What are sedimentary rocks?
Sedimentary rocks are best recognized by their layers. These bedding planes, or strata, result from the way the sediment has been deposited.
•These rocks are formed in a wide variety of environments on the Earth's surface, and in many cases are easier to study than igneous rocks, which often form at great depth in the Earth's crust.
•Many sedimentary rocks are made of particles that have been eroded or weathered from pre-existing rocks. For this reason, they are often referred to as secondary rocks.
•The particles are transported by rivers, wind, glaciers, and gravity. During this journey they are changed and may become smaller, more rounded, and broken.
•Eventually, when the transporting medium is no longer able to carry the sediment particles, they are deposited. A river, for example, can only carry large pebbles when it is flowing swiftly. As it slows down, so this material is left behind.
•Most sedimentary rocks are formed on the seabed. A river entering the sea slows down, depositing its load. The continental shelves have a great thickness of sediment, but much also reaches deeper parts of the oceans.
•Sedimentary rocks are of great importance. By looking at their detailed features and comparing these with how modern sediments are being formed, geologists can work out what our past environments were like.