River Dee Geology

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The River Dee (Welsh: Afon Dyfrdwy) flows 110 miles from it's source to Hilbre Island. Travelling through Wales and England and also forming part of the international border between them, the river rises in Snowdonia, flows north via Chester and discharges into an estuary between Wales and The Wirral. The lower reaches of the river are unusual in that comparatively little water occupies so large a basin. One theory of a contributory factor to the large basin is that once the River Mersey and/or the River Severn flowed into the Dee. A more recent theory, however, is that the estuary was not formed by water, but by ice being pushed southwards by the pressure of an icecap over the Irish Sea. The total catchment area of the River Dee up to Chester Weir is 1,816.8 square kilometres (701.5 sq mi). The average rainfall over the catchment is estimated to be 640 millimetres (25 in) yielding an average flow of 37 m³/s.

The River Dee - roughly 110 miles from source to sea seen here at the "Geology of Britain Viewer".

The history of the Dee has many interwoven layers. There is the geological record, from the Ordovician rocks at its source to the modern deposits in the estuary. There is a historical record starting with it's use as a trade route in pre-Roman times. A path of myth which places the young Arthur at it's source. And an industrial tale bringing gold, stone, wool and water downstream as the railways and canals crept ever upstream towards it's head. We make no apology for mashing-up these strands for the Dee is a river to be explored in its many moods on many levels.

This geological cross-section from William Smith's 1815 map (Snowdon to London) illustrates Smith’s observation that strata across much of southern England dip towards the southeast, with younger strata lying on top of older ones.

River Dee geology through time

The Earth formed around 4,600 million years ago. After some 500 million years, heating of the newly accreted Earth by radioactive elements increased the temperature until heavy elements such as iron and nickel became molten. These elements then began a journey to the center of the Earth which released enough gratitational potential energy to melt almost all of the planet. Its once molten surface hardened to give a relatively thin layer of solid rock (the crust) floating on a mass of molten and semi-molten rock (the mantle) surrounding a core of nickel/iron alloy and other heavy elements. The crust consists of about a dozen, quite separate, irregularly-shaped ‘plates’ that float on the mantle. These plates can collide when one plate will crumple to form mountains and the other will be pushed down into the mantle where it will melt and be recycled, or the plates can drift apart leaving gaps filling with new rock welling up from the mantle and solidifying. The Earth’s cooling caused water vapour in the atmosphere to condense into rain to create lakes, seas and oceans. The action of wind and water began to erode the first igneous rocks into grains which settled to form the first sedimentary rocks - such as mudstones and sandstones. The weather still continues to wear down mountains to debris which are transported elsewhere. The continual movement of the Earth’s crust gradually transformed some of these early igneous and sedimentary rocks into metamorphic rocks, such as slate formed from mudstone/siltstone. Some rocks have an organic origin: limestone is formed from skeletal fragments of marine organisms, coal from plant life. Faulting of the crust can mean that rocks from different ages can border each other, and can allow mineralisation of rocks with elements such as lead, copper, manganese and gold.

A very "rough and ready" view of the geology of the River Dee. The river cuts through Silurian and Carboniferous deposits before emerging into the graben of the Cheshire plain with its characteristic red sandstone and entrapped salt deposits.

The geology of Britain is such that the oldest rocks are exposed to the west and the youngest to the east. This means that the River Dee crosses rocks from several geological periods on its journey from source to sea. This fact was first noticed by William Smith, who, in 1815, published the first geological map of Britain. In 1817 Smith drew a remarkable geological section from Snowdon to London. Unfortunately, his maps were soon plagiarised by the Geological Society of London and sold for prices lower than he was asking. He went into debt, and despite the sale of his geological collection to the British Museum, he finally became bankrupt in 1819 and was sent to debtor's prison. On 31 August 1819 Smith was released from King's Bench Prison in London. He returned to his home of fourteen years at 15 Buckingham Street to find a bailiff at the door and his home and property seized. Subsequent modern geological maps have been based on Smith's original work, of which less than 40 copies have survived, one of which is in the Grosvenor Museum at Chester.

The plain south of Chester is a "graben" or "rift valley" formed between a pair of faults. This has allowed the New Red Sandstone from the Triassic and Permian to survive, whereas to either side these rocks have been eroded away exposing the Carboniferous coal measures at Ruabon and Macclesfield.

