - South GloucestershireA journey across 400 million years 4 A journey across...

A journey across 400 million years

1P a l a e o z o i c E r a M e s o z o i c E r a

Ediacaran Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic CretaceousC a e n o z o i c E r a

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The story of

geology & landscapein South Gloucestershire


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Introduction

This booklet tells the story of the geological history of South Gloucestershire, spanning some 400 million years.

It involves vast, almost unimaginable earth movements and dramatic, radical changes in continents, climate and the landscape. The evidence for these processes lies all around us, albeit unobtrusive and, more often than not, overlooked – in the rock of quarries, road cuttings, stone walls and buildings; and in the mud of the Severn Estuary.

The booklet directs you to a series of specific sites in South Gloucestershire, some of which have their own geological information boards.

Some sites, such as Aust Cliff, will require an hour or two to explore fully. Others – where the rocks are physically inaccessible or in a dangerous location, like a working quarry – can only be viewed from a specific, safe vantage point.

A 6-figure Ordnance Survey Map grid reference (NGR) for each of the sites is provided in the text. Sites mentioned in the text are shown on the map opposite. There is also a geology map on pages 22 and 23.

Please visit the web site www.avonrigs.org.uk which provides greater detail on the geology and landscape of South Gloucestershire.

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Must see sites

Aust Cliff ST565895

Huckford Quarry Local Nature Reserve ST657799

Wick Golden Valley Local Nature Reserve ST709729

Information boards

Brandy Bottom, Shortwood ST683773

Chipping Sodbury Car Park ST724823

Codrington Quarry ST727783

Cromhall village ST698905

Slickstone Quarry ST703915

Tytherington Road Cutting ST655876

Other sites

Brinkmarsh Quarry ST674913

Buckover Road Cutting ST665906

Cullimore’s Quarry ST721927

Hawkesbury Quarry ST772873

Pithay, Thornbury ST633906

Sites mentioned in the text

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© Copyright South Gloucestershire Council All rights reserved LA100023410, 2007

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Skull and front flipper of an Ichthyosaurus (x0.2)

One of the early Jurassic marine reptiles known as ‘Sea Dragons’ (Bristol Museum)

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Basics…

In geology, as in all other sciences, there are some important principles that underpin everything else. One of these is the Rock Cycle (see opposite).

Plate Tectonics - the Earth’s crust is divided into plates composed of lighter rocks, the continents and shallow seas, which float on heavier, denser rocks beneath the oceans.

Continental Drift - throughout geological time, the position of the continents has shifted slowly but dramatically – for example, some of the rocks now making up South Gloucestershire may have been formed south of the equator.

Rocks - are formed from minerals and may be extremely hard like granite or soft like clay.

The position of the continents 280 million years ago

eroded materialtransported towards the sea

sedimentary, metamorphic or

igneous rocks may be exposed at Earth’s surface

by uplift and erosion

may cool at depth to form

an intrusiveIGNEOUS

ROCK(eg granite)

sediments deposited(eg mud/sand/lime)

burial, compaction, cementation

SEDIMENTARY ROCKS(eg mudstones, sandstones and

limestones)

may then be subjected to deeper burial with considerable

heat and pressure

may cool and recrystalise into aMETAMORPHIC ROCK

(eg slate and marble)

may be subjected to even greater heat and pressure to melt, deep in

Earth’s crust

volcanic extrusive

IGNEOUS ROCKS

(eg lava, basalt)

weathering and erosion of all

rocks exposed at Earth’s surface

MAGMAdeep in Earth’s crust,

magma provides the ‘raw material’ for

new rocks.

Follow the Rock Cycle

North America

IndiaAfricaSouth

America

PANGAEA

AntarticaAustralia

Asia

Europe

Geology fieldwork tools of the trade

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Old Rocks, mostly buried - the Silurian, 440 to 410 million years ago

The oldest visible rocks in South Gloucestershire belong to the Silurian Period.

These occur mainly in the Tortworth area and belong to the Llandovery Series, so-called because similar rocks containing the same types of fossils were first recorded at Llandovery in Wales.

The Series consists of a succession of sedimentary rocks interrupted by two bands of basalt, a fine-grained igneous rock, formed from cooled lava that was forced through the sedimentary rock.

