(Folds in the Clogherhead Formation.)
These rocks there are three basic categories of sedimentary rocks. Students should know how each type is formed and be able to give examples of each. Sedimentary rocks form in layers (bedding planes). The three groups of sedimentary rocks are:
Sedimentary rocks are formed on or near the surface of the earth. They develop in layers called strata these are formed by compressed sediments or fragments and often form on lakebeds or under the sea. Successive layers of strata are separated by bedding planes. Sedimentary rocks are generally divided into 2 categories:
1. Organic sedimentary rocks - these are formed from the compacted remains of living things. Limestone and Peat are examples.
2. Inorganic (Mechanical) sedimentary rock - these are formed from the compacted remains of the broken down particles of other rocks; Sandstone and Shale are examples.
Sedimentary Rock examples and their formation:
Sandstone is the second most common rock found in Ireland. It consists of grains of sand which were deposited on land or in shallow seas and lithified (particles are cemented together to become a rock) over a period of time. Old Red Sandstone has a brown-red/ purple colour as it was formed with particles of non-rusting over time. Old Red Sandstone was laid down 400 million years ago when Ireland had a desert climate and the Caledonian Fold Mountains were being weathered and eroded.
Mudstone, like other sedimentary rocks, is formed through the process of lithification. Limestone is formed by cementing and compacting sediments (remains of bones and dead plants life in shallow warm waters). It is formed naturally. Most limestone is formed in shallow seas across the world. Greywacke is a variety of sandstone generally characterized by its hardness, dark color, and small rock fragments set in a compact rock. It is a sedimentary rock generally found...