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Geology of Chester
In 1831 a young Charles Darwin spent a week tramping around North Wales studying geology. With the benefit of hindsight the geology of North Wales and the Dee valley is quite straightforward, with older rocks at the head of the Dee and younger ones following in sequence down the river. In Chester, and indeed most of Cheshire, the rocks are red sandstones of varying colour and hardness. This abundance of visible geology led Charles Kingsley, while a canon at Chester cathedral, to give a series of lectures for the Chester Society for Natural Science, Literature and Art (he was a founder member). In 1872 he produced a book: “Town Geology” about what can be seen of geology without having to tramp round Wales like Darwin. Darwin got home ftom his travels to find a letter inviting him to join HMS Beagle, and the society which Kingsley helped found evolved into the Grosvenor Museum.
Five Billion Years In One Day
If the geological history of the earth were compressed into one day the first few hours would be terrifying as it would be raining rocks the size of France. After about four in the morning of this notional day things improve a little, but there is not much to see. Any life (which emerged quite soon) is microscopic, and it stays that way until about ten in the evening. Then suddenly there is an explosion of diverse life forms, almost as if life decided it was time to experiment. This was the “Cambrian” geologic age, named after Wales, where rocks of this period can be found. However the diverse forms which appeared were almost all confined to the sea. The land had little other than microbes and perhaps towards the end of the period something like a mollusc which fed upon the microbes. The lower boundary of the Cambrian was originally held to represent the first appearance of complex life, represented by Trilobites. The recognition of small shelly fossils before the first trilobites, and of Ediacaran biota] substantially earlier, led to calls for a more precisely defined base to the Cambrian period. Eventually it was decided that this should be set at the first appearance of what may have been a burrowing worm which lived in the sea bottom.
The next age is the “Ordovician” (named after a Welsh tribe) and it is on Ordovician rocks that the River Dee rises. 450 million years ago Hoole and Upton were part of a string of volcanic islands as far south as the Falkland isles are today. Slowly this “micro-continent” drifted northwards at about the speed at which fingernails grow. Geologists have named it “Avalonia”. Remains of the volcanic activity can be found around Dduallt and Rhobell Fawr. The Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya (million years ago). Most boundaries between geological periods are marked by an extinction of some kind - as it is generally the fossil record, and changes in it, which determines the start end points of a geological period. The Ordovician came to a close in a series of extinction events that, taken together, comprise the second largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct - about 49% of genera of fauna disappeared forever. The most commonly accepted theory is that these events were triggered by the onset of cold conditions, followed by an ice age, that ended the long, stable greenhouse conditions typical of the Ordovician. The late Ordovician glaciation event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm). The dip was probably triggered by a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode.
Below Bala the river Dee crosses onto “Silurian” rocks (named after another Welsh tribe). These rocks are about 430 million years old. At the time these rocks were forming all that lived on land were some primitive plants and something like millipedes and spiders. Avalonia was still moving northward and about to enter a warmer climatic zone, comparable to that of the southern hemisphere deserts. The type of rocks being formed changed to red desert sandstone – the “old red sandstone” of which none is exposed along the river Dee. Around this time Avalonia collided with another continent - todays north America - the rocks of which today comprise Scotland and Ireland. The rest of Europe joined in the collision, pushing up the Scandinavian and Caledonian mountains, like a lasagne squashed from the ends. Heat and pressure baked some rocks into slate.
The drift of the continents northwards continued. Limestone was laid down by shallow equatorial seas and later coal, giving this period the name “Carboniferous”. Today these rocks can be found along the Dee from Langollen to Ruabon. Further north still the megacontinent drifted into another desert zone – comparable to the Sahara and once again the rocks were formed as sandstone – the familiar “new red sandstone” of Cheshire. Rivers like the modern Nile crossed this desert of 250 million years ago and both wind and flash floods shaped layers of sandstone into characteristic “wavy” patterns.
There are no younger rocks along the Dee than these, which date from the time just before the dinosaurs (about an hour before midnight on the geological “day”). Eventually the huge continent split apart almost but not quite along the earlier join. Parts of Avalonia stuck to North America and parts separated with Europe.
Ice ages also influenced the shape of the land. The “Backford Gap”, north of the zoo, was cut by meltwater. The ice also cut channels through the Sandstone Ridge and helped to form the meres of Delamere. These ice-cut features influenced the routes of roads, canals and railways. The low ridge that extends from Beeston Castle down the Wirral is the home to many communities safe from floods and marks a route which would have been used by marching armies – as it crosses fewest streams. The Romans built a string of marching camps along what is now Long Lane – possibly even pre-dating their fortress at Chester – on what is the highest part of the ridge locally. They located their fortress on a sandstone bluff above the river Dee with convenient springs as a Water Supply and stone to build the fortress walls of Roman Chester.
After the Romans departed the local geology continued to influence Chester. Aethelflaed, daughter of Alfred the Great, refortified the city as part of a string of defendable towns across the gap between the hard gritstones of the Welsh hills and the Pennines. This protected her flank while she and her brother drove the Danish invaders out of the east of Britain.
Geology has influenced Chester's history in many other ways. In 1658 when Chester butcher Owen Jones died he left a piece of “almost worthless” land to the Corporation with the provision that the rents should be given, each year, to the poor of each guild in the order they walked in the Midsummer procession. The land later turned out to be a metaphorical gold mine – or rather a lead mine. The small bequest became a bonanza. In 1798 the distribution of the funds from the charity to the "Poor Bretheren" was £19.50 per head: at least £20,000 in present money. The "Poor Bretheren" included the then Mayor of Chester (Thomas Barnes Esq), his son, and possibly at least one other family member. The only beneficiary who did not get £19.50 was "Widow Dawson", who got £1.00. This handout led to corruption, and became one reason which guild membership was restricted. Lead and the canal also attracted the Leadworks to Chester, a mixed blessing. So much toxic lead was released into the environment that children living near the leadworks in 1980 had some of the highest blood levels of lead in the country.
From almost its beginning the leadworks made paint. The ease with which white lead paint could be obtained may be one reason for the popularity of black and white architecture in Chester. The local red sandstone was also used for building with the rarer, harder-wearing yellow sandstone brought in for wealthier clients (such as banks). In Ruabon the local geology produced the characteristic dense red bricks and terracotta used in many Chester buildings by John Douglas and others. Slate, formed by ancient continental collisions, could be brought in from Wales.
Charles Kingsley lived in cobbled Abbey Square. His “Town Geology” is very dated and quite “politically incorrect”, but his basic premise is still true. Just looking at the older buildings around us can tell us much about the local geology.
A geological time chart
|Eon||Era||Period - and what happened (green = land, blue = sea, aquamarine = mixed)||Extent, Million
|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.|
Sources and Links
- “Town Geology” as a free e-book;
- Surface Geology;
- The Flood Maps;
- Geology Viewer;
- 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;
- Holt Geology;
- Roman remains in Hoole - marching camps built for practice;
- Coal, Iron and Lidar;