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The
majority of Penwith is comprised of a large mass
of granite. This hard rock gives this
part of Cornwall its characteristic scenery of
steep cliffs and rocky outcrops. Most of the areas not covered by the granite have still
been affected by it. The heat of the molten
granite changing the surrounding geology into to hard metamorphic
types.
This
granite based geology has led to this area being
comprised of rocks and minerals that were
commercially valuable. They have been extensively
worked, forming the cultural identity of
the region.
At
Pool only 14 miles from Penzance, is the Camborne
School of Mines Geological Museum. This has a
fantastic display of rocks and minerals. This
museum has a good educational slant. |
Granite
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Granite
is a beautiful building material, which until
recently was heavily employed in construction. Granite is a coarse
to medium grained rock. In Penzance its use can
seen everywhere in buildings and in the older
pavements of Chapel Street and the raised causeway
of Market Jew Street.
Granite
is not an homogenous rock, its composition varies
depending on its location. The primary components
of granite are feldspar and quartz; and to a lesser
degree biotite and muscovite. Depending on the
proportion of these minerals there is a wide range of
composition, colour and crystal size.
Other less common components gives the granites a wide
variation in looks and colour, as seen
below. |
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Granite
is formed deep under ground, but over millions of
years, the rocks laying on top of it have eroded
away, so now it lays on the surface.
The
crystal size of granite varies, the size depending
on the time the granite took to cool. The larger the
crystals the longer it took to
cool. |
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Xenolith in
Granite |
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Large
Feldspar
Crystals In Granite at Praa Sands
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composition of granite gives it a distinctive
appearance. The proportion of the minerals that
make up the granite varies from location to
location. The main constituents of granite are,
feldspar, quartz, biotite and muscovite. Also
commonly found are pieces of other rock that fell
into the granite before it cooled. These are
xenoliths.
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Orthoclase |
KAlSi3O8 |
Good examples of these crystals can
seen in some granites. They can be the
largest crystals by far. |
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Quartz |
SiO2 |
A very common mineral,
the
main constituent of
granite. |
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Biotite |
K(Mg,Fe)3AlSi3O
10(OH,F)2 |
A black to brown mica, found in nearly
all granites. |
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Muscovite |
KAl2(AlSi3O10)(OH,F)
2 |
A lighter coloured mica found in
granite. |
Landscape Features of
Granite
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When first formed granite is deep
underground and under immense
compaction by the weight of the
overlying rocks. When the overlying
rocks are eroded away this weight is
removed and the granite slightly
expands and cracks to form
joints.
The weathering of the granite is
escalated at the joints, creating
distinctive landscape features such as
tors and logans (rocking
boulders). |
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Jointed
Granite Outcrop of Kendijack
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are very interesting rock formations, comprising of
groups of precariously balanced rocks. Tors are
natural formations, but once were thought
to be man made. Theories were
proposed, many involving mystical
rites.
In Cornwall, tors are composed of
granite, a rock originally
formed at great depth. Over the
millennium the material covering was
gradually removed and the graniteeventually
reached the surface.
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Granite Tor On Carn
Brea |
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With the release from this immense weight, compression on thegranite was relaxed. Expansion occurred,
leading to cracking and jointing.
The
jointing allowed natural processes
such as frost, wind and rain to enter
the granite, enhancing weathering in
these areas and producing these
wonderful shapes.
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Logan
Rock SW397220
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The
horizontal joints in granite can erode faster than
the boulder over it. Some of these boulders
eventually become unstable and will be able
to be rocked. The most famous example locally is
that of Logan Rock.
Logan
Rock is a huge boulder that can be rocked by one
person. In 1824 a band of sailors rocked it so
much it moved, stopping it from rocking. Atgreat expense they were forced to put it back in
place. |
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Logan
Rock
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Hornfels
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Hornfels a tough rock metamorphosed by the granite, was
mined up to recently at Penlee Quarry Newlyn, primary for
use as building aggregate. |
Minerals
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In
this part of Cornwall, the rich metal deposits
are associated with the local granite formation.
While the granite was hot magma, the minerals
became concentrated in high temperature liquids
and gases, which as the temperature fell formed
mineral veins.
Due
to the igneous geology of this area, mineral lodes
of tin, copper, iron, zinc, lead and to a lesser
degree, silver were formed. All of these metals
were in sufficient amounts to be
mined.
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Fossils
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As
the majority of the geology is metamorphic and
igneous, fossils are not typically associated with
Penwith. However there are late Caenozoic clay and
sand deposits at St Erth. This site is
sufficiently important to be a Site of Special
Scientific Interest. At this site many species of
marine organisms have been found. It is
particularly noted for the large variety of
ostracods.
As
with many
important sites, before they were known for their
scientific interest, St Erth had a commercial value. Clay and sand was still being quarried
after the Second World War. The sand was used for
foundry molding, while the clay was used by
potters. |
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Land
that directly fronts onto the sea is directly
affected by erosion through it. The composition of the
rock and the geological features, such as faults,
determine the differing rates of erosion,
and hence the type of coastal features
formed. |
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The cliffs of Penwith are primarily
composed of hard rock. However the
rocks themselves are not of a
homogenous composition, there are
structural weaknesses which erode
faster than the rest and can lead to
the production of caves.
Cave formation can be be extended
inside the cliff away from the cliff
face. Eventually it can reach the
upper surface forming a blowhole. When
waves hit the cave air is forced up
the blowhole producing a
bellowing sound. |
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Sea Action On
Blowhole |
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Cliffsare eventually eroded by the sea. A
typical sequence being shown in the diagram below.
The cliff face crumbles forming a stack, which
itself is eroded into a stump. |
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During
the period of the Ice Ages the climate varied. There were
interglacial periods when weather warmed, the sea
rose and beaches formed that were higher than they are
now.
Just outside Penwith is the small
village of Praa Sands well known for
its sandy and surfing
beach. It is also very interesting
geologically as on the beach can be
seen an ancient raised beach that
represents a previous much older
landscape when sea levels were
higher.
At
one end of the beach is a small cave, this is an adit. Adits
were used to drain water from mine workings. In the adit walls can
be seen rounded pebbles, these are the remains of a raised beach.
The importance of such a area has been
recognised and Praa Sands is now a Regionally
Important Geological/Geomorphical
Site. |
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Pebbles
in Raised Beach at Praa Sands
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Adit
on Praa Sands beach
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