Geology

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Bedrock

The rocks which form the shoreline of the Severn Estuary range in age from about 400 million to about 200 million years old1. The Estuary marks a boundary between the older rocks of Wales and the Forest of Dean and the younger limestones, deposited 350 million years ago in the Carboniferous period. The oldest rocks – sandstones – deposited in the early and middle parts of the Devonian Period, occur along the Somerset and North Devon coast from Minehead westwards2. These are the rocks which form the uplands of Exmoor and the Quantock Hills. Rocks of a similar age underlie part of the upper Estuary near Lydney in Gloucestershire. Jurassic limestones and mudstones, about 200 million years old, form the spectacular cliff scenery of the Glamorgan Heritage Coast from Barry, westwards to Southerndown3, and the cliffs at Lavernock Point and Penarth Head. These rocks also occur at Watchet in Somerset4 and around Burnham-on Sea, but there they are mostly hidden by more recent deposits. The limestone also forms the islands of Flat Holm and Steep Holm and the headlands of Brean Down and Sand Point and were formed from calcium-rich shells and skeletons of tiny animals which accumulated on the floor of a tropical ocean.

The bedrock of much of the shoreline of the Severn Estuary upstream of a line from Lavernock Point to Brean Down near Weston-super-Mare is formed of soft red mudstones of the late Triassic Period (about 220 million years old5). This is most often covered by the more recent deposits of the Wentloog, Caldicot and Somerset Levels; but in several places, such as at Penarth, Aust, Westbury on Severn and at Blue Anchor Bay in Somerset, it is capped by harder rocks, forming cliffs6. Between Barry and Sully on the Glamorgan coast, and at Sudbrook in Monmouthshire, the Triassic rocks are red or yellow sandstones and form low cliffs3,7.

Protruding through the cover of red Triassic mudstones in places along the coast are hard grey limestones deposited about 350 million years ago during the early part of the Carboniferous Period. These form the prominent headlands of Sand Point, Worlebury Hill and Brean Down at Weston-super-Mare5, and extend inland as the Mendip Hills. The beds of limestone are usually tilted as a result of compression and folding of the rocks at the end of the Carboniferous Period about 300 million years ago1. To the west, these folded limestones form Steep Holm and Flat Holm as well as Barry Island and Sully Island on the Glamorgan coast8. The limestones are also present along the Welsh coast at Ogmoreby- Sea and Porthcawl and upstream at Chepstow7. On the English side, the Carboniferous limestones, along with some late Devonian sandstones, form the coast between Portishead and Clevedon. Some early Carboniferous volcanic rocks are present within the limestone sequence at Middle Hope at Weston-super-Mare5. This complete sequence of rocks can be seen today in the cliffs at Aust (see Fig. 1) and Westbury on Severn.

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Bedrock offshore

Offshore, the bedrock underlying the floor of the Severn Estuary is largely a continuation of the shoreline geology9. Upstream of the Lavernock Point-Brean Down line, the rock beneath the Estuary is mainly Triassic red mudstones. Around Flat Holm and Steep Holm, folded Carboniferous limestone forms the floor extending towards Sand Point and Brean Down (see Fig. 2).

Downstream, the rocks beneath the waters are mainly early Jurassic limestones and mudstones like those at Watchet or along the Glamorgan Heritage Coast, but in the western part, off Porlock and Foreland Point in Somerset, middle and late Jurassic rocks are present (see Fig. 3). These youngest rocks (about 160 million years old), are preserved in an east-west dip where the layers of rock have been bent and folded downwards. The Jurassic rocks beneath the Severn Estuary, west of Weston-super-Mare are cut by many east-west fractures or faults and bent into a series of roughly east-west folds10. These structures in the rocks have influenced the shape of the coastline westwards from near Burnham-on-Sea on the Somerset side, and between Lavernock point and Nash Point on the Glamorgan coast (see Fig. 4). Other faults, running southeast-northwest, perhaps extending across Geology the Severn Estuary from the western side of the Quantock Hills, intersect the Welsh coast at Nash Point and turn the coastline north-westwards. Upstream of Lavernock Point, the Severn Estuary runs north-eastwards. This is probably controlled by a much older fault system, extending back to early Carboniferous times, the Severn Estuary Fault Zone.

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The Ice Age and after

At the height of the last ice age, about 20,000 years ago, much of northern and western Britain was covered by a thick ice sheet whose southern limit lay through South Wales. So much water was locked up in the great ice sheets of the northern hemisphere at this time that sea level fell by about 120 metres1. The Severn Estuary at this time would have been a wide, cold plain with a central river channel flowing to the sea which would have been situated far to the west, beyond Land’s End. Rivers of fast moving meltwater from the ice sheet flowed south to join the Severn, transporting coarse sands and gravels from the rock debris carried by the ice and cutting valleys deep below present sea level. The main channel of the Severn now lies submerged at the bottom of the estuary and provides a deep water route for shipping.