The River Dee rises on Ordovician volcanic rocks which formed when North Wales was a sea dotted with volcanic islands near the south pole over 400 million years ago. These mineralised rocks include deposits of much mined gold and silver with copper, lead and zinc sulphides. It flows northwards along the Bala fault over Ordovician and Silurian rocks laid down as sediments in an ancient ocean and estuaries, which were raised into the massive Caledonian mountain range when Wales collided with North America. Over 20,000 feet of these mountains have since been eroded away.

The Bala area is rich in geological features and interest, including a major fault line, a major syncline flanked by domes and all the classical rock types, i.e.: sedimentary, metamorphic and igneous formations, as well as significant glaciation. The area has given names to geological features or time-scales including Arenig time scale, the Bala Series, and Hirnant time scale. The principal geology dates back to the Silurian period (410-430 million years ago) and Ordovician period (430-500 million years ago). The Bala Lake/River Dee valley which runs north-east to south-west was originally caused by a fault line (The Bala Fault) which extends south-west to Tal-y-Llyn with a westerly separate fork forming what in now the Mawddach estuary, extending to Cardigan Bay.

Near Llangollen the Dee has cut through Carboniferous limestones, grits and coal measures which fueled the Industrial Revolution in the region. The floor of the valley is the "Dinas Bran Formation" - mudstone and sandstone, sedimentary bedrock formed approximately 419-423 million years ago in the Silurian period beneath shallow seas. It emerges from the Vale of Llangollen, crossing a fault line onto a range of 240-320 million year old sandstones from the Permian and Trassic. The coal measures (from ~310 million years ago) and later the sandstones seem to have been formed in the vicinity of a river flowing northwards from what is now France, wearing down the long-vanished Variscan mountains. All more recent rocks have been eroded away, and in the last few million years glaciation has left a layer of boulder clay in the Cheshire plain "rift valley".

A sandstone "monolith" in the Roman Quarry at Edgar's Field shows the characteristic pattern of river-deposited sandstone. The coloured banding is due to a differing iron content in the deposits, which influences the rate at which they erode. Features include "cross-bedding" and "slumps" which forms as the braided river system forms banks and shoals in its bed and changes course. The sand and pebbles in this bed come from the slow wearing down of the Variscan Mountains in France, formed when the Iberian/African plate collided with the Avalonia.

The Sandstone

A section through a small part of the "Type Reference" of the Chester Pebble Beds formation in a railway cutting near Northgate.

The Chester basin rock system is part of the "Sherwood Sandstone Group" which extends from Devon northwards as far as Armagh in Ireland and Gretna, Dumfries and Galloway in Scotland. This "New Red Sandstone" was deposited as a result of the erosion of the Variscan mountains in what is now France. The "rock-group" has economic importance as the reservoir of the Morecambe Bay gas field, the second largest gas field in the UK. The full sequence of the Cheshire basin rock system is:

  • Helsby Sandstone Formation: around a 250m thickness of mid-Triassic sandstone with conglomerate and siltstone. Two major faulted blocks of these rocks are largely responsible for the prominent west facing escarpment of the Mid Cheshire Ridge. The Newgate in the City Walls is faced with Helsby Sandstone;
  • Wilmslow Sandstone Formation: an up to 900m thickness of early Triassic sandstones with occasional siltstones. The 60m thick "Thurstaston Sandstone Member" and the 2m thick "Thurstaston Hard Sandstone Bed" are found in the central ridge of the Wirral;
  • Chester Pebble Beds Formation: from less than 90m to over 220m of river-formed sandstones with some conglomerates and siltstones of early Triassic age. One of the internationally recognised "reference sections" for these strata is the railway cutting near Northgate stadium, but this type of rock can be seen in several places around Chester. The formation extends from the south Devon coast northwards, up to the Cumbrian coast on the west side of England, and to the Doncaster area on the east side.

The Carboniferous shales which lie below the Chester sandstone have been proposed as a source of "shale gas", which could potentially be extracted by the controversial method of "fracking" - something which led to a good deal of local debate and protest. Discussion of this somewhat polarised subject is beyond the scope of this site.

The Dee in its "modern" form has existed over these few million years, either as a river or a glacial flow in the ice sheets which covered North Wales and Cheshire. The last period of glaciation is named the Devensian derived from the Latin Dēvenses, people living by the Dee (Dēva in Latin), along the Welsh border near which deposits from the period are particularly well represented. The river continues to erode in the Upper Reaches, and transport material through its Middle Reaches to deposit silt in the Lower Reaches. Geology had influenced history, with the Bala corridor and the Upper Reaches of the River Dee being used for settlement, cattle raising and communication since Neolithic times and the fertile Cheshire plain supporting a dairy industry that exported its surplus as Cheshire Cheese.

