Natural History of Upper Weardale
3 - BEDROCK GEOLOGY
By Brian Young: the Editor’s synopsis
Fluorite: White’s Level, Westgate © The Trustees of the Natural History Museum, London.
Most of the Dale’s rocks visible today and which have been critical in shaping its physical, industrial and social character, and even the style of its buildings, formed between around 360 – 295 Ma (Million years ago) during the Carboniferous period of earth history. By then relentless movement of the earth’s tectonic plates had brought our area to lie almost astride the equator.
During early Carboniferous times much of what is now Northern England lay beneath a warm tropical sea with mountainous land to the north over what is now the Scottish Highlands and parts of Scandinavia.
Within these waters, abundant marine organisms accumulated to form thick beds of limestone in which fossils of corals, shells and other creatures are locally common. Huge rivers draining the northern mountains carried vast amounts of mud, silt and sand into this sea creating extensive river deltas and low-lying swamps upon which lush forests of gigantic trees and other vegetation developed.
We see these deposits today as beds of sandstone, shale and coal seams. For reasons that are still not fully understood, but are almost certainly related to a complex interplay of crustal instability and climatic fluctuations, over millions of years there was a repetition of submergence beneath the sea and encroachment of these enormous deltas. This resulted in the formation of regularly repeated individual sequences of rocks, known as ‘cyclothems’, each consisting of - in idealised upward succession - limestone, shale, sandstone and coal, though in detail many cyclothems are more complex.
The Dale’s early miners and quarrymen named individual beds from their distinctive features. Thus we have, e.g. the Tynebottom, Scar, Three Yard, Five Yard and Four Fathom Limestones and thickest in the Dale, the Great Limestone.
Simplified geological section from Copthill to Rookhope, illustrating the relationship of the main rock units. Note that the form of the Burtreeford Disturbance and the relationship of the Great and Little Whin sills are conjectural. Diagram created by M Byron, based on original images by B Young and modified from British Geological Survey material.
The latter includes the very distinctive bed known as the ‘Frosterley Marble‘; not a true marble but a black limestone crowded with striking white fossils of the coral Dibunophyllum. Long valued as an ornamental stone, fine examples can be seen in local churches and in Durham Cathedral.
Whereas most of the Dale’s limestones have been worked for making lime, by far the most important was the Great Limestone, which is the rock exposed in the large quarries around Frosterley, Stanhope and the former cement works, and which is still worked today at Heights Quarry.
Around 295 Ma molten rock from within the earth’s mantle was forced between beds of the Carboniferous rocks, cooling to form the roughly horizontal body of rock known as the Whin Sill. Although conspicuous in Teesdale and the Roman Wall country, this lies deep beneath Weardale but was reached in several mines: two small offshoots from it may be seen near Cowshill and Stanhope.
Soon after the formation of the Whin Sill, warm mineral-rich ground-waters began circulating in cracks and faults deep beneath the surface. As they cooled they deposited layers of minerals creating the veins that were the basis for the dale’s long history of mining. Ores of lead, iron and zinc are accompanied by much larger amounts of ‘spar’ minerals, dominated here by quartz, calcite, ankerite and fluorite, many of which were found as spectacularly beautiful crystals. Weardale is internationally famous as the source of some of the world’s finest examples of fluorite, to be seen in museums across the world.Download Chapter in PDF