Friday, 21 June 2013

tour de force

The Lake District has seen extensive mining activity since the mid-sixteenth century, when German miners were hired during the reign of Elizabeth I (1558–1603) to plug a skills gap in the local population. During the subsequent centuries, hundreds of mines were opened across the district, extracting mainly ores of copper, lead, zinc and barium. However, The industry was in steep decline by the start of the twentieth century, and the last mine to operate, Force Crag Mine in Coledale, closed in 1991 following a major roof collapse. Only two other mines survived into the second half of the century: Greenside Mine above Glenridding, where millions of tons of lead and significant quantities of silver were extracted; and Carrock Mine in Mosedale, which was reopened for only a few years to meet demand for tungsten.

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:

An ideal location for a quiet afternoon stroll: the road to Force Crag Mine.

Approaching the mine: the crushing mill, built in the late 1930s, is the most obvious feature. Force Crag itself is the dark area in the background; it is impossible to make out any details on the face when it is in shadow.

Looking back down the valley: the distinctive U-shaped cross-section is indicative of glaciation during the last Ice Age.

A representative of the local wildlife: this large, fat caterpillar (about 3.5cm long) was crossing the track in front of me. Any help in identifying the species would be appreciated. Food must be sparse around these parts, so my guess, based on its size, is that it was looking for somewhere to pupate.

Coledale Beck: it looks harmless, but try crossing it after a few days of heavy rain, which is the default condition in the Lake District. You may die in the attempt. Note the gravel deposits on the inside of the bends, where the current is always weaker than on the outside, where most of the erosion occurs.

4 comments:

  1. This is the kind of place my wife and I love to explore. The beautiful scenery would be an added bonus! Great info about the science of it all. Thanks...

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    1. Before making up your mind Pat, you should know that the Lake District is almost as wet as your local deserts are dry, although that doesn't seem to deter many visitors. It’s also good to know that my time at university studying geology wasn’t entirely wasted!

      By the way, you’ve just reminded me that I had intended to comment on your latest post, also about mining.

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  2. I can't even remember the date of the last time I went to the Lake District but I DO remember that it rained, and rained and rained again. It is a beautiful place for sure but a little too wet for me. I don't like the feeling of being drenched all day long.

    Lovely photos of all that greenery but wouldn't want to get stuck on those long winding roads though. It looks so serene and isolated there.

    Dennis, how's your knee doing now? Did the walk aggravate it at all?

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    Replies
    1. The wettest place in England Rum! But we locals are used to it.

      This was a very gentle walk, mainly because I haven’t done much walking since I hurt my knee, but it held out well. I don’t think I’ll be straying onto rough ground though. If you saw me walking now, you wouldn’t notice anything wrong, but I’m aware that the knee isn’t quite right. Thank you for asking.

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