With the singular exception of the wadd (graphite) mine at the end of Borrowdale, the ore bodies that were exploited formed in broadly similar circumstances. The district is underlain, at great depth, by granite, which consists overwhelmingly of silicate minerals, with the presence of quartz indicating a surfeit of silica in the original magma. Crystals of these minerals have a basic skeleton in which the silicon and oxygen form a chain, a sheet or a three-dimensional lattice, and there is room in the structure for other atoms, notably those of aluminium, magnesium, calcium, iron, sodium and potassium.
However, as the various magmas cooled and silicate crystals began to form, the fraction that remained liquid became an increasingly concentrated mix of ions that were too big to fit into the available vacancies in the silicate structures. These included commercially valuable metals such as copper, lead and zinc. Although tin is frequently associated with granite intrusions (cf. the Cornish tin-mining industry), no significant deposits occurred in the Lake District.
While the granite was cooling and solidifying, the overlying rocks were being squeezed, twisted and fractured by the forces associated with plate tectonics, so large numbers of faults and fissures opened up. Under pressure from below, the concentrated soup of large metal ions then migrated upwards through the cracks until it could rise no further. The commonest nonmetal in such a soup is sulphur, so the most common minerals to be found are sulphides and sulphates.
Having provided the technical background, I can now describe a visit yesterday to Force Crag Mine. I’d been there only once, in 1971, to collect mineral specimens, but I thought that it would be a good test for my knee, because I haven’t done much walking during the past 18 months. The mine lies at the head of Coledale, a glaciated valley that runs southwest into the mountains from the foot of Whinlatter Pass. It is slightly less than 4km from the road, to which it is connected by a well-maintained dirt track.
The main minerals extracted here were galena (lead sulphide), sphalerite (zinc sulphide) and barytes (barium sulphate). Galena, together with the small amount of silver that is often associated with this mineral, was the first ore to be extracted, in the first half of the nineteenth century; both sphalerite and barytes were discovered later, but they became increasingly important in later years as the output of galena declined. The mine is now officially abandoned.
The following photographs illustrate what is a very pleasant afternoon walk: