The geology of an area is usually considered as two structures, the bedrock (“Solid”) and the surface features (“Drift”).
The bedrock of the Land of Oak and Iron is formed by “Coal Measures” rocks laid down during the Carboniferous Period, 360 to 300 million years ago, when this area was covered by tropical forests. It is these rocks that have formed the landscape that provided the raw materials for the early coal and iron industries.
The “solid” geology is mainly seen where the River Derwent has removed the “drift”, at Shotley Bridge and at Chopwell, and where the rocks have been quarried.
Fossils remains from this period are scarce, mostly leaves of trees and sections of the bark of trees, still to be found in stream and riverbeds and in disused quarries. Some better examples, from coal mines, are in the Hancock Museum.
Typically, the rocks were laid down as layers of sandstone, coal and shale, as local climate conditions changed. Some of the older sandstones around Shotley Bridge are quite hard, and were once used to make millstones for grinding corn. Some of these were quarried out of the riverbed just upstream of the bridge. The River Derwent has exposed the bedrock at Shotley and Chopwell Crags. Nearby at Consett, a type of rock called “ironstone”, found in the shale layers, had a high percentage of iron ore, and, though local supplies were rapidly exhausted, it was this stone that started the early iron works. Other sandstones have been quarried for building stone, the stone being used for example, at Gibside, including the Statue to Liberty, and at Friarside Chapel.
Much more importantly, the coal has been mined and has fuelled local industries, as well as being shipped down the coast to London. This trade provided vast wealth for some of the local landowners, who created the houses and estates at Hamsterley Hall, Axwell Park and Gibside. Crowley’s Ironworks at Winlaton Mill and later the Derwenthaugh Cokeworks became major sources of local employment.
Another legacy of the mining of coal is the network of former railways and waggon ways used to carry the coal down to the River Tyne. The Derwent Walk and several other coal ways are now in use as countryside walks and cycle paths.
“Drift”, surface geological features
Almost everywhere in the Land of Oak and Iron the bedrock is concealed by layers of material deposited during and at the end of the ice ages. There are large deposits of glacial sand and gravel in the Ryton/Greenside/Crawcrook area and at Broad Oak, north of Ebchester. These have been quarried for building materials for many years, with the larger quarries later used for landfill.
In the areas that have not been quarried, parts of the landscape are formed by glacial material left behind by the ice, including conical hills called drumlins and linear features called eskers. These can best be seen in the Beda Hills area, near High Spen.
Post Glacial Features
The most curious and special feature of the post-glacial geology of the Land of Oak and Iron are the “Tufa” formations. While a few of these are known in other parts of the country, the Land of Oak and Iron has a unique cluster of sites where tufa dams form in streams.
Micro-organisms in the streams extract calcium from the water and deposit it on the bed of stream, not uniformly, but as a series of dams and pools over a length of a hundred metres or so. Each dam is 20 to 60 centimetres in height, with a pool of similar depth and two to three metres long. The dams are sometimes fragile, and can be damaged by floods, but the micro-organisms quickly repair them, incorporating tree roots, rocks and leaves into the structure.
The dams have the effect of oxygenating the water and the pools provide habitats for many invertebrate animals. The tufa formations are, on a small scale, analogous to the biodiversity enhancing effect of coral reefs or beaver dams.
The origins of the tufa dams are still a bit of a mystery.