The Geology of the Cotswolds AONB
The Cotswolds AONB stretches for nearly 60 miles, forming part of an outcrop of Jurassic rocks that runs NE from the Dorset coast to the North Sea off Yorkshire. The escarpment can be identified from the area around Bath, right up to near Market Weighton in Yorkshire and is the largest continuous landform feature in lowland England. It is a classic example of a scarp and dip landscape. The steep western scarp of the Cotswolds exposes sections through Lower and Middle Jurassic rocks that dip gently eastwards towards Oxford and London.
At Leckhampton Hill and Cleeve Common the thickest sections of Jurassic rocks anywhere in the country are exposed, a feature recognised by their designation as Sites of Special Scientific Interest. Towards the south-east the rocks get gradually younger and their different lithologies and erosional histories produce different types of features in the landscape. The relationships between geology and landscape can be clearly seen.
The rocks that form the Cotswolds are made up of three different stages of the Jurassic period and date from between 210-140 million years ago. The Lower Jurassic is made up of the Lias Group, the Middle Jurassic Rocks are divided into the Inferior Oolite Group, and the Great Oolite Group and the Upper Jurassic is represented by the Ancholme Group. Each Group is subdivided into Formations and Members distinguished from each other according to differences in the constituent parts of the rock, the types of fossils found in the rocks and by erosional surfaces that mark breaks in deposition of the sediments.
The geology of the Cotswolds as everywhere has a very strong influence on the landscape, vegetation and wildlife, as well as on the industry and heritage of the people living in the area. The soils and vegetation noticeably change as the underlying geology changes from one type of rock to another, influencing growth of different species of plants and trees, which in turn provide habitats for a variety of different animal and bird species..
The landscape of the escarpment is relatively young, as the wearing back of the escarpment has taken place over the last 1.6 million years during the Quaternary period. Evidence of periglacial activity comes from gravel fans at the edge of the scarp and some areas continue to show active geomorphological processes. The Cotswolds escarpment probably has one of the highest inland concentrations of landslips in the country. The gentle undulations of the Cotswold landscape were formed by numerous streams cutting down through the rocks. Some of these streams still flow but many were the result of melting snow and ice and higher levels of precipitation following the Ice Ages that have left dry valleys behind them.
The Cotswolds are famous for the variety and abundance of fossils that can be found. The types of fossils and the ways in which they are preserved can tell us a great deal about the environments in which they lived and died. The fossil content of the rocks is a useful tool in establishing relative ages of the strata, separated into different ‘zones’ by the particular species of ammonites or brachiopods.
Ages of different rocks can therefore be correlated over very great distances. The variety is so great in the Cotswolds and includes brachiopods, bivalves, ammonites, belemnites, echinoids, crinoids, fish, reptiles and mammals.
Awareness and understanding of the sedimentary deposits and landforms developed during the climatically ‘restless’ Quaternary Period (1.64 Ma – present), with its glacial and interglacial episodes, is highly significant to the geoscientist working in Great Britain. Because of prolonged episodes of weathering and erosion during this time, deposits formed only in the last 500,000 years or so are the most widespread in the Cotswolds. Remnants of some older, early Pleistocene river terrace deposits, known as ‘Northern Drift’ occur in places, believed to be evidence of a proto-Evenlode/Thames river system that drained much of the Midlands and possibly parts of central and north Wales.
The most extensive younger deposits include the river terrace complexes of the Severn, Upper Thames and the Warwickshire and Wiltshire Avons. Most of these were laid down during glacial Stages, with some during interglacials. Till and associated glacio-fluvial and meltwater lake deposits from the Anglian Glaciation (480,000 – 430,000 years Before Present) are present in the Vale of Moreton. Sediments laid down during the latest glaciation, the Devensian (75,000 – 10,000 years BP), and the following temperate climatic stage, the Flandrian (10,000 years – present), occupy the largest areas. Major Devensian deposits include terraces along the main river systems, fan deposits in the Severn Vale, and head in the dip slope valleys of the Cotswold escarpment. The post glacial sediments include river alluvium, very widespread and spectacular landslip along the Cotswold Scarp (possibly the largest in Britain), and local occurrences of tufa and peat.
Although the essential character of the modern topography is thought to have developed during the preceding Tertiary Period, major modifications occurred during the Quaternary. The terrace deposits record episodes of shallow but progressive river incision and planation; meltwaters and post-glacial precipitation have carved valleys on the Cotswolds which are now dry or occupied by smaller misfit streams. The land-slipped areas on the Cotswold Scarp have a characteristic set of topographic features and are highly responsive to environmental change in and on them, such as caused by increased rainfall or ground engineering projects.
The above is from ‘The Geological & Geomorphological Importance of the Cotswolds Area of Outstanding Natural Beauty’. Click here to download full report.
Click here to download the general information leaflet 'Geology & the Cotswolds'