Started by gash, April 16, 2018, 11:15:44 pm

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Fossil hunting is a fascinating pastime enjoyed by families and individuals of all ages and levels of experience throughout the year. With just a little time spent learning the basics anyone can enjoy the thrill of finding evidence of prehistoric creatures and the environments they lived in. The following page offers some guidance to getting started, including the best places to look and techniques for fossil hunting effectively and safely.

The modern use of the word 'fossil' refers to the physical evidence of prehistoric life that is preserved from a period of time prior to recorded human history. There is no universally agreed age at which the evidence can be termed fossilised, however it's broadly understood to encompass anything more than a few thousand years. Such a definition includes our prehistoric human ancestry and the ice age fauna as well as more ancient fossil groups such as the dinosaurs, ammonites and trilobites.

Fossils occur commonly around the world although just a small proportion of former life made it into the fossil record, perhaps less than a billionth. Most living organisms simply decayed without trace after death. Thus, the abundance of fossils reflects the immense number of organisms that have lived and the vast length of time over which the rocks have accumulated.

The earliest fossils discovered date from 3.5 billion years ago, however it wasn't until approximately 600 million years ago that complex multicellular life began to enter the fossil record, and for the purposes of fossil hunting the majority of effort is directed towards fossils of this age and more recent.

The geologic timescale is divided into eras which are further divided into periods, of which the most frequently quoted is the Jurassic period (from the Mesozoic era) - famous for the abundance of dinosaurs at this time.

Where to look for fossils?
The first step towards understanding where to look for fossils is to appreciate the distribution of fossil bearing rocks and the conditions that led to their formation and subsequent exposure. The rocks reveal the conditions present at the time of their formation and the forces that subsequently influenced their character.

There are three primary rock types: sedimentary, formed from accumulated sediment, e.g. sand, silt and skeletal remains; igneous, formed from molten rock that has cooled and hardened; and metamorphic, sedimentary or igneous rocks that have been altered significantly by heat and/or pressure.

Fossils are most commonly found within sedimentary rocks due to the favourable conditions of burial and limited alteration through time. Sedimentary rocks form on the Earth's surface as sediment accumulates in rivers, lakes and on the seafloor in particular. Among the common sedimentary rocks include: sandstone, composed predominantly of grains of eroded rock; limestone, composed predominantly of shell debris and planktonic skeletons; and shale, formed from hardened clay (originally deposited as mud).

Sedimentary rocks may undergo considerable change millions of years after deposition resulting in a new rock type, e.g. slate. These 'altered' rocks are collectively known as metamorphic. Slate was originally laid down as a muddy sediment which was then compacted and hardened to form shale (a sedimentary rock), over time the shale was exposed to greater pressure and heat within the ground, a result of continental movement and/or tectonic activity. Over time the fabric of the shale was altered, replacing the original fabric and converting it to a metamorphic rock, consequently fossils within the slate are often flattened and distorted.

On very rare occasions fossils can also be found within igneous rocks where molten rock escapes to the Earth's surface and envelops organisms in its path, such as a tree. In this example if the molten rock cools and hardens in less time than it takes to turn the tree to ash, then the hardened rock may form a solid mould around the tree. Over a short period of time the tree tissues decay leaving an empty chamber inside the rock, some examples even preserve the texture of the outer bark on the walls of the mould.

Having recognised unaltered sedimentary deposits as the main source for fossils, the next step is to understand where such rocks are located. Geology maps are a useful place to start as they reveal the age and type of rocks present at the surface; note that the surface rock is generally underlain by older rocks unless significant geological forces have caused buckling/folding of the landscape.

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