At the end of the last ice age, as the ice sheet melted and water returned to the world’s oceans, sea level rose in stages, reaching its present height about 4500 years ago1. In the Severn Estuary, this rise in sea level inundated a wooded coastline. Beds of peat with trees were buried beneath fine grey clays. These ‘submerged forests’ are visible at low tide along the shoreline in a number of places along the Severn Estuary coast (see Fig. 5). The spread of clays form the Wentloog and Caldicot Levels on the north side of the Estuary and the Somerset Levels on the south where the clays can be up to 30 metres thick6. The hilltops of Flat Holm, Steep Holm and Brean Down also became islands.

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Geologically important sites

The rocks along the Severn Estuary coast record changes of environment and climate through time, from the river-deposited sandstones of the Exmoor coast in the Devonian Period, the tropical seas of the Carboniferous, the desert plains and hills of the Triassic, and the spread of the Jurassic sea over the region1. The places which best display these rocks are protected as Sites of Special Scientific Interest (SSSI) which should be preserved for education and science. They include sites which are important for the fossils they contain, or for the occurrence of particular rocks, structures, or minerals11 (see Fig. 6).

The Estuary today

Today, the estuary is known as a ‘drowned valley’. The bedrock floor of the Severn Estuary upstream of a line from Lavernock Point to Brean Down is covered with areas of gravel, sand and mud. These also cover bedrock in the Bridgewater Bay area, but downstream of the Lavernock-Brean Down line, bedrock is exposed on the floor of the Estuary9. Movement of sediment within the Severn Estuary is complex due to the high tidal range and the strong tidal currents that prevail here. Not only are silt and mud carried seaward into the Estuary by the rivers, they are held in the Estuary, deposited and moved again over considerable periods of time. In addition, sand is carried up the Estuary and landward9. This sand is not derived from erosion of the shoreline rocks, but from the deposits of glacial rivers during the last ice age. The surface layers of the sandbanks are largely unconsolidated and may be moved and redeposited by the strong tides. Some of the near-shore sandbanks provide an element of protection against coastal erosion by reducing wave energy. The dredging of sand for the aggregates industry is therefore closely monitored.

The Future

The Estuary is constrained at points by harder rocks such as along the Glamorgan and Somerset coasts, however much of the upstream shoreline is formed of softer rocks or river muds, such as the Wentloog, Caldicot and Somerset levels8. Where there are no defences or hard geology, the Estuary will continue to widen and retreat upstream under rising sea levels. However, there are very few stretches of coastline that are fronted by neither hard geology nor sea defences. It may therefore be necessary for certain stretches of sea defences to be moved landward in the future, thus allowing the Estuary to broaden in a managed way.

For more information see the following links:

References

Tom Sharpe, National Museum of Wales.

  1. North, F.J., 1964. The evolution of the Bristol Channel with special reference to the coast of South Wales. Third edition. Cardiff: National Museum of Wales, vii+110pp.
  2. Edwards, R.A., 1999. The Minehead district – a concise account of the geology Memoir for 1:50 000 Geological Sheet 278 and part of sheet 294 (England and Wales). London: The Stationery Office, xii+128pp.
  3. Williams, A., Davies, P. & Caldwell, N., 1997. Coastal processes and landforms. The Glamorgan Heritage Coast. Southerndown: The Glamorgan Heritage Coast Centre, iv+90pp.
  4. Robinson, E., 2006. The geology of Watchet and its neighbourhood, Somerset. Geologists’ Association guide No 66, London: The Geologists’ Association, 27pp.
  5. Whittaker, A. & Green, G.W., 1983. Geology of the country around Weston-super Mare. Memoir for 1:50 000 geological sheet 279, New Series, with parts of sheets 263 and 295. Institute of Geological Sciences. London: HMSO, x + 147pp.
  6. Hardy, P., 1999. The geology of Somerset. Bradford on Avon: Ex Libris Press,
    223pp.
  7. Welch, F.B.A. & Trotter, F.M., 1961. Geology of the country around Monmouth and Chepstow. (Explanation of One-inch Geological Sheets 233 and 250. New Series). London: HMSO, viii+155pp.
  8. Perkins, J.W., Gayer, R.A. & Baker, J.W., [1997]. Glamorgan Heritage Coast. A guide to its geology. Southerndown: The Glamorgan Heritage Coast Centre, 40pp.
  9. Kellaway, G.A. & Welch, F.B.A., 1993. Geology of the Bristol district. Memoir for 1:63 360 geological special sheet (England and Wales). London: HMSO, xii+199pp.
  10. Howe, S.R., Owen, G. & Sharpe, T., 2004. Walking the rocks. Six walks discovering scenery and geology along the Glamorgan coast. Cardiff: Geologists’ Association South Wales Group, 116pp.
  11. Green, G.W., 1992. British Regional Geology, Bristol and Gloucester region.
    Third edition. London: HMSO, x + 188pp.
Last Updated 2013

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