Discover Bristol and Explore the Fossils and Geodiversity of Avon Gorge
- Wayne Munday
- Jul 6
- 5 min read
Updated: Sep 11
Sip back and discover Bristol and explore the fossils and geodiversity of Avon Gorge a picturesque natural landmark whose steep fossiliferous limestone cliffs rise high above the River Avon. Carved by the forces of sea level change, tectonic movement, glacial erosion and flowing water. Avon Gorge tells a story that spans more than 350 million years back to the Carboniferous Period. Avon Gorge is flanked by the open expanses of Clifton and Durdham Downs to the east and the ancient woodlands of Leigh Woods to the west. The iconic Clifton Suspension Bridge, completed in 1864, spans the gorge, offering commanding views both north and south. Avon Gorge in Bristol offers the visitor stunning views, striking limestone cliffs and the famous Clifton Suspension Bridge all within a short distance from the heart of the City of Bristol.
The underlying geology of Avon Gorge was laid down during the Lower Carboniferous period in an age when what is now southwest Britain lay beneath a shallow tropical sea. This ancient sea deposited layer upon layer of limestone now visible in the cliffs of Avon Gorge. Two prominent formations of limestone are exposed including the older Black Rock Limestone and the overlying Clifton Down Limestone and both are part of the Pembroke Limestone Group.
The Pembroke Limestone Group of southern Wales and northern Somerset including Bristol forms part of the broader Carboniferous Limestone Supergroup that spans much of England and Wales including the Peak District, North and South Wales, the Bristol area and also extends underground into southern England and parts of Ireland. This significant geological formation was deposited between 363 - 325 million years ago during the Dinantian Epoch. The fossil record within these rocks is remarkably rich featuring well-preserved marine organisms such as brachiopods, crinoids, corals, and bryozoans.
The alternating sequences of limestone, shales, and sandstones across Avon Gorge reflect fluctuations in sea level over millions of years. Tectonic deformation has left a clear mark on the area most notably in the form of the Avon Thrust a significant geological fault where older rocks were pushed over younger ones causing uplift and folding. This structural disturbance is linked to the formation of the nearby Mendip Hills and set the stage for the River Avon and glacial meltwater to carve through layers of Carboniferous Limestone, Sandstone and Shale especially during the Pleistocene interglacial periods.
The gorge itself lies within what geologists call locally the "Avonian" sequence of the Dinantian Epoch a well-preserved Lower Carboniferous sedimentary succession that has fascinated geologists for over two centuries. Pioneers of geology such as William Smith, Henry De la Beche, William Conybeare, Louis Agassiz and William Buckland all of whom studied this region in the early 19th century.
The Black Rock Limestone was laid down between 359 - 344 million years ago during the transitional period from the Tournaisian to the Visean stages. This formation consists mainly of dark grey packstones packed with corals, brachiopods and crinoids. Its lower layers are dominated by brachiopods like Cleiothyridina and Syringothyris and the middle layer by a varied coral fauna of Caninophyllum and Caninia and the upper layers by Siphonophyllia cylindrica. These fossils and sediments suggest that they were deposited in a warm, shallow lagoon or shelf sea environment.
The Clifton Down Limestone formed between 344 - 337 million years ago. This layer contains a variety of limestones including oolitic, bioclastic and oncolitic limestone mixed with calcite, dolomite mudstones and silicified fossils. Thiese silicified fossils include the now extinct and stony colonial rugose corals of Carcinophyllum vaughani recognisably wrinkled or folded in appearance and Siphonodendron martini with a cylindrical, flattened, spaghetti-like fossil form. These fossils and sediments were likely to have been deposited in a more energetic shallow marine environment such as a barrier reef system or shelf lagoon.
Avon Gorge tells a story that extends far beyond the Carboniferous Period. Within the fissures and cavities near the Clifton Suspension Bridge geologists have uncovered layers of sediment from the Late and Early Triassic periods including the Rhaetic bone beds of thin layers containing the fossilised remains of fish, reptiles and early dinosaurs including Thecodontosaurus, nicknamed the "Bristol Dinosaur", washed out to sea to be buried by sediment to be preserved as fossils. One remarkable feature from this period is the Cotham Marble, a decorative limestone formed by the interplay of algal mats and burrowing organisms on ancient mudflats.
Further up the timeline, Avon Gorge has revealed its Ice Age past. In 1842, during excavation work for the Clifton Suspension Bridge a cavern known as the Durdham Down Bone Cave was discovered. This cave contained the remains of Ice Age megafauna. Evidence such as gnawed bones and broken skulls suggested that the cave may have once served as a den to a clan of Cave Hyena, probably Spotted Hyenas (Crocuta crocuta), during the Ipswichian Interglacial period also known as the Eemian in northern Europe around 128,000 - 116,000 years ago.
Avon Gorge is not just a geological destination but also a rich ecological habitat. Its steep slopes and unique limestone soils support a range of rare and endemic plant species, including the Bristol Rock-Cress, several varieties of Whitebeam (Sorbus), Round-Headed leek and Autumn Squill. The ancient woodlands of Leigh Woods shelter native Oak, Ash, and Lime trees and comes alive in Spring with carpets of Bluebells and Wild Garlic (Ramsons). Wildlife also thrives with Peregrine Falcons now nest in the gorge’s cliffs and are often seen stooping over the River Avon and the nearby Bristol Harbour and docks.
Avon Gorge has earned its designation as both a SSSI and Regionally Important Geological and Geomorphological Site (RIGS). It is protected and managed through a partnership of organisations including the National Trust and the Avon Gorge & Downs Project. These groups work to conserve this fragile habitat and geological features while promoting public education and access to this extraordinary landscape.
Avon Gorge is also steeped in local folklore. One popular legend tells of two giants called Goram and Vincent who both loved a woman named Avona. To win her hand, she challenged them to drain a great lake that once covered the Bristol area. Goram began digging but paused to rest and fell asleep, creating a rocky hollow known as "Goram’s Chair". Meanwhile, Vincent worked tirelessly and succeeded in carving the gorge winning Avona’s love. When Goram awoke and saw he had lost and in disappointment he threw himself into the middle of the Bristol Channel and his body according to legend now forms the Carboniferous Limestone islands of Flat Holm and Steep Holm.
Today, it is understood that Avon Gorge was formed during the last Ice Age when a glacier blocked the flow of the River Avon causing a large lake to form upstream. When the lake overflowed it began cutting down into the limestone plateau. As the softer rocks were gradually eroded away the river incised deeper into the resistant Carboniferous limestone carving the steep-sided gorge we see today.
For anyone visiting Bristol the Avon Gorge offers a story of ancient marine life written in stone where fossilised corals and crinoids lie beside traces of prehistoric deserts, Triassic reptiles, dinosaurs and Ice Age beasts. Yet it is also a living landscape, filled with birdsong, wildflowers and legends.
