The earth science lesson being demonstrated by this earthcache is the classification of rocks. There are three types of rock, each category depends on their formation, aesthetics and properties.
Sedimentary
Sedimentary rocks are formed on the surface of the Earth, either in water or on land. They are called secondary, because they often result from the accumulation of small pieces broken off from pre-existing rocks. Most sedimentary rocks become cemented together by minerals and chemicals present when they are formed, and others are held together by electrical attraction. Some, however, remain loose, crumbly and unconsolidated.
Gravel is an overall name for large sediment grain size, which includes boulder, cobble, and pebble. Sand includes sediment grains ranging in size from 2mm to 0.625 mm. Silt is the name of a sediment grain that range in size from 0.625 mm to 0.0039 mm. Mud is the smallest grain size and is also known as clay. It is important to distinguish the grain size called clay from the mineral called clay. Clay sized grains are too small to see individually without the aid of a microscope. Layers are often seen as the sediments are dropped in cycles and disturbances between the cycles often causes a break in the rock formation and a new layer begins when it resumes. Fossils are another easy way to identify sedimentary rocks. Sandstone and limestone are examples of sedimentary rocks.
Igneous
Igneous rocks get their name from the Latin word ignis, meaning "fire." Under the surface of the Earth the magma is kept liquid by high temperature and high pressure. As the volcano erupts hot magma reaches the surface. Afterward the lava rapidly cools down and solidifies. The crystals formed by cooling magma are usually small. Magma doesn't always reach the surface. Sometimes it is trapped underground in pockets inside of other rocks. In this case the magma cools down more slowly forming larger crystals and coarse-grained rocks. How the rocks form will depend not only on the different cooling temperatures of the magma but also its chemical composition.
Igneous rocks have individual crystals that are visible to the naked eye. Crystals can range from less than a millimeter to centimeters in diameter. Igneous rocks with a porphyritic texture show two or more distinct crystal sizes—coarse-grained crystals, visible to the naked eye, surrounded by fine-grained crystals. There would be no evidence of layers or foliation in an igneous rock. Visible crystals or phenocrysts are an easy way to identify an igneous rock. An example of an igneous rock would be granite.
Metamorphic
Sedimentary and igneous and metamorphic rocks which were subjected to intense pressure and/or heat and as a result underwent a complete change. Metamorphic rocks form deep within the Earth's crust. The process of metamorphism does not melt the rocks, but transforms them into other rocks which are denser and more compact. New minerals are created either by the rearrangement of a mineral's components or by reactions with fluids that enter the rocks.
There are two distinctive metamorphic textures. The first is Foliation. This represents a distinct plane of weakness in the rock. Foliation is caused by the re-alignment of minerals when they are subjected to high pressure and temperature. Individual minerals align themselves perpendicular to the stress field such that their long axes are in the direction of these planes (which may look like the cleavage planes of minerals). Usually, a series of foliation planes can be seen parallel to each other in the rock. Well developed foliation is characteristic of most metamorphic rocks. Metamorphic rocks often break easily along foliation planes. Slate or gneiss are good examples of this type of metamorphic rock.
The second type is Granular. This describes a metamorphic rock consisting of interlocking equant crystals (granules), almost entirely of one mineral. A granular texture is developed if a rock's chemical composition is close to that of a particular mineral. This mineral will crystallise if the rock is subjected to high pressure and temperature. A granular texture is characteristic of some metamorphic rocks. Marble and schist are examples of this type of metamorphic rock.
As the grade of metamorphism increases (more temperature and pressure), both crystal size and the coarseness of foliation increase. Therefore, gneiss represents more intense metamorphism (or a higher grade) than schist.
In order to log a find against this earthcache you will need to answer some questions and then contact me with your answers through my geocaching profile or the Messenger facility.
1) Please examine the rocks. Describe the texture, grain orcrystal size and colours seen in the rocks.
2) From the three rock types outlined above, what type of rock is this stone circle made from? Please explain your answer and reference the information provided to explain your conclusion.
3) Is the middle or "altar stone" the same as the other rocks? And please explain your answer
4) Do you believe this stone circle to be a prehistoric stone circle, or one erected in relatively modern times? Again explain your answer