The Jurassic Period - 208 to 135 million years ago

The full succession of Jurassic rocks found in South Gloucestershire is listed below. They mostly dip at a low angle to the east, so that the Lower Jurassic rocks are only exposed in the centre and west of the area, while The Middle Jurassic rocks form the higher ground of the Cotswolds to the East. Most of the succession consists of clays but there are also several limestones which have been an important source of building stone and water supply. The succession of rocks is based on the use of characteristic fossils, especially ammonites, a technique first discovered and used by the 'Father of English Geology', William Smith in 1799.

There are no rocks of Upper Jurassic age in South Gloucestershire.

Lower Lias

The name Lias was used by local quarrymen as far back as records exist, and adopted by the pioneer geologist William Smith in his 'Order of Strata' in 1799. It was mainly used for the oldest Jurassic rocks, which are usually a succession of blue-grey coloured mudstones and thin limestones. In fact the Blue Lias looks similar to Triassic rocks which occur directly beneath, but are separated by the first appearance of fossil ammonites, marking the start of the Jurassic period.

Jurassic Ammonites

Above these rocks is a sequence of mudstones, which also contain fossil ammonites and the remains of other marine animals. They are not permanently exposed since they quickly break down with weathering at the surface, and best known from boreholes.

The total thickness of the Lower Lias varies from less than 100m at Chipping Sodbury to 200m a few miles to the northeast.

It is difficult to see exposures of the Lower Lias as it is not quarried and it forms areas of low relief. Because it gives rise to heavy, difficult soils it tends to underlie commons and woodlands, where ploughing for arable farming has not been possible. The rock can be seen in the stream bed at Wetmoor ST746875.

Middle Lias

The Dyrham Silts are also rarely exposed but mostly occur at the foot of the Cotswold escarpment, and are named after one of the villages in the area. They are only about 20m thick at Dyrham.

The Marlstone Rock Bed is usually a shelly iron-rich limestone, but has only been found north of Horton where it is probably less than 1.5m thick. However since it is so much harder than the rocks above and below, it forms a small bench which stands out from the steeply eroded escarpment. Several settlements have been built on these areas of level ground. Horton, Hawkesbry Alderley and Wotton-under-Edge. (The latter is not in South Gloucestershire, but it can be seen from the vale and the top of the escarpment). The rock also acts as an aquifer and streams emerge where the rock is exposed, which is also a situation leading to the development of the villages.

Old Sodbury Church on the Marlstone Bench

Upper Lias

Traces of the Junction Bed have only been found near Old Sodbury. It is a thin bed of limestone, which also produces a small bench on the escarpment.

The overlying Cotteswold Sands consist of fine-grained silty sand, which is only weakly cemented. It has been much eroded to form part of the steep scarp slope of the Cotswolds, and has also been cut through by a series of sunken lanes. Its thickness varies from about 60m at Hawkesbury, thinning to about 50m in the south.

The Cephalopod Bed is a thin series of marls and limestones, often rich in fossils, but only about 3m thick in the area.

Inferior Oolite

There are only three recognisable units of the Inferior Oolite found in South Gloucestershire, and the cream coloured limestones involved are only about 12m thick. However it is the rock of this age which is most responsible for the Cotswold escarpment, and it usually caps the main scarp slope.

The Scissum Beds are sandy limestones, and are named after its zonal ammonite. Then there is a gap in the succession, followed by the fossil-rich Upper Trigonia Grit, which is named after a distinctive fossil bivalve. Finally comes the Clypeus Grit, named after a fossil sea urchin. Both Grits are in fact limestones, despite their name.

The absence of other units of the Inferior Oolite, found further north in the Cotswolds, seems to be the result of their erosion on the sea floor of the time.

Fuller's Earth

William Smith named this clay after its use around Bath to remove grease from wool during the fulling process, when the woven cloth was compressed into a more dense fabric. In South Gloucestershire the Fulleršs Earth consists of both clays and limestones, and in some cases different types of rock were formed in different places.

The Lower Fuller's Earth consists of clay and is about 15m thick. Above is the Doddington Ash Rock, which is a fine grained limestone which erodes to form a marked bench. Above again is the thin Hawkesbury Clay. Finally there are three rocks of the same age. The clay Upper Fulleršs Earth is found in the south, but near Hawkesbury this changes into a fine-grained limestone known as the Tresham Rock. Further north again this limestone changes into the Athelstan Oolite.

Great Oolite

The famous 'Bath Stone' building stones still quarried around Bath, consist of the Combe Down Oolite and Bath Oolite. Both are found in South Gloucestershire, and were quarried and mined in the area, but they are thin and die out northwards.

Forest Marble

Another name used by William Smith, from the occurrence of the rock in Wychwood Forest, Oxfordshire, near where Smith was born. It is not a true marble but a shelly limestone, once quarried for ornamental use in buildings. In South Gloucestershire, the rock sometimes contains fossil coral reefs, and in one area is an oolitic limestone known as the Acton Turville Beds.

Although there are few exposures of the oolitic limestones in South Gloucestershire there is one very important site at Hawkesbury. (Grid reference ST772873).

This is a large disused quarry located a short distance west of Hawkesbury Upton and can be seen from the road. The quarry exposes rocks of Middle Jurassic age, in particular the Lower Inferior Oolite. This is the most southernmost exposure of this kind of rock in the Cotswolds area; it is absent from the Bath area and only reappears in the Eastern Mendips. It is a very important locality in the interpretation of Middle Jurassic environments and geography, forming a link between the rocks exposed in the Mendips and the Cotswolds areas. For this reason it has SSSI status (Site of Special Scientific Interest).

Hawkesbury Quarry

The rocks exposed at Hawkesbury quarry indicate that they formed in a shallow marine environment. During much of this time, Britain was largely covered by a shelf sea. The rocks are coarse shelly Oolitic limestones often rich in fossils. Oolites are rocks that contain many small pellets or ooliths. Ooliths formed on the sea floor under special conditions where small fragments are rolled back and forth by the currents and gradually accrete layers of calcium carbonate. They look rather like herring roe (eggs), hence the name oolith which is Latin for egg. The most abundant fossils are those of bivalve molluscs, such as oysters as well as gastropods (snails) echinoids (sea urchins) and brachiopods. The fossils and ooliths indicate that the rocks exposed at Hawkesbury Quarry formed under sub tropical conditions in a shallow shelf sea environment.

Oolites
Jurassic Brachiopods

The rocks would have provided a good quality building stone to a wide area over several centuries. Some buildings and walls in Hawkesbury village have been constructed in this stone. If good quality stone was available locally, it was often cheaper to exploit this, close to where it was needed using local labour, rather than importing stone at a higher cost from elsewhere in the country. With the advent of improved road and rail transport, the greater expense of using local natural stone when compared to good alternatives to stone such as bricks and concrete, many of these quarries became uneconomical to work and closed down.

Hawkesbury Church
A wall built of Jurassic Limestone

[Home] [Top] [Geological History of South Gloucestershire - Main Page]