Saltford Environment Group
Detail from an ammonite (Metophioceras caesar) found in Saltford.
SEG Home > Saltford's Geology
This page highlights and provides a showcase for Saltford's fascinating geological features that will be of interest to those who are new to this topic as well as to experienced geologists.
An accessible circular geology trail is planned for Saltford so that residents and visitors can explore the local geology for themselves. We shall publish details on this page when the trail is ready.
SEG is particularly grateful to SEG members field naturalist and geologist Simon Carpenter (who grew up in Saltford and who now lives in Somerset) and Dick Stabbins, a local geologist, for contributing most of the information and related research to enable us to publish this geological record of Saltford.
New rock exposure on the Railway Path - clearance completed
Phase 1 of the Saltford Railway Path Geology Project to create a permanent rock exposure on the former railway line was successfully completed on 3rd October 2014. This initial clearance with a mechanical excavator involved the removal of talus (loose rock fragments) and vegetation covering a section of the rocks in the cutting. This project on the Railway Path at Saltford (Grid Ref: 687678) received the go-ahead following a successful funding application to the Geologists Association.
Phase 2, the final clearance, was completed by hand using community volunteers (Conservation Volunteers - Avon Branch) using hand tools and brushes over 2 days in November. The plan is to install an illustrated information sign during 2015.
Simon Carpenter, field naturalist and geologist, is co-ordinating this project to create a new rock exposure on the Railway Path at Saltford. The rocks here are approximately 190 to 200 million years old and were formed during the Sinemurian age of the Lower Jurassic period. A superb exposure of these rocks can be seen along the length of Mead Lane, Saltford. These rocks are not accessible to the public as the outcrop is in private gardens. The new exposure on the railway path will link to other rock exposures in Saltford to form an accessible circular geology trail so that visitors can explore the local geology for themselves.
The Lower Jurassic rocks in Saltford are particularly interesting as they contain an abundance of well-preserved fossils. The coiled shells of ammonites are particularly common and can be seen built into walls around the village.
The site is located between the Avon Lane gated access and the Avon Lane footbridge (if joining the railway path at the 'Bird in Hand', turn left towards Bitton and the exposure can be seen after a short distance on the right hand side).
Simon Carpenter, grew up in Saltford but now lives in Frome, Somerset. He has led geology walks around Saltford on several occasions in recent years and is very excited by the new project which has been funded by a grant from the Geologist's Association.
Description of Saltford's Geology
Saltford has a fascinating geological history with Lower Jurassic* rocks seen across ploughed fields and in old quarries and it has been used in many of the older houses in the village.
* The Lower Jurassic period is the first geological period of time in the Jurassic period that began 201.3 million years ago and ended 174.1 million years ago.
The rocks, mainly limestone, have abundant fossils and provide evidence that this region was once under a warm, shallow, sub-tropical sea, 180 million years ago. Some limestone exposures in Saltford are extremely important in our understanding of local and regional geology and have been protected as Regionally Important Geological Sites (RIGS) in recognition of this. The GWR, the former Midland Railway Path and the River Avon provide some of the best exposures of these rocks anywhere in the West of England. These RIGS sites include:
1. Avon Lane Railway Path Cutting
2. Mead Lane Saltford
3. Saltford Railway GWR Cutting (north west of tunnel)
4. Saltford Railway GWR Cutting (south east of tunnel)
5. Avon Farm storm water cutting
The limestone exposed in Saltford is mainly Blue Lias. Blue Lias comprises decimetre (10 cm) scale alternations of muddy limestone containing argillaceous minerals (fine grained minerals containing clay-like components and that can appear silvery upon optical reflection) and mudstone (hardened mud). Blue Lias is useful as a building stone, and as a source of lime for making lime mortar. It is famous for its fossils, especially ammonites, and the blue-grey colour arises from its iron content, enclosed to a large extent in pyrites (mineral pyrites are an iron sulfide and the metallic luster and pale brass-yellow hue give them the nickname 'fool's gold').
Around Saltford there are now no natural rock exposures, most being man-made. These include the railway cutting on the Bath to Bristol railway line, some cuttings and a new exposure along the railway path, and the cliff section in the gardens along Mead Lane, which may be an old quarry. Many small quarries, from which the stone for Saltford's older houses, their garden walls and the field walls was extracted, have now been filled in.
Terrace gravel deposits of a much younger age occur in the Avon Valley and accumulated here during the last ice age that ended 11-12,000 years ago (the Pleistocene Period). One gravel deposit, near Avon Farm has national protection as a Site of Special Scientific Interest (SSSI)*.
*English Nature citation sheet for this SSSI site (1991) at ST 682684, due west of Avon Farm:
At least 2 metres (7 ft) of sandy gravels are recorded, consisting of limestone clasts mainly, but also with Millstone Grit, Pennant Sandstone, flint and chert clasts. The site is of considerable importance for studies relating to the possible glaciation of the area, and of the terrace stratigraphy, particularly as it is one of only two accessible terrace deposits in this part of the Avon valley.
The Geology surrounding Saltford
Other than the very young (Holocene, less than 10,000 years old) muds and silts which form the banks of the River Avon, and the river terrace gravels around Stidham Farm / Avon Farm (Grid ref: ST 677685 / ST 682684) which are of Pleistocene age and less than 1 millon years old, the youngest rocks in our area occur at Kelston Round Hill. The hill top consists of weathered, yellow or pale brown limestone of Middle Jurassic age, sometimes known as the Bath or Great Oolite. This is identical to the stone seen in many of Bath's Georgian buildings. It is an easily worked, shelly, "oolitic" limestone which was laid down in a shallow sea about 165 to 170 million years ago.
