Dartmoor is an area of raised moorland formed by the
intrusion of granite, an igneous rock from the the
earth's mantle. The semi-molten rock forced its way
through the surrounding country rock; later mineral
fluids emanating from the mantle flowed through the
fissures in the granite. These then cooled and
crystallised forming mineral veins termed lodes.
Generally, the lodes are vertical or near vertical.
In this part of the Dartmoor environs the lodes contain iron oxide, in the form of micaceous haematite, with
smaller quantities of red oxide, iron sulphide or Pyrite
(Fool's Gold) and tin oxide or Cassiterite (tin ore). The lodes also
contain gangue (unwanted) minerals such as quartz. The
micaceous haematite, locally known as 'Shiny Ore' due to
its splendent lustre, occurs in the form of minute,
greyish-purple mica-like flakes.
A number of mines successfully worked these deposits, the
largest of which was the Great Rock Mine at Hennock.
Great Rock was run during this century by the Ferrubron
Manufacturing Company, who also acquired several other
mines in the area at the same tirne. All except Grear Rock and Kelly Mine
were immediately closed, presumably to ensure a monopoly
on the production. Later, when British Standards were
imposed on the paints used by the Royal Navy and the
railways, the material produced by Ferrubron was found to reach the
specification without further processing. Kelly Mine
closed in 1951 and Great Rock Mine closed in 1969 bringing the
production of 'Shiny Ore' in Britain to an end.
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