River Dee Geology

The River Dee (Welsh: Afon Dyfrdwy) flows 110 miles from its 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 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 its use as a trade route in pre-Roman times. A path of myth which places the young Arthur at its source. And an industrial tale bringing gold, stone, wool and water downstream as the railways and canals crept ever upstream towards its 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.



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 centre of the Earth which released enough gravitational 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, all of which have been mined along the Dee.



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. 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.

A geological period is one of several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisions into which geologists have split the Earth's history. The Dee rises on rocks from the "Ordovician" period. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.



Following the course of the Dee takes us through almost all of the geological periods of the early part of earth's history.

Cambrian
The Cambrian Period was the first geological period of the Paleozoic Era (it means "ancient life") which spans the period from 540 million years ago to 250 million years ago. There are no Cambrian rocks along the River Dee, but geology of this period has influenced the course of the river. The Dee rocks are from the Ordovician, Silurian, Carboniferous and Permian periods. These are truly ancient periods, before the age of the dinosaurs.

The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago (mya) to the beginning of the Ordovician Period 485.4 mya. The period was established (as the “Cambrian series”) by Adam Sedgwick, who named it after Cambria, the Latinised form of Cymru, the Welsh name for Wales, where Britain's Cambrian rocks are best exposed. Complex, multicellular life gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized — hence readily fossilized — life became common. Creatures like algae evolved, but the most ubiquitous of that period were the armored arthropods, like trilobites. Almost all marine phyla evolved in this period. During this time, the supercontinent Pannotia begins to break up, most of which later became the supercontinent Gondwana.

Nowadays, the Cambrian rocks are found near Harlech and pre-Cambrian rocks in Angelsey, where some of the oldest rocks in the UK are exposed at Newborough. These very distinctive rocks formed a result of underwater “smokers” spewing out lava during periods of eruption. The seawater proved very effective at cooling the liquid lava, which quickly formed lobes, or “pillows”, on the sea floor. As the eruptions continued the pillows would overlay each other, creating the structures seen today. The rocks of the Harlech region consist of conglomerate, sandstone, and mudstone (now slate), which have been compressed into large folds, raising them into a huge dome, known as the "Harlech Dome". The Rhinog Mountains were initially deposited as marine sediments around the margins of a deep sea-filled basin that lay across central Wales in Cambrian times. As they accumulated around the basin margins these sediments were periodically shaken by earthquakes, or simply collapsed under their own weight, and slumped or flowed down the submarine slopes into the deeper parts of the basin, forming plumes of suspended sediment known as ‘turbidity currents’. These sediments eventually settled out in the deeper parts of the basin to form thick layers and fan shaped wedges of often poorly sorted sandstones called ‘turbidites’. Gradually, through later Cambrian, Ordovician and Silurian times this deep marine basin became filled with sediments, which were later compressed by the mid-Devonian Caledonian Orogeny to form the Harlech Dome. This structure was later unroofed and exposed to erosion by further uplifts associated with the opening of the adjacent Atlantic Ocean, Irish Sea and Cardigan Bay basins, so that the thick beds of resistant grits and sandstones now form the spectacular terraced escarpments of the Rhinog Mountains.

Ordovician
What is now Snowdonia was a volcanic region during the Ordovician which had at least one "Krakatoa" type eruption. The River Dee rises on other Ordovician (but earlier) 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 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 systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, and placed them in a system of their own.

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.

Silurian
With the supercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe. The high sea levels of the Silurian and the relatively flat land (with few significant mountain belts) resulted in a number of island chains, and thus a rich diversity of environmental settings.

During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late-Ordovician glaciation. The southern continents remained united during this period. The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity. The continents of Avalonia, Baltica, and Laurentia drifted together near the equator, starting the formation of a second supercontinent known as Euramerica.

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 is the Caledonian orogeny, a spate of mountain building that stretched from New York State through conjoined Europe and Greenland to Norway. At the end of the Silurian, sea levels dropped again and the new mountain ranges were rapidly eroded.

Carboniferous
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.

Permian
The Dee 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".



The Sandstone


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.


 * Kinnerton Sandstone Formation: from 0m to over 150m thickness of largely aeolian (wind/dune formed) sandstones of early Triassic age.

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.

sources and links

 * Cheshire Trove on the Geology of the area;
 * Environmental Change and Mineral Formation in Wales;
 * Mineral Resource Maps of Wales;
 * Bala Geology;
 * Brenig Way Geology;