THE SCIENCEY BIT
There are three main types of rock...
Sedimentary rocks are formed by small particles (sand, shells, fragments etc - or sediments!) which accumulate in layers over millions of years, and eventually hardens into rock. This type of rock can be fairly soft and can crumble. It can contain many types of fossil. An example of this type of rock is coal, and another is limestone - the local(ish) Oolitic Limestone is famous for giving many Cotswold villages their distinctive honey-coloured building stones.
Metamorphic rocks are formed from already existing rocks, under under the earth's surface. Intense heat and pressure causes physical or chemical changes to the rocks. These rocks may have obvious strata, lines or ribbons, and contain crystals. Examples of this includes slate and marble.
Igneous rock is formed from the cooling and solidification of magma or lava, which forms crystals to make granular rocks. Small crystals indicate rapid cooling whilst larger crystals show the magma cooled over a much longer period. Once cooled this type of rock is extremely hard, and as such it is a very durable building material - for example granite used in many buildings or for monuments. In the UK North and Mid Wales is known as having hard, volcanic igneous types of rock.
THE ICE AGE AND GLACIAL ERRATICS
A glacier begins when snow doesn't completely melt away during the summer. Each winter new snow falls on top of the old snow. Thick layers of snow are gradually compressed into glacial ice. A glacier might look like a solid block of ice, but it is actually moving very slowly. The glacier moves because pressure from the weight of the overlying ice causes it to deform and flow. Meltwater at the bottom of the glacier helps it to glide over the landscape.
Glaciers are made up of more than just ice and snow. They contain water, rocks and sediments. This can make the ice look very dirty. When a glacier melts, all of the rock, sand and mud that it was carrying gets left behind. Geologists call this mixture of sediment till.
Glacial erratics - or rocks moved many miles by a glacier - are part of this 'till'.
So our definition could be: An erratic is a boulder transported and deposited by a glacier having a makeup different to the bedrock upon which it is now sitting.
Erratics are useful indicators of patterns of former ice flow. In this area, near to the River Avon, it is difficult to see any of the local bedrock in order to compare our erratic to this. However, the grand and imposing church is made of the local Cotswold Limestone.
LOGGING REQUIREMENTS FOR THE EARTHCACHE
As this is an EarthCache there is no physical container to find.
Instead please answer the following questions and send them to me via email, geocaching.com message centre or clicking my user profile.
1. Describe the colour and texture of the boulder.
2. How does this differ to the (local) stone of the church?
3. What type of rock do you think our erratic is? And why do you think this?
4. There is a small plaque above the rock. Where does it suggest the boulder came from?
If possible, it would be nice to see photos in logs of cachers at GZ, but this is not a logging requirement.
Please feel free to log your find, although logs with no emailed answers will be checked and possibly deleted.
Happy EarthCaching & Congratulations to Pixie & The Giant on their FTF! Well Done!