Between the two layers of basalt there is a succession of sandstones, siltstones and lime-rich rocks that reflect the changing environments at the time that they were laid down – shallow seas that deepened as time progressed.

Later, these seas grew shallower, with coral reefs and shellfish that became fossils. These fossils enable the rocks to be correlated with those at Wenlock Edge, Shropshire - they are thus part of the Wenlock Series.

The basalts can be seen at Cullimore’s Quarry ST721927.Fossiliferous limestones (the Brinkmarsh Beds of the Wenlock Series) containing corals and other fossils can be seen in Brinkmarsh Quarry ST674913.

Silurian Period rock can also be seen in walls at Tortworth, Avening Green and Damery.

Complex Geology Revealed - the change from Silurian to Devonian Buckover Road Cutting ST665906 Site of Special Scientific Interest (SSSI) on the A38 near Thornbury illustrates the relationship between rocks of two different geological periods – the Silurian and Devonian.

A break in deposition is called an ‘unconformity’. The unconformity at Buckover Road represents a period of about 35 million years between the Silurian and the Devonian (Upper Old Red Sandstone).

Red rocks such as the Upper Old Red Sandstone are strongly indicative of a desert environment.

Frequent powerful flash floods then eroded these desert mountains, depositing large boulders, chiefly quartzite, from the older harder rocks. These became cemented together to form the massive Quartz Conglomerate. The deposited beds were once horizontal but colossal earth movements subsequently caused them to become tilted and faulted.

Wenlock limestone showing corals and brachiopods (x1.3)

Photo of Charfield Pin Mill with a variety of local Silurian rock

Footpath

Massive Quartz Conglomerate

Thick bed of Quartz Conglomerate

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UnconformityDevonian

Buckover Road Cutting

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Mountains and Coral Seas - the Lower Carboniferous about 350 million years ago

The limestone formed during the Lower Carboniferous Period (Carboniferous Limestone) is one of the most important rocks for commercial purposes, because of its hardness and accessibility.

This limestone is 900m thick, forming a ridge that extends from Cromhall to Chipping Sodbury and from Tytherington to Almondsbury (see diagram on opposite page) and is extensively quarried. See boards at Codrington Quarry ST727783, Chipping Sodbury car park ST 724823 and Wick Golden Valley Local Nature Reserve ST709729.

The hard rocks began as the soft, lime-rich mud and the remains of marine creatures, such as coral, on a tropical sea floor. Over time, layer upon layer of sediment was compressed into rock by its own weight.

These beds are not, however, horizontal. A massive collision of continental plates folded this part of the Earth’s crust into a chain of mountains that would have extended across present day Europe. This chain was then ‘planed off’ by extreme weathering and erosion, slowly and over vast periods of time – the tilted, angled beds of rock in these limestone quarries are evidence of these processes.

In South Gloucestershire the rocks form a great basin, filled with younger rocks.

A section through the North Bristol Coalfield Synclinevertical scale exaggerated

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È Information leaflets are available from South Gloucestershire CouncilTel 01454 863592 or visit www.southglos.gov.uk

Wick Golden Valley Local Nature ReserveThere are spectacular views of the Carboniferous Limestone quarry at Wick from the Wick Golden Valley Local Nature Reserve.

Frampton CotterellAlmondsbury Chipping Sodbury

Upper Carboniferous

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Palaeosmilia regia (x1) Fossilised coral from the Carboniferous Period

Lower Carboniferous

Younger Rocks

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Between Limestone and Coal - the later part of the Lower Carboniferous and the early part of the Upper Carboniferous, about 325 million years ago

Gradually the environmental conditions changed. Clay, silt and grit from the land muddied the clear, tropical waters and formed a river delta. This sediment slowly settled and was compressed into mudstones, sandstones and gritstones.

As the rivers of the delta changed course, the patterns of this

deposition changed too - land became flooded, then reverted back to land again, with swamps and tropical forests covering the area.

Vegetation slowly built up on the forest floor and was compressed under its own weight to form thin seams of one of man’s most important natural resources - coal.

This sedimentation went in cycles as the rivers changed course, alternating between thin beds of coal and thicker beds of sandstone.One such bed near Cromhall is over 300m thick and is known as the ‘Cromhall Sandstone’. It is worked in a large quarry and, while there is no public access, samples of the rock can be seen on the green next to the B4058 in Cromhall village ST698905.

