Sedimentary rocks are formed from overburden pressure as particles of sediment are deposited out of air, ice, or water flows carrying the particles in suspension and often contain fossils.
Igneous rocks form from molten rock, and rarely have fossils in them.
Metamorphic rocks have been put under great pressure, heated, squashed or stretched, and fossils do not usually survive these extreme conditions.
Fossils are the preserved remains or traces of animals, plants, and other organisms from the remote past that can be found in rocks. There are many factors that can contribute to the likelihood of an organism being preserved as a fossil. Fossils are most common in limestones. That is because most limestones consist partly or mostly of the shells of organisms.
The most common fossils in Irish limestone are Corals, Brachiopods & Crinoids.
(A) CORALS are common in the limestone, and are often concentrated at specific levels in the limestone. Where present, the corals show that the sea was shallow enough to allow sunlight to penetrate the water to allow algae to photosynthesise. If the corals are cut in cross section, each branch appears as a small circle with white lines radiating out from the centre, like the spokes of a bicycle wheel. If the corals are cut in long section, they appear as long pencil-like features.
(B) BRACHIOPODS or “lamp shells”, are very common fossils in the limestone. They are a type of shellfish with two valves and are still alive today. They live attached to the sea floor by a tough ligament-like stalk, and feed by filtering tiny particles of food (e.g. plankton) out of seawater. In cross section, the valves of the shell look like large white circles. In long section, the valves of the shell look like white semicircles. They are the most common type of fossils to be found in Irish limestone.
(C) CRINOIDS are a relative of the starfish have a radial array of feathery arms on top of a vertical stem. They use their feathery arms to pick or pluck food particles from the seawater and place it in their mouth. After the animal dies, these soft tissues decay and the pieces of the skeleton fall apart. Where the crinoid pieces are cut in cross section, they appear as scattered white circular structures, about 2-10 mm wide. Where they are cut along their length, they appear as two parallel white lines with a jagged edge where they face each other.
To log this earthcache, please send me the answers to the following:
1) Fossils are most common in which type of sedimentary rock?
2) At GZ, there are lots of fossils to be seen. Which of the above described fossils is most dominant?
3) At GZ, estimate the diameter of the largest fossil?
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