Tresayes Quarry - Roche Glass Mine
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Tresayes Quarry is a little-known geological nature reserve
hidden in Roche. Many thanks to Sean O’Hea of Cornwall
Wildlife Trust for granting permission to set up this
Earthcache.
Access
To access this Earthcache, take the B3274 southwards out of Roche.
Turn down Prosper Road by the old chapel. At N50°23.775
W004°49.353, the tarmac road will finish. There is space to leave a
car here – Please do not block access to any of the tracks at
this point. Walk up the track to the right of Prosper Pit and turn
left into the reserve just before the gates at the house. The
tracks can be slightly overgrown and uneven. There is a small
section of boardwalk leading to the rock face and interpretation
boards and this can be very wet and muddy – even in the
summer, long trousers and wellies will be the best option!
Geology
Around 270-295 million years ago, a huge body of granitic rock
(called a batholith) was being formed a few kilometres beneath much
of southwest England. Millions of years of weathering and other
earth processes have now brought lots of that rock to the surface
allowing geologists to study it in more detail. We now recognise
that granites in southwest England are not all uniform in
appearance, composition, texture or crystal size. The type of
granite that can be seen in this old quarry is called a pegmatite.
A pegmatite is a special type of granite with very large crystals.
The pegmatite here lies just outside of the now exposed main body
of granite, having been squeezed into the hot country rock (known
in Cornwall as killas), that lay above the granite batholith.
There, the magma cooled very slowly forming the huge crystals that
we can see today. The continental crust material that had been
melted to form the magma was very rich in silica (SiO2). Whilst
some of this silica cooled and crystallised to form quartz, some
joined with other elements (including the metals aluminium,
potassium, sodium, iron and magnesium along with water, boron and
fluorine) to form other minerals such as feldspars, micas,
tourmaline and cordierite. Though not rare, pegmatites are
geologically important as they often yield gemstones and are
enriched in rare earth elements.
History Feldspars of the best quality (grade 1) were
produced at Tresayes and other nearby pits at Kernick and Trelavour
– both sadly now consumed by china clay extraction and buried
beneath the china clay waste. Consumers demanded that the mined
feldspar was clean and free from any impurities. It had to be
separated from the unwanted minerals and broken into smaller
pieces. Although men mined and quarried the material from the
ground, the subsequent refining was done by women, in common with
many other mines at the time. These 'bal maidens’ broke the
mined blocks by hand to isolate the feldspar from the waste
materials. Once done, the final product was then transported
up-country and to the continent, via the port of Fowey, to be used
for glass making, as a pottery glaze and in ceramics. As a
consequence, this site was known as the Roche Glass Mine. The works
opened in the late 1870s and in August 1879, people were invited to
buy shares in a 'bona fide and safe investment' some six months
before it was put up for auction! The exact reason why quarrying
ended is uncertain, but an article in the Mining Journal records
that production was hampered by severe winters on this exposed
common – Perhaps the ‘bal maidens’ felt the cold!
Feldspar quarried in Sweden and Norway required less processing, so
was considered better quality. The quarry reopened briefly in 1917,
during the First World War, to provide feldspar for electrical
porcelain.
In order to claim this Earthcache you need to visit the site and
do a little research on pegmatites to find the answers to the
following questions and email them to me via my profile:
1)Study the crystal textures exposed in the rock
face…Try to work out which crystals grew first in the
pegmatite and how long they are.
2)What rare earth element used in
mobile phones can be found in tiny crystals of mineral at this
quarry?
3)Please suggest which mechanism (from metamorphic, magmatic and
metasomatic) best explains how the Tresayes Pegmatite was formed.
Bear in mind, it outcrops in the aureole zone around a large
granite intrusion!
In keeping with the recent changes to Earthcache rules, photographs
are now optional and I cannot require these to be posted. A
photograph says a thousand words though, so I hope you will choose
to post one to the cache page anyway!
Thanks for taking the time to visit this Earthcache and I hope you
enjoy your visit.
If you enjoy the geology here, you may wish to visit Roche Rock
nearby – there is even a cache there (GC77E4 –
Hermit’s Hideaway)!
Additional Hints
(Decrypt)
Svaq gur vagrecergngvba obneqf - gurer ner gjb bs gurz!