River Dee Geology

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 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.



River Dee geology through time


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,[13] 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, a debtor's prison. 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 River Dee rises on Ordovician volcanic rocks which formed when North Wales was a sea dotted with volcanic isalands 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 Silurian rocks laid down as sediments in an ancient ocean, 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".



The full sequence of the Cheshire basin rock system is:


 * Helsby Sandstone Formation: around a 250m thickness of 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;


 * 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;


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

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;