At or near Earth's surface, sedimentary rocks form in two ways: by the accumulation of rock grains or by the formation of a solid from minerals dissolved in water. The fragments that go into making sedimentary rocks can be as big as boulders or as small as clay particles. Over long periods of time, the upper layers of debris compress the lower layers, squeezing out excess water or air trapped between the rock fragments. Under the pressure, individual fragments eventually dissolve and stick together, or the remaining fluid within the sediment brings in other substances that act as a cement, until the sediment has turned into rock. Scientists classify many sedimentary rocks based on the size of the particles that built the rock; mudstone and sandstone, for example, originally came from fine-grained mud and sand deposits that hardened over long time periods.

After a body of water deposits layers of sediments, the grains in those layers, or strata, undergo various changes in response to the pressure of the layers on top of them. Scientists use the word diagenesis from the Greek roots dia (change) and genesis (origin) to encompass all these changes. In the process called cementation, minerals grow or precipitate out of solution in the pore spaces between rock or mineral grains. Compaction is the collapse of pore spaces within a sedimentary rock due to the weight of the rock above it. Lithification is the process of hardening sediment into rock through compaction and cementation. Diagenesis takes place at temperatures of less than 200 degrees C (392 degrees F); at higher temperatures, scientists say that the rocks are undergoing metamorphism.