Tropical forests, the basis of an industry - the Upper Carboniferous Coal Measures, about 300 million years ago

The rocks at the end of the Carboniferous Period were all formed by this continuing process of river delta sedimentation, alternating between layers of limestones, mudstones, shales and coal.

Together these are called the ‘Coal Measures’, although coal does not make up the greatest proportion of the rocks (seams are all less than 2m thick). In fact, the thickest band of rock is a sandstone, known as ‘Pennant Sandstone’.

There were once scores of small coal mines across the area where these

rocks are found. All of these are now disused, but the best preserved remains are at Brandy Bottom Colliery, Shortwood ST683773.

The mudstones of the Coal Measures are still used to make bricks at Cattybrook, near Almondsbury, but the rocks themselves are rarely seen since they are soft and quickly weather where they are exposed.


Cromhall Quartzite Quarry

Miners underground at Parkfield South Colliery, Shortwood. The colliery closed in the 1940s.

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Red, Pink and Grey - the Pennant Sandstone

Most of the southern part of South Gloucestershire is underlain by Pennant Sandstone. It was the basis of a thriving local industry, being used extensively for building around Bristol and being shipped to other parts of the country.

These sandstones were formed in thick beds by rivers, with gaps between beds indicating when the supply of sediment was interrupted. The sediment varied in grain size, from coarse sand down to fine clay. Changes in the river currents meant that the beds were partially eroded and a new direction of

bedding formed over the top. This happened repeatedly and is called ‘cross bedding’ as distinct from true bedding.

Pennant Sandstone is jointed and faulted due to the great earth movements which took place after it had formed. The vertical joints may contain iron minerals – goethite, limonite and hematite.

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È Information leaflets are available from South Gloucestershire Council 01454 863592 or visit www.southglos.gov.uk

Huckford Quarry Local Nature Reserve The reserve is next to the River Frome in Winterbourne

Development of cross bedding

Pecopteris cyathea (x0.15)Fossil plant from the Coal Measures

Huckford Quarry Local Nature Reserve ST 657799 in Winterbourne is a good place to view these rocks (follow the steep track from Hick’s Common Road near the railway viaduct). The quarry opened to provide stone to build the viaduct-the stone was encased in brick to form the pillars.

Broken rocks on the quarry floor may contain the fossil remains of plants.

Winterbourne Railway Viaduct circa 1900

Pennant Sandstone dry stone wall, Huckford Quarry Local Nature Reserve

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Deserts Again - the Permian, 290 million years ago & the Triassic, 210 million years ago

There are no rocks of Permian Age in South Gloucestershire.

However, we know from elsewhere that desert conditions prevailed and that the continents merged into one single land mass (super continent) extending as far as the South Pole, where there was glaciation over a large area (see page 4).

Our own area lay about 15 degrees north of the equator and was dominated by desert conditions. Great thicknesses of the Coal Measures, as well as the chains of mountains created by earth movements at the end of the Carboniferous Period, were eroded away.

When new rock was formed in the next geological time period - the Triassic - it was deposited directly on top of the rocks at the surface of this now heavily eroded landscape.

This can be seen at Slickstone Quarry, Cromhall ST 703915 (see information board in layby off B4058) where red Triassic rocks can be found in fissures in the Carboniferous Limestone (see photograph on opposite page).

One Triassic rock type – Dolomitic Conglomerate – is derived directly from the erosion of these (Carboniferous Limestone) mountains and was formed by boulders, sand and clay being deposited as debris on desert valley sides and at the base of the mountains.

You can see this rock off the Pithay in Thornbury ST 633906 - walk down the lane west of the parish church and immediately right is a low cliff of Dolomitic Conglomerate to the rear of a small field.

Finer sandy beds can be seen at Tytherington Road Cutting ST 655876. There is an information board where the lane goes under the M5 motorway.

Part of the upper jaw of Agnosphitys cromhallensis found in Slickstone Quarry in 1990. It is thought to have been a medium-size dinosaur that may have eaten insects and other small animals. The bones are in the Virginia Museum of Natural History, USA.