Nowadays, the Dee is one of the most regulated rivers in Europe, it supplies more water for public supply than the whole of the English Lake District and two-thirds of the river water is abstracted before the River Dee reaches the weir at Chester. The natural flow of the River Dee during most summers is insufficient to sustain this rate of abstraction, so a series of reservoirs have been constructed to store excess water available in wintertime and release it back into the River Dee during drier months. This system of low-flow regulation was used by Thomas Telford at the beginning of the 19th Century in order to guarantee a supply of water to the Ellesmere Canal. Telford constructed sluices at the outlet of Bala Lake to control the flow of the Dee downstream so that there was always sufficient water to supply the canal where it started at Horseshoe Falls.

The eventual silting of the Dee Estuary led to a downstream migration of Chester's port and the eventual canalisation of the River from Chester to Connahs Quay with "Sealand" reclaimed - on the English side so that land prices would be better, and the river channel cut so shallow that Liverpool became the dominant port in the region.

A geological time chart

Eon Era Period - and what happened (green = land, blue = sea, aquamarine = mixed) Extent, Million
Years Ago
Where on the Dee are the rocks from this period exposed? (pink = glacial, brown = sandstone, silver = limstone and coal, yellow = mudstone, orange = volcanic)
Phanerozoic Cenozoic Quaternary (Pleistocene/Holocene) - During this time, substantial glaciers advanced and retreated over much of North America and Europe, parts of South America and Asia, and all of Antarctica. The Great Lakes formed and giant mammals thrived in parts of North America and Eurasia not covered in ice. These mammals became extinct when the last glacial period ended about 11,700 years ago. Modern humans evolved about 190,000 years ago 2.588–0 The superficial boulder clay found along many parts of the River Dee is an ice age deposit. The ice-sheets deposited huge amounts of what is termed "drift": till (rock fragments in a clayey matrix), outwash sands and gravels (deposited by meltwater streams). Sub- and post-glacial erosion accentuated the pre-existing valleys such as the Dee. The present day topographical features of the Denbigh Moors, Clwydian Range, Vale of Clwyd and Wrexham were produced by glacial processes, where many typical glacial features such as cwms, drumlins, eskers, kettle holes and hummocky topography may be found.
Neogene (Miocene/Pliocene) - During this period, mammals and birds continued to evolve into roughly modern forms, while other groups of life remained relatively unchanged. Early hominids, the ancestors of humans, appeared in Africa. Some continental movement took place, the most significant event being the connection of North and South America at the Isthmus of Panama, late in the Pliocene. This cut off the warm ocean currents from the Pacific to the Atlantic ocean, leaving only the Gulf Stream to transfer heat to the Arctic Ocean. The global climate cooled considerably over the course of the Neogene 23.03–2.588 There are no rocks from this period along the Dee valley. By the end of the Neogene, the Dee was probably already flowing along much of its present course and as futher uplift occurred erosion continued. By the end of the Pliocene the Eurasian Plate continued to drift northwards from the Tropics towards its present location. The climate was cooling quickly and the conditions under which the deep tropical weathering could occur had come to an end.
Paleogene (Paleocene/Eocene/Oligocene) - mammals evolved from relatively small, simple forms into a large group of diverse animals in the wake of the Cretaceous–Paleogene extinction event that ended the preceding Cretaceous Period. Britain had a very warm climate in the Palaeogene but it gradually cooled as Britain drifted northwards. As a side effect of the Alpine Orogeny Britain was uplifted. The tropical climate promoted erosion. 66.0–23.03 Much of the course of the Dee may have been covered by Jurassic and Cretaceous deposits at the start of the Palaeogene, but these were lost through erosion.
Mesozoic Cretaceous - T. Rex and the really great dinosaurs flourish ("Jurassic Park" should have been called "Cretaceous Park") - then, at a time when the dinos had lots of problems anyway, along comes a major comet impact to wipe them out. 145.5–66.0 No geology from this period in the Dee valley, it has all been eroded away. The removed layer probably comprised a few hundred feet of chalk, rather like that which still survives at Dover. Samples of chalk (Upper Cretaceous) can occasionally be found in Irish Sea Ice glacial deposits in the Wrexham area and some areas of the Irish Sea are floored by chalk.
