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.

Consequences
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.

Related Pages



 * River Dee;
 * River Dee Geology;
 * Sandstone Ridge;
 * Roman Chester;
 * Dark Ages;
 * City Walls;
 * Civil War;

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;