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Black Rock Limestone exposure at Slickstone Quarry

Orange iron-stained stripe - ancient desert fissure

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More deserts - the Triassic 245 to 210 million years ago

As well as this erosion and the deposition of the Dolomitic Conglomerate there were salty lagoons, similar to the Dead Sea or the Great Basin of the USA.

The sediments that were deposited in these lagoons consisted of fine muds, rich in salts. The hot, dry climate meant that evaporation left the dried-out mud with bands of minerals called evaporites - the residues left after evaporation has taken place.

Rocks formed by these processes can be seen at Aust Cliff ST 565895.

The lower part of the cliff is a red mudstone containing bands of pinkish-white nodules. These are alabaster, a form of the mineral gypsum. The red colour of the mudstone is typical of a desert environment, while the green

band further up the cliff face indicates a change to a more humid environment.

Above the green mudstone, black shale containing fossil sea shells marks a change from desert to marine conditions. A thin bed at the base of the shale is called the Rhaetic Bone Bed and is world-famous for its fossilised bones and teeth of fishes, marine reptiles and even dinosaurs.

On top of the shale are cream-coloured limestone beds. These formed in lagoons at the edge of the sea and land at the very end of the Triassic Period.

Fragments of these rocks can be found on the beach below the cliff. If you break open pieces you may find fossil teeth embedded in them.

There are also broken pieces of limestone belonging to the next geological period.

Take great care when you are there. The cliff is dangerous and the rocks are slippery. Children should be accompanied by adults. Beware of the incoming tide.

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È Information leaflets are available from South Gloucestershire Council 01454 863592 or visit www.southglos.gov.uk

Aust CliffAust Cliff is well worth a visit and there is an information board to help you identify the various rock layers which can be seen in the cliff.

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Limestone & Shale

Limestone & Shale

Green Mudstone

Red Mudstone

View of Aust Cliff and Severn Estuary

© Copyright South Gloucestershire Council

All rights reserved LA100023410, 2007

© Sharper Image Photography Tel: 08702 646646

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The Age of Ammonites and Great Building Stones - the Jurassic, 210 to145 million years ago

Rocks of the Jurassic Period are divided into three sections - the Lower, Middle and Upper Jurassic - although only the first two are present in this area.

Ammonites flourished in the warm seas during this period and some species are used to age and differentiate between these sub-divisions because they were around for just a brief time.

The rocks of the Lower Jurassic Age in South Gloucestershire are chiefly those of the Lias Clays. Exposures are rare, but the clays make a significant contribution to the landscape, forming the heavy soils of pasture and woodland beneath the Cotswolds.

By the Middle Jurassic, conditions had changed to clear seas

abounding with life. Rocks of the same age don’t necessarily contain the same fossils as different environments occur in different places at the same time. In the Cotswolds, the Middle Jurassic rocks are mainly oolitic limestones in the north, whereas those further south are made up of fossil fragments. However, a disused quarry at Hawkesbury contains both types of rock and thus provides the link. This site, Hawkesbury Quarry ST772873, is important in establishing the relationships between the two different areas and has been notified as a Site of Special Scientific Interest.

The rocks exposed at Hawkesbury Quarry indicate that they were formed in a shallow marine environment, being coarse oolitic limestones rich in fossil shells. The most abundant are animals with two shells - brachiopods and bivalve molluscs (such as oysters), as well as gastropods (snails) and echinoids (sea urchins).

The rocks have provided a good quality building stone which has been used extensively in the Cotswolds and elsewhere over several centuries.

1 Drawing of section through oolitic limestone

2 Oolitic limestone wall, Hawkesbury

3 Fossilised shells of Rhactorhyncia witchelli - Inferior Oolite, Stroud, Glos.

4 Ammonites, Arnioceras semicostatum - Lias, Yorks.

5 Fossilised shells of Terebratula cortonensis - Inferior Oolite, Dundry

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Skirroceras macrum (x0.3) Ammonite from Middle Jurassic

(x5)

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The ‘Gap’ - end of the Jurassic to the Present Day

Remarkably, there is little geological evidence between the youngest Jurassic rocks and now – a period spanning about 146 million years.

Much of the area was probably above sea level and so subject to erosion. Later, when the sea level rose, a succession of clays, chalk and sands were deposited over much of the British Isles. Vast earth movements in the Miocene Period lifted the rocks above sea level again.

