|Introduction | History
of lead mining | History
of zinc mining | Lead
ore & mines
Zinc ore & mines | Iron
ore, ochre & mines | Coal mining
ore and ochre mines
of Iron ore
Iron ore is most commonly found as haematite (Fe2O3),
pyrite (FeS2) and goethite (FeO.OH), or as a mixture of
hydrated iron oxides known as limonite. In the Mendips, haematite
and goethite commonly occur together as either massive, granular,
often siliceous masses, or as altered, amorphous, loosely compacted,
earthy varieties known as ochre. Two type of ochre occur: red ochre,
which is mostly impure earthy haematite; and yellow ochre which is
earthy goethite, clay and other iron hydroxides. Ochre can take a
variety of forms depending on its mode of deposition. It often occurs
as encrusting, botryoidal or stalatitic forms.
Manganese is a common mineral associated with iron and ochre deposits,
and is typically found as dull black, soft oxides known as ‘wad’,
a mixture chiefly of pyrolusite (MnO2) and psilomelane,
another manganese oxide.
Stalactitic yellow ochre, which once infilled a pre-existing cavity,
Shute Shelve, Axbridge.
Unlike the lead and zinc ores, many of the iron ore deposits are
secondary deposits. Intense weathering of the iron pyrite-rich
Coal Measures, and other iron bearing rocks during Permian and
Triassic times released the iron into the groundwater. The iron
was subsequently redeposited as many thin discontinuous veins of
haematite or pyrite, within the Carboniferous Limestone and the
Dolomitic Conglomerate, and especially along the unconformity between
the two. Many of these pyrite veins have now been altered to form
limonite or ochre. Ochre also occurs infilling cavities in the
Carboniferous Limestone and Dolomitic Conglomerate, or as a replacement
ore-body, where metal-rich ground-waters have chemically replaced
the host rock with iron ore.
Following deposition, solutional lowering of the limestone has caused
the iron-rich ore to accumulate in the subsoil. Some of this material
has then sludged into caves and open fissures. Many of the ochre
deposits are actually cave fillings, and splendid examples of these
can be seen above Shute Shelve on Axbridge Hill.
Haematite has been mined here and there throughout the Mendips, but
the lower grade ochre ores are especially common in the hills surrounding
Winscombe and the Lox Yeo Valley. Ochre was being mined on Axbridge
Hill in the 1920s, and old workings exist on Banwell Hill
and on the northern side of Sandford Hill. Most of the ore was
mined from small shallow pits and adits. A large ochre mine, now
in a state of collapse was worked at Compton Martin. Various types
of iron ore, and some rare oxychlorides of lead, were worked from
a mine at Higher Pitts Farm near Wookey Hole.
Small amounts of manganese was worked for paint making in the Wadbury
valley near Mells, around Harptree and elsewhere.