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Notes from 6.2 Types of Sedimentary Rocks pages 141-144 Sedimentary rocks are classified by their mode of formation. Clastic Sedimentary rocks – these are the most common and are formed from the abundant deposits of loose sediments that accumulate on Earth’s surface. Clastic means broken Coarse-grained sedimentary rocks contain gravel sized rock and mineral fragments a. Conglomerates – have rounded, gravel-sized particles this is because mountain streams, flooding water, ocean waves, and glacial meltwater have a high energy flow and can carry larger pieces of sediment. The gravel becomes rounded and abraded as the pieces scrape off each other. This is why beach and river gravel-rocks have well rounded edges. b. Breccias – are composed of angular, gravel-sized particles. This is because the sediments did not have time to become rounded. Therefore the particles were transported a shorter distance. Medium-grained sedimentary rocks have sand-sized rock and mineral fragments. These are often found in stream and river channels, beaches and deserts. Sandstone can be used to map ancient river and stream channels. These rocks are called sandstone. Porosity is the percentage of open space between grains in a rock. Sandstone have porosities of up to 30%. Fine-grained sedimentary rocks consist of silt and clay-sized particles. These can be found in swamps and ponds which have slow moving water. The sediments settle at the bottom and form thin layers and have a very low porosity. Shale is an example Biochemical sedimentary rock are rocks that have the remains of once living organisms in them. These are most common in limestone.