The drainage follows a pattern which is essentially at odds with the geological structure, so it is assumed that the pattern was established on a covering of rocks (possibly chalk) which has since been eroded away. We know that erosion removed most of the Jurassic sandstones and limestones covering these older rocks. The fretted edge of the Cotswold scarp shows that, even

today, springs are causing erosion of the scarp edge.

Another unknown is what happened during the various Ice Ages – there have been over a hundred during the last 2.5 million years. Ice came into the area along the Bristol Channel as an extension of the South Wales ice sheet and another sheet came in from the north, reaching as far as the Cotswolds (Cheltenham).

Ice also diverted rivers. It caused the River Severn to drain much of mid-Wales, which in turn means that the Severn carries a vast quantity of sediment in the form of mud.

One other effect of the Ice Ages was the creation of a dense network of rivers on the dip slope of the Cotswolds. The courses of these can be seen as dry valleys, as the once frozen ground is now permeable. These constant changes in temperature also weakened the rocks, meaning that land slippages are a common occurrence today.

Mud…

Most of the rocks in South Gloucestershire are sedimentary - that is, rocks made from sediments which in turn were eroded from other rocks, mostly by water, wind or ice.

Erosion is a very slow process because the climate is temperate. Additionally, most sedimentary rocks are formed under water and, because of this, it is within the tidal waters of the Severn Estuary where this sedimentary process is still continuing.

The estuary carries enormous quantities of sediment, brought down by rivers, streams and through the soil. However, while each falling tide takes sediment out towards the sea, each high tide brings it back in again. The salt in the seawater makes suspended clay particles stick together (flocculate), settling on the bottom and building up to form mud flats.

Over millions of years, under its own weight, this mud will gradually form rock.

It is a phenomenon that, if nothing else, shows us that we are witness to, and part of, an on-going, dynamic geological process that preceded us by hundreds of millions of years: and that will continue for millions of years to come.

Mud banks on the Severn Estuary

The Cotswold escarpment from Toghill

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South Gloucestershire Landscape Character Assessment

GENERALISED GEOLOGY

ChippingSodbury

Kingswood

FramptonCotterell

Coalpit Heath

Winterbourne

Patchway

StokeGifford

South Gloucestershire Boundary

Alluvium

Great Oolite Limestone

Fuller’s Earth

Inferior Oolite Limestone

Lower Lias

Jurassic Rocks undiferentiated

White and Blue Lias

Penarth Group

Mercia Mudstone

Upper Coal Series and Pennant Series

Lower Coal Series

Upper Cromhall Sandstone

Carboniferous Limestone

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This map is reproduced from Ordnance Survey materialwith the permission of Ordnance Survey on behalf of

the Controller of Her Majesty's Stationery Office Crowncopyright. Unauthorised reproduction infringes Crown

copyright and may lead to prosecution or civil proceedings.South Gloucestershire Council Licence No 100023410, 2006

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Source Information based upon Avon County Mineral Plan

Geological Periods Main Groups of Rocks

Holocene

Jurassic

Triassic

Carboniferous

Devonian

Silurian and Cambrian

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THE SITES DESCRIBED IN THIS BOOKLET ARE SPECIAL PLACES– please respect and conserve

them.

ß Always seek permission before entering private land.

ß Never enter a working quarry without obtaining permission from the quarry owners and visiting the quarry office prior to entry.

ß Always consider your personal safety and the safety of others.

ß Wear appropriate clothing and footwear.

ß Beware that rock falls and collapses of sand and gravel faces can occur at any time and without prior warning.

Follow the Fieldwork Codes from The Geologist’s Association www.geologist.demon.co.uk and The Geological Society www.geolsoc.org.uk.

Countryside Code

ß Be safe – plan ahead and follow any signs

ß Leave gates and property as you find them

ß Protect plants and animals and take your litter home

ß Keep your dog under close control

ß Consider other people

For bus service and timetable information contact Traveline or www.traveline.org.uk

Acknowledgements

Special thanks to Hilary Davies and Roger Clark for editorial help and to the Bristol City Museum and Art Gallery for advice and supply of many of the fossil photographs.

Illustrations by Shane Feeney

www.southglos.gov.uk (access is free from your local library) Design & Print Services CE&CR 1717 Printed by South Gloucestershire Print on recycled paper 0

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