Kelston Round Hill's summit with its grove of trees is a natural feature and not an archaeological site. The small, remaining cap of hard limestone has protected and preserved the softer, Lower Jurassic shales which form the hill's lower slopes. Lansdown Hill nearby also consists of the Middle Jurassic Great Oolite and is the site of several Roman and early British archaeological features.
The oldest rocks exposed in the Saltford area are the Rhaetic White Lias limestones. These are of youngest Triassic age, and consist of buff coloured, very fine grained limestones, with almost no fossils. They can be seen beside the railway path near the former Kelston Station (Grid ref: ST689671). These rocks are thought to have been laid down in an extremely shallow sea or a lagoon, or perhaps in brackish water, some 200 to 205 million years ago. These Rhaetic rocks mark a transition to the Lower Jurassic rocks on which Saltford is built.
Note: See also our West to East geological cross-section of Saltford and Kelston Round Hill at the bottom of this page.
A small area of even older rocks has been found in the fields on either side of the A4 near Corston (around Grid ref: ST 699655). They are not exposed now but have been proved in a borehole to consist of Upper Carboniferous Coal Measure shales of the Pennant Series. These beds are a very small outlier of the Bristol coal field and have been dated at 300 to 305 million years.
The few remaining exposures confirm that Saltford and its immediate surroundings lie on the Blue Lias which is of Lower Jurassic age. These rocks were laid down some 190 to 200 million years ago in a shallow, sometimes muddy sea, perhaps near to land, as evidenced by occasional bivalve fossils and fragments of fossil wood or lignite. They consist mainly of grey to brown, thin to medium-bedded, often somewhat soft, shaly limestones containing frequent fossils. The limestones alternate with beds of dark grey shale of variable thickness.
When exposed to weathering for a long time, both the limestones and shales turn a much paler grey, or sometimes brown or yellow, as seen in some of our older buildings and surrounding walls.
Fossils found in Saltford
Note. See the artistic impressions of Saltford's ancient marine reptiles below.
Fossils from the Lower Jurassic period typically found in Saltford include:
AMMONITES: Agassiceras (4-10 cm), Arietites (15-20 cm), Arnioceras (5cm) and Schlotheimia (4-6 cm).
More rarely, pieces of bone and teeth are found. These belonged to sharks and also ancient marine reptiles such as Ichthyosaurs and Plesiosaurs (see below) but, as they occur as isolated fragments, a precise identification is rarely possible.
The images of the 8 fossils shown below have been reproduced courtesy of the Natural History Museum. They are from "British Mesozoic Fossils" published by the British Museum (Natural History) in 1975 and should not be reproduced by third parties without the NHM's prior permission. They are not to scale (for size range see list above).
Ancient marine reptiles
An ancient sea covered our region during the Lower Jurassic Period and evidence of some of the creatures that lived here can be found as fossils in our local rocks. The sea water was warm, shallow and teeming with life and included a number of successful sea-going reptile predators. These reptiles, some with powerful flippers, flew through the water much as penguins and sea lions do today. They were chasing and eating ammonites, fish and possibly other reptiles.
We know a lot about them from a few exceptionally well preserved fossils, particularly from Germany and England. These fossils, in some instances, have baby reptiles preserved within their body cavities suggesting that the mother reptile was pregnant when she died. It also clearly illustrates that these marine reptiles were not laying shelled eggs like the majority of other reptiles but gave birth to live young in the same way as modern day marine mammals. This shows an adaptation to a fully marine existence without the need to return to land to breed and reproduce.
The most common of these reptiles were the Ichthyosaurs (translated means 'fish-lizards'). These streamlined predators (some were just a few metres long) were dolphin-like in form and air breathing. The second group were the Plesiosaurs. They were not as common as the Ichthyosaurs and had quite a different body plan - tending to have longer necks and smaller heads but also had a smooth and streamlined profile. Both Ichthyosaurs and Plesiosaurs became extinct with the dinosaurs, about 60 million years ago.
Superficially many of the marine animals from the Jurassic Period look like mammals in the oceans today, but we shouldn't forget that these were reptiles and reptiles dominated the planet at sea and on land at this time.
During the construction of the Midland and Great Western Railways a few complete skeletons of marine reptiles were found and some were donated to local museum collections. Sadly, most of Bristol Museum's marine reptile collection was lost when the building housing them was bombed during World War 2. Bristol has since built up a more recent collection of stunning reptile fossils that are worth a visit. The national museum collections held in Cardiff and London also have spectacular displays of marine reptiles, many found on the Dorset coast.
Just occasionally individual bones and teeth are found in rocks picked up on the edges of fields in Saltford or from rocks discarded from building sites.
These 1824 drawings by William Conybeare of the skeletal anatomy of an Ichthyosaur and a Plesiosaur give an indication of what these pre-historic creatures looked like:-
These artistic impressions of Ichthyosaurs and Plesiosaurs when Saltford was a shallow sea some 200 million years ago have been produced for SEG by local artist Julian Balsdon:-
"There is a way that nature speaks,
West to East geological cross-section of Saltford and Kelston Round Hill (from sea level)
All content © Saltford Environment Group unless stated otherwise
W3C compliant website