Sedimentary rocks form from sedimentation; sedimentation is the process through which minerals and organic particles collect and settle together. These particles are called sediment. Sedimentary rocks are categorized into three different main types that include many different rocks. Sedimentary rocks are found across the Earth's land masses and in the Earth's oceans.
Clastic sedimentary rocks are composed of rocks that moved because of fluids and settled when the fluid stopped; for example, rocks in a river or stream that then dried up. These rocks are made up of silicate and rock fragments that have become cemented together. The materials found in clastic sedimentary rocks are quite variable, but includes: feldspar, mica, clay fragments and quartz. Clastic sedimentary rocks are further classified into groups based on the size of the fragment that makes up the rock. These three groups are breccias, sandstones and mudrocks. Breccias are made almost entirely of rock while sandstone is composed of sand and mudrocks of mud. This group is sometimes also known simply as the breccias.
Biochemical sedimentary rocks are made up of particles from air or water. These rocks are also known as organic sedimentary rocks because they form as result of something that grew. For example, coal is a biochemical sedimentary rock that comes from a plant. The plant intakes carbon as it grows and when it dies that carbon accumulation eventually creates coal. Some types of limestone are also classified as biochemical sedimentary rocks; these rocks form from the skeletons of corral, mollusks and other organisms.
The final group of sedimentary rocks is the chemical rocks. These rocks form as materials evaporate. For example, over time seas and ancient oceans dried up. The salt in these seas gathered to form chemical sedimentary rocks including other types of limestone, dolomites and rock salt. The rocks are different than the other types because they don't form by energy or by current. These rocks are classified by the chemical make up of the materials that form them. The three categories are: carbonates, evaporites and chert.