The Triassic Period - 245 to 208 million years ago
An informal name for both the Permian and Triassic periods is New Red Sandstone. This older name has survived partly because of the difficulty in sub-dividing these rocks. There are no series names. The two divisions of rock type found in South Gloucestershire are the Mercia Mudstone Group and the Penarth Group.
In order to understand the pattern of deposition it is necessary to have a picture of the palaeo-environment at the end of the Permian. Massive amounts of erosion had reduced much of the area to a plain, with a few remnants of mountains.
Steep valleys like wadis formed in these mountains and flash floods from time to time brought the eroded bedrock, mostly of Carboniferous age, to be deposited in the valleys or on the lower slopes. This happened spasmodically both in the late Permian and in the Trias so that no single age can be given for this rock, which is known as the Dolomitic Conglomerate. It is inter-bedded with Triassic rocks at the margins of the saline lakes and rivers where sandstones and mudstones formed. Because of the variable stratigraphic position the Dolomitic Conglomerate is termed diachronous i.e. crossing different time periods.
The large amount of erosion which took place means that the Triassic rocks lie unconformably across the whole region. Sometimes they are above the Coal Measures and in other places above the Carboniferous Limestone. The conglomerate may be found in patches at Aust where it sits on the Carboniferous Limestone seen in the river at low tide. Other outcrops are around Thornbury. Grid Reference ST633906. Walk down the lane, west of the Parish Church (The Pithay). Immediately right is a small field backed by a low cliff formed of Dolomitic Conglomerate. Here a massive bed supports others, which are eroded to display large boulders up to 0.5 metres across. The angle of the beds reflects the way the debris flowed down a former hillside. It can be seen that the beds get finer from south to North showing that the flows came from a southerly direction. Dolomitic Conglomerate varies from massive boulders cemented together to much finer sandstone. It also varies in colour from red to yellow. The much finer beds can be seen at Itchington where the lane goes under the motorway ST 655876. An interpretation board is located here. This site is excellent for seeing the unconformity between the Dolomitic Conglomerate and the underlying rock.
The Mercia Mudstone Group
Not all of the rocks are mudstones. In Stoke Park ST 617767 on the boundary between South Gloucestershire and Bristol, there is an outcrop of coarse sandstone which may have formed in a delta which spread across the mudflats, but the most commonly seen examples of the Mercian Mudstones are red mudstones and the most famous place to see them is at Aust Cliff. ST 565895 where there are an interpretation boards on the foreshore and at the entrance to the Services.
In the area which is now Aust cliff a shallow lagoon formed. The seawater was continually evaporated leaving minerals to collect with mud and silt on the lagoon floor. The water was replenished from time to time and the thickness of the evaporite deposits increased. These are represented by the red and green mudstones seen in the lower part of the cliff. Within these sediments crystals of gypsum and calcite can be found together with cube shaped voids where crystals of halite, (table salt) have dissolved.
Triassic rocks filled fissures in the bedrock of the older limestone (Carboniferous). One place where there are significant deposits is Slickstones Quarry ST 703915. Photo from Slickstones Board. Here the fissures are filled with red Triassic rocks, which can be seen as red staining on the quarry faces. Important fossils were found in a cave which had formed in the limestone and which was exposed by quarrying. These are vertebrates, ancestors of modern lizards and are some of the earliest land reptiles as distinct from those, which lived partly in water. There is an interpretation board at Slickstones.
The Penarth Group Late Triassic
Rocks belonging to this group can be seen in the upper part of Aust cliff, though they are inaccessible here. Samples which have fallen from the cliff can be found on the beach at Aust. Within these rocks there are fragments of bone and the teeth of fish.
The change of colour of the rock from red to yellowish grey with bands of black signifies a change in the environment of deposition from desert lagoon to deeper sea. Fossils of bi-vales and teeth of fish can be found by a careful examination of the rocks.
The rocks which are in the highest part of the cliff at Aust can be inspected more closely at Manor farm GR ST577894. Please contact the landowners before visiting the site.
The rocks exposed at the Manor Farm, Aust site replicate the highest part of the Aust Cliff site (approximately 800m to the west). The exposure was created in 1995 to provide construction materials for the new Severn Bridge and associated works. The landowners, RMC Roadstone Ltd and the restoration contractors all agreed to keep a rock face exposed for the benefit of geologists and interested members of the public.
The rocks have been excavated to form wide ledges, allowing easy access to most layers. All the rocks are of Late Triassic age and were formed in shallow lagoons in arid or semi-arid conditions. The majority of rocks are soft, crumbly shales and mudstones, but there are also harder layers of limestone. At the base of the exposure and only preserved in places is the famous Ceratodus bone bed, so named because it contains the teeth of a now extinct air-breathing lungfish called ‘Ceratodus’. The bone bed also contains abundant isolated bones of marine reptiles and fish, coprolites (fossil dung) and very occasionally the bones of dinosaurs. Above the bone bed, the exposure consists largely of shales, mudstones and thin limestone layers. These also contain bones but not in the same concentrations as the bone bed. The most numerous fossils in these layers belong to the hard calcite shells of shellfish.
At the gated entrance to the site, close to the interpretation board, are several large blocks of the bone bed. Visitors to the site are encouraged not to hammer these blocks. Fragments of bone, teeth and coprolites of marine reptiles and fish can be seen protruding from the surface of the rock. Collecting fossils from the remaining exposure is permitted; most are small and can be collected from close inspection of the weathered surfaces of clay and shale.
[Home] [Top] [Geological History of South Gloucestershire - Main Page]
