2. Inorganic or mechanically formed sedimentary rocks These rocks form from the lithification of rock fragments.
Sandstone is formed when particles of eroded igneous, metamorphic and other sedimentary rocks are carried by rivers and accumulate on the seabed or in lakes. During lithification silica glues the grains together. Sandstone is a porous, permeable rock. The spaces between sand grains are called pores. These pores may be filled with air, water, oil or gas. Liquids and gases pass easily through the pores, making sandstone a permeable rock. Irish sandstone formed 400 to 350 million years ago when Ireland lay 30° south of the equator. At this time and latitude Ireland experienced very dry desert conditions. Huge rivers flowed across this dry landscape carrying the sediments that eventually formed the Old Red Sandstone which is now found in the Cork and Kerry mountains.
Shale or mudstone: Layers of fine particles of silt and mud build up in deep water and become dark grey or black rock. Shale is found in south County Clare.
Conglomerates: When a mix of sand grains and pebbles stick together, a conglomerate is formed. The pebbles may be rounded or angular and have many different colours. In some places this rock is known as pudding stone. Conglomerates may be found in Skerries and Rush in north County Dublin.
3. Chemically formed sedimentary rocks
These are rocks formed from the evaporation of sea water from enclosed lakes or seas in hot climates.