Smithfield Market is located in central London close to St Paul’s Cathedral and St Bartholomew’s Hospital. The covered meat market buildings date from the middle of the 19th Century and were designed by architect Horace Jones, who also designed Tower Bridge. He also designed Billingsgate and Leadenhall markets. There has been a meat market in the area since medieval times. The area has been under threat of redevelopment and even demolition in recent years, although some of the buildings are now listed. The area is well worth exploring.
The coordinates take you to a street corner, and you should be able to see a prominent stone plaque above your head. This commemorates the opening of the Fish Market building in 1888. Much of the fish market building is red brick, although this is badly weathered in places. At GZ you should be able to see three different types of stone:
- Rough granite just above pavement level.
- Polished stone between waist and head height
- Stone pilasters above, which are cream in colour
The polished stone is granite, a hard igneous rock. Granite is made up of crystals of three main minerals: quartz, which is colourless or grey, feldspar which is usually pink (orthoclase) or white (plagioclase) and mica which can be silvery grey (muscovite) or black (biotite). Granite it very resistant to weathering and erosion.
Look at the polished stone. It has some very large crystals set within a matrix of smaller crystals. These are called phenocrysts. Granite, and other igneous rocks often also contain xenoliths. These are pieces of ‘foreign rock’. You may be able to spot one of these if you look carefully. They form when rocks surrounding a magma chamber underground fall into the magma, but don’t fully melt away. Answer the following questions using your observations and the information in the description:
- Find the largest phenocryst you can and measure its length and width.
- Describe the shape of the phenocryst you measured. Does it have a regular crystal shape or is it just an irregular mass?
- Which mineral are the phenocrysts made of do you think?
- Granite solidified underground from molten magma, and crystals grow as the magma cools. Do you think the magma that formed this granite cooled quickly or slowly?
- Lastly, the stone used for the cream coloured pilasters is a sedimentary rock. Have a guess at the type of sedimentary rock (hint: it’s used extensively at St Paul’s).
Please email your answers using the email in my public profile, then you can log your find immediately.
Please feel free to upload a photo of you / your GPSr at the site, but this is not a requirement. No close-up photos of the phenocrysts please.