According to local folklore the Avon Gorge was constructed by a giant called Vincent but it is actually testimony to the erosive power of water cutting its way down through the huge thicknesses of limestone rock over many thousands of years. The geology and geomorphology of the Avon Gorge, Downs and Leigh Woods, tell a remarkable story.
Geology of the Avon Gorge
The spectacular scenery of the Avon Gorge owes its remarkable features to the amazing forces of geology spanning over 300 million years. The Gorge is famous for the shear amount of Carboniferous rock exposed over such a small area. These limestones and sandstones are teaming with the record of ancient life and it was this rich diversity of life that made the Avon Gorge the site of the first major studies into Carboniferous fossils.
Not surprisingly, the 2.5km long gorge is designated as a Site of Special Scientific Interest (SSSI) – a reflection of the truly outstanding fossil flora and fauna. It is also a Regionally Important Geological and Geomorphological Site (RIGS); designated because of its value in teaching about Carboniferous Limestone.
The Gorge’s underlying limestone rock is responsible for the development of carboniferous limestone soils, upon which the rare plants of the Gorge depend.
Rocks of the Avon Gorge
The rock sequence begins with Devonian Old Red Sandstones at its base. Imagine a warm and arid climate, as Britain once lay south of the Equator. Sadly these Devonian rocks are not seen in the gorge itself, but are important as they hold clues to the evolution of vertebrates. This time period came to end as the land was slowly inundated by rising seas.
It was at this time that the famous Carboniferous sediments were deposited. These rocks show a gradual passage from Sandstone, through Fossiliferous Sandy Limestones and Shales to Limestones laid down in a relatively shallow sea. These rocks include a vast diversity of life, such as algae, plants, sponge spicules, annelids, crinoids, echinoids, lamellibranches, gastropods, pteropods, cephalopods, ostracods, conodonts, as well as corals and brachiopods.
In the late Carboniferous, conditions changed as the Bristol area became humid allowing the deposition of Sandstones and Gritstones, followed by the well-known Coal Measures which lie south of the Suspension Bridge. The rocks were folded into great mountain ranges by the Variscan Orogeny around 290 million years ago, giving rise to the 30° South-East dip of the rocks. This huge collision event also produced the large fault seen under the Suspension Bridge.
Erosion of these huge mountain ranges left a 70 million year time gap (unconformity) where no rocks of younger age occur before the deposition of the Triassic Rocks 220 million years ago – Permian rocks are effectively missing. By the Triassic, Britain was 20-30° north of the equator and evidence of the climate lies within the red Dolomitic Conglomerate seen on Bridge Valley Road. This represents the upper limit of rocks exposed within the Gorge.
Forming the Avon Gorge
For a long time it was unclear what caused the Avon to cut through the limestones, rather than run south west through towards Weston-super-Mare. However, during the last ice age Bristol was at the southern edge of glaciation and it has been suggested that ice blocked the river’s natural route through Ashton Vale to the west, allowing it to cut down through the huge thicknesses of limestone.
Where to look
Several sites at the edge of the Downs and Leigh Woods, afford stunning views of the Gorge. Observatory Hill, the Peregrine watch, Sea walls, Stokeleigh view point, Peregrine view point (Leigh Woods) and the Tow Path, will enable the viewer to see the massive bedding planes in the Carboniferous Limestone.
Details of the Clifton Down Limestone can be seen along Bridge Road, here there are superbly exposed masses of fossil corals and shellfish, as well as the mineral calcite with a few geodes of Quartz.
Geology of the Downs
Most of the rocks found in the Gorge also underlie the Downs, with the oldest group being of carboniferous limestone. However, in places the Downs also demonstrates several younger rocks, such as the Westbury Beds from the Triassic period.
There have been many quarries on the Downs, where the Carboniferous Limestone was extracted for use as a building stone and for making lime mortar. The working of these quarries also led to a number of geological discoveries, such as the ‘Bristol Dinosaur’ Thecodontosaurus antiquus. This was discovered when its fossilised remains were found in a limestone quarry near the top of Blackboy Hill, in 1834. The youngest deposits known of on the Downs are those that were found in the Durdham Down Bone Cave, which was discovered in one of the quarries on the Downs, in 1842. These deposits included the remains of long extinct animals, which are believed to have been the prey of a group of hyenas that had inhabited the cave. A model of the cave is preserved in the Geology Department of the City Museum and Art Gallery.
There are also a number of caves in the area, predominantly found in the sides of the Gorge. The only remaining cave entrance from the Downs, is that of Observatory Hill Cave, however its entrance has been blocked up.
For more detailed information on the geology of the Downs see The Geology of Clifton and Durdham Downs by Andrew Mathieson. This article has been reproduced from the Journal, Nature in Avon (The Proceedings of the Bristol Naturalists’ Society) 2013, by kind permission of the Bristol Naturalists’ Society.
Information about the tufa dam will be coming soon.
Photographs © Avon Gorge & Downs Wildlife Project, © Denice Stout, Quarry to the west of the junction of Ladies Mile and Stoke Road (now filled in); watercolour by William Arnee Frank, c.1862 (© Bristol Culture K6317).