Jurassic - Dinosaurs doing really well, and give rise to the original "early bird": Archaeopteryx. The burial of algae and bacteria below the mud of the sea floor during this time resulted in the formation of North Sea oil and natural gas, much of it trapped in overlying sandstone by salt deposits formed as the sea levels fell to form swamps and salty lakes and lagoons. 201.3–145.0 Cheshire 30-40 degrees north of the equator and probably under water. No geology from this period in the Dee valley, it has all been eroded away.
Triassic - Early dinosaurs (Archosaurs) diversify at the expense of the Sysnapsids. 95% of the species existing in the Permian period are "missing" in the Triassic - there was a mass extinction between the two periods which eclipses later 'mass' extinctions such as that of the dinosaurs. Mineralisation during the Triassic was dominated in northeast Wales by calcite, together with the lead and zinc ores, galena and sphalerite, fluorite, quartz and a variety of scarcer minerals. 252.17–201.3 The remnants of the Variscan uplands in France to the south were eroded down, resulting in layers of the New Red Sandstone being deposited across central England, and in the faulted basin in Cheshire. The "Chester Pebble Beds" indicate that they were transported by prototypic river Dee: a large and powerful braided-river system, probably on the margin of an arid, desert mountain range. Cheshire was at the time between 20-30 degress north of the equator.
Paleozoic Permian - The world was dominated by a single supercontinent known as Pangaea, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Later, much of Great Britain was submerged in shallow waters as the polar ice sheets melted and the Tethys Ocean and Zechstein Sea formed, depositing shale, limestone, gravel, and marl, before finally receding to leave a flat desert with salt pans. The Permian Mass Extinction almost wiped all life from the earth. 298.9–252.17 The sandstone rocks around Erbistock were formed from rivers depositing mainly sand and gravel detrital material in channels to form river terrace deposits, with fine silt and clay from overbank floods forming flood-plain alluvium, and some bogs depositing peat and includes estuarine and coastal plain deposits. There is comparatively little Permian rock exposed at the surface in Britain, and most of the Cheshire basin is Triassic
Carboniferous - Cheshire at or near the equator with a similar latitude to the Amazon river basin of the present day. At first the Dee valley was under the equatorial Rheic Ocean leading to extensive limestone deposits. Later, the land uplifted and the Dee became a river delta system forming millstone grit. Later still, vast swathes of forest covered the land, which would eventually be laid down and become the coal beds characteristic of the Carboniferous system. The atmospheric content of oxygen reached their highest levels during the period, 35% compared with 21% today. This increased the atmospheric density by a third over today’s value. 358.9–298.9 Towards the lower end of the Vale of Langollen Dinantian limestone is followed by floodplain alluvium (millstone grit) and then the coal-measures of the Denbighshire Coalfield in the valley sides, indicating a transition from a carbonated ocean to a coal-forest swamp. These vast tropical rainforests collapsed suddenly as the climate changed from hot and humid to cool and arid. This was most likely caused by a combination of volcanism (possibly near modern Oslo), intense glaciation and a drop in sea levels of over a hundred metres. Though the exact speed and nature of the collapse is not clear, it is thought to have occurred relatively quickly in geologic terms, only a few thousand years at most.
Devonian - the "Age of Fishes" - the first bony fishes and tetrapods (four limbed animals) appear. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. The first tetrapods, evolving from lobe-finned fish, appeared in the coastal water no later than middle Devonian, and gave rise to the first Amphibians. 419.2–358.9 Below Llangollen there are no Devonian rocks, which implies that the Dee valley was above the sea at the time. This was due to the collision of Avalonia with Laurentia which crumpled up the Cairngorms, the Southern Uplands, the Lake District, Snowdonia, the Appalachians and the Irish mountains. The Welsh mountains at the time would have been huge, comparable with the Himalayas of today. However, the Caledonian mountains had largely been eroded away by the end of the period during which the country would have experienced an arid desert climate as it was located close to (but not at) the equator at between 10° and 15° south.
