Sedimentary rocks form in three ways and are usually classified as three types of deposits. The following lists these types of rocks. (Note: The origins of some rocks, such as limestone, chert, and dolomite, are sometimes difficult to differentiate between chemical and organic processes):
Clastic-Sedimentary rocks that form by the mechanical accumulation of rock fragments are referred to as clastic deposits. These include sandstones, conglomerates, and shales. The most common constituent of clastic sedimentary rocks is quartz.
Chemical-Sediments deposited through precipitation from solution (or particles that drop out of a liquid solution) are called chemical deposits. For example, evaporites are chemical sedimentary deposits.
Organic-Deposits involving activity by living organisms to create sedimentary rock are called organic. They include coal and oil shale.