You have chosen visible grains

Updated Thursday, 28th September 2006
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What rocks are these?

You have identified your rock as having a structure of visible grains.
This means it is a sedimentary rock
Is this correct? If not, start again.

Does your sample appear to contain fossils or shelly fragments?

If there are fragments of fossils or shell in your rock, we have identified your rock as limestone

If there are no fossils or pieces of shell, to identify your rock we need to ask further questions

More help with this question:
What are fossils?
The word fossil comes from the latin word ‘fossilis’ which means 'dug up'. A fossil is the remains or impression of any plant or animal which has been buried in sediment and preserved in a sedimentary rock. Less commonly, fossils can also be preserved in ice, e.g. woolly mammoths, or even in lava flows and volcanic ash, e.g. charred tree stumps.

There are two main kinds of fossil: a body fossil which is the actual remains of a plant or animal and a trace fossil, which records the activities of the animal, like footprints, burrows and bite marks.

How do fossils form?
In principle forming a fossil is quite easy. A plant or animal just has to get buried in sediment. Sadly the process is not that simple. Not all organisms get buried, and not all of those that are buried are preserved as fossils.

Normally the first thing that happens is the death of the organism. The soft body parts then decay or get eaten by carnivores. Only in very rare and exceptional cases are the conditions right to preserve the soft parts of an organism.

If the remaining hard parts, like the skeleton or shell, are then quickly buried by sediment they have a chance of being preserved. If not, they may be scattered by predators or broken up by wind and water.

Even when buried, potential fossils may be destroyed by chemical solutions percolating through the sediment and dissolving the hard parts.

As the sediment continues to compact and turn into sedimentary rock, the remaining hard parts may themselves undergo change. For example, the original skeleton may be dissolved and replaced by a new mineral.

Why are fossils important?
Fossils tell us something about the biology of now extinct plants and animals and sometimes also about the environment in which they lived. By comparing fossils with present animals and plants we can also begin to map out parts of the evolutionary tree of life. Fossils are also useful for the relative dating of sedimentary rocks and in some cases they also serve a practical economic purpose. The oil, gas and coal industries make extensive use of fossils in rock formations to help them date and correlate rocks so that they can then locate and exploit reserves.

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