Geology on the High Street – St Austell EarthCache
Geology on the High Street – St Austell
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This EarthCache is designed to provide a tour of the various rock types used as building stones along the main High Street in St Austell. Unlike some of the quarries from which the stones were sourced, the localities in this EarthCache are accessible to all and do not require you to leave the pedestrianised zone or enter any private property.
Start at the western end of Fore Street at the headline coordinates.
The building to your right has 2 vertical polished pieces of stone on either side of its main shop front. This igneous rock is known as Larvikite as it originates from near the Norweigan town of Larvik. The rock was formed between 250-300 million years ago, during a time known as the Permian Period. The unusual mixture of alkali feldspar and plagioclase feldspar crystals in this rock gives rise to an interesting property when viewed from different angles.
Question 1 - Look at the larger crystals in this rock. Observe and describe what appears to happen to the crystal as you move your head to look at it from different angles. Suggest what may cause this.
Continue eastwards along the pedestrianised zone to the building next door.
Look up to your right to take-in the whole flavour of the building. This ornate stone and red brick building was designed by local architect Silvanus Trevail and built in 1890. It has a St Austell Town Trail location sign on it. The lower columns of this building are made from a local stone called Luxulyan granite. Luxulyan granite outcrops to the east of St Austell and was formed from magma that cooled underground around 285 million years ago, again during the Permian Period. This distinctive rock contains some very large crystals known as megacrysts. In the right hand column of the left archway, there is a xenolith (torn-off inclusion of the country rock through which the granite was intruded) present at knee level as shown in the photo below -
Question 2 – Look at the megacrysts (largest crystals) and the xenolith - Estimate their size and describe what they look like.
Proceed a few more metres eastwards along the main street to WP2.
You should be at a bench so feel free to take a seat! The vertical pillar between the large windows of the building behind you has been faced with a very pretty polished green stone. This stone is a brecciated serpentinite, quarried from near Genoa in Italy. Sometimes called ‘green marble’, this rock is actually formed by the metamorphism of an olivine-rich igneous rock called peridotite. In Cornwall, serpentine can be found on the Lizard Peninsula, but has a different colour and texture to the building stone used here. This stone is dissected by veins of a lighter coloured mineral, probably quartz or calcite.
Question 3 – Observe the mineral veins in the two lower pieces of stone and explain how you can tell that they were once joined as part of a larger piece of stone (Hint – Follow the vein down from the upper piece to the lower piece of stone…What do you notice?) Also, suggest a possible reason why this stone wasn’t used to clad the whole of the building.
Continue eastwards to WP3.
You should be close to the start of the passageway called Chandos Place. The brick-built building on the right as you look down the passageway is unusual in its construction in that it is built from recycled waste material from the China Clay industry! Unlike some other areas in the UK, Cornwall lacks deposits of suitable materials to create conventional clay-based bricks. However, the granite to the north and west of St Austell suffered extensive kaolinisation (chemical alteration) that altered most of the feldspar it contained into China Clay. This China Clay is too valuable to turn into bricks, but all the waste minerals such as quartz, mica and any un-kaolinised feldspar are effectively worthless and have to be dumped in big piles known as skytips. See GC4NTQR for more information! Available in large quantities, this waste material is exempt from the Aggregates Levy (a tax on rock, sand and gravel extracted from the ground) so the bricks and blocks that are created locally are formed using a mixture of cement and this aggregate. The bricks in front of you were created in this way. In these days of Reducing, Reusing and Recycling, this novel solution that creates an outlet for the China Clay waste and eliminates the need to dig up new aggregates has seen it used in some high-profile construction showcases in London. Most notable examples include the offices at One Coleman Street (Winner of the 2007 Concrete Centre Award for Sustainability) and a number of the London 2012 Olympic Venues.
Question 4 – Observe the texture of the bricks. Describe the appearance of the grains and crystals that you can see in them. What are they likely to be? (Hint – Which minerals in the granite didn’t suffer kaolinisation?)
Continue eastwards to the final location - WP4.
You should be at a bench so feel free to take a seat once more and face the building to the right of Biddicks Court. This building has utilised two major stones in its construction – large blocks of an imported limestone and smaller cut blocks of a local slate. The limestone was deposited in a generally low-energy marine environment occasionally affected by storms. Although the slate looks less impressive, the story of its origin certainly is! During much of the Carboniferous and Devonian Periods (300 to 400 million years ago), the area around St Austell was submerged below a deep sea. Muddy sediment built up in the sea and hardened over time to form rocks such as mudstones and shales. Throughout most of the Carboniferous Period, Cornwall also experienced violent forces and earth movements as a result of the mountain building event called the Variscan Orogeny. Two huge pieces of the earth’s crust collided to form a supercontinent, called Pangaea. The slate rock that you see in front of you was metamorphosed from softer mudstone into harder slate during that event.
Question 5 – Observe and describe the colour and texture of the limestone that makes up the lower part (3 blocks tall) of this building. If you look closely, you will see fossils – especially in the two blocks to the left of the cash machine. What do you notice about these fossils and how does this support the idea that the limestone sea in which this rock formed occasionally suffered from higher energy storm events? (Hint – look at the shapes and sizes of the fossils – do they all look to be complete?)
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Additional Hints
(Decrypt)
Cyrnfr yrnir lbhe trbybtvpny unzzre ng ubzr!
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