Fossils of Northern Ireland - Introduction

Rocks of Northern Ireland

What Rocks are Fossils found in?

Nearly all fossils are found in sedimentary rocks, which form by the accumulation of particles of rock or organic material in the sea, lakes, rivers and on land. Sediments usually are deposited in horizontal layers but these may be tilted by later earth movements. Common types of sedimentary rock in Northern Ireland include sandstone, mudstone and limestone.

Rocks that (almost) never contain Fossils

Some other rock types were once molten liquids at >1000°C, since cooled and solidified. Some of these molten rocks came to the surface at volcanoes; others cooled deep beneath the surface. Occasionally the remains of fossil animals or plants are found beneath lava flows, sometimes severely burnt, or in layers of volcanic ash. In general fossils are never found in these igneous, or 'fire-formed' rocks. Common volcanic rocks in Northern Ireland are basalts, while those that cooled at greater depth include the granite of the Mourne Mountains.

Some sedimentary rocks have been so deformed and altered by heat and pressure that we can no longer recognise any fossils that they might once have contained, though in some that have not been too severely altered the fossils may still be visible. These are metamorphic rocks.

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