Silurian - jawless fish become widespread and bony "modern" fish appear. Life began to appear on land in the form of small, moss-like, vascular plants that grew beside lakes, streams, and coastlines, and also in the form of small terrestrial arthropods. However, terrestrial life would not greatly diversify and affect the landscape until the Devonian. The Silurian system was first identified by British geologist Sir Roderick Impey Murchison, who was examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s. He named the sequences for a Celtic tribe of Wales, the Silures. 443.4–419.2 From Corwen to the bottom of the Vale of Llangollen the bottom of the Dee valley is Silurian rock deposited under estuarine conditions. Cheshire (as part of Avalonia) has drifted northwards and was then about 20 degs south of the equator, but just about to collide with Laurentia (North America). When the proto-Europe collided with North America, the collision folded coastal sediments that had been accumulating since the Cambrian off the east coast of North America and the west coast of Europe. This event was the Caledonian orogeny, a spate of mountain building that stretched from New York State through conjoined Europe and Greenland to Norway. Welsh slate was created by the impact, as is that in Spain and Belgium, by the effects of heat and pressure on the mudstone.
Ordovician - first fish evolve, they are armoured and jawless like a cross between a dogfish and a lobster. The Ordovician came to a close in a series of extinction events that, taken together, comprise the second largest extinction event in Earth's history in terms of percentage of genera that went extinct. The C02 in the atmosphere was at fifteen times recent pre-industrial levels, while oxygen was only at two-thirds of the present levels. At this CO2 concentration, International Space Station crew experienced headaches, lethargy, mental slowness, emotional irritation, and sleep disruption. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods, respectively. 485.4–443.4 The River Dee rises on Dduallt, which is volcanic rock ("Aran Fawddwy Formation" - Felsic Tuff formed from compacted volcanic ash) from this period. By this time Cheshire (as part of Avalonia) had drifted to about 70 degs south and the Iapetus Ocean was narrowing, pushing crustal plates down beneath the Welsh Basin and creating a line of volcanos including Rhobell Fawr. The young river flows over the volcanic Tuff and sedimentary "Nant Ffrancon" subgroup Siltstone, with the occasional patch of Microgabbro and crosses onto Silurian mudstones near Corwen. The Ordovician sedimentary rocks were deposited under a succession of mostly deep and sometimes shallow seas.
Cambrian - while diverse life forms prospered in the oceans, the land was comparatively barren – with little more complex than a microbial soil crust. 541.0–485.4 Rocks of the "Harlech Dome" laid down. These are some of the oldest rocks in the UK.
Proterozoic Neoproterozoic Ediacaran - global ice-age ends. Ediacara biota bear little resemblance to modern lifeforms. 635.0–541.0 Cheshire is on a continental plate somewhere around the South Pole and drifting north. No rocks from this time survive along the course of the Dee.
Cryogenian - the greatest ice ages known to have occurred on Earth, occurred during this period. There is much debate whether these glaciations covered the entire planet (the so-called 'Snowball Earth') or if a band of open sea survived near the equator (termed 'slushball Earth'). 850–635 No rocks from this time survive along the course of the Dee.
Tonian - rifting leading to the breakup of supercontinent "Rodinia". 1000–850 No rocks from this time survive along the course of the Dee.
Mesoproterozoic Stenian - The supercontinent Rodinia assembled. 1200–1000 No rocks from this time survive along the course of the Dee.
Ectasian - The first taxonomically resolved eukaryote (Bangiomorpha pubescens - a type of red algae, invents sex) 1400–1200 No rocks from this time survive along the course of the Dee.
Calymmian - The supercontinent Columbia broke up. 1600–1400 No rocks from this time survive along the course of the Dee.
Paleoproterozoic Statherian - By the beginning of the Statherian, the supercontinent Columbia had assembled. 1800–1600 No rocks from this time survive along the course of the Dee.
Orosirian - Two of the largest known impact events on Earth occurred during the Orosirian. At the very beginning of the period, 2023 Mya, a large asteroid collision created the Vredefort impact structure. The event that created the Sudbury Basin structure occurred near the end of the period, 1850 Mya. 2050–1800 No rocks from this time survive along the course of the Dee.
Rhyacian - The first known eukaryotes began to evolve. 2300–2050 No rocks from this time survive along the course of the Dee.
Siderian - Banded iron formations were formed as anaerobic algae produced waste oxygen that combined with iron, forming magnetite (Fe3O4, an iron oxide). This process cleared iron from the oceans, presumably turning greenish seas clear. Eventually, without an oxygen sink in the oceans, the process created the oxygen-rich atmosphere of today. This event is known as the oxygen catastrophe 2500–2300 No rocks from this time survive along the course of the Dee.

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