When sediments are first deposited, they are unconsolidated and are not considered a rock. Lithification is the process of converting unconsolidated sediments into sedimentary rock. Lithification involves primarily the processes of compaction and cementation. Recrystallization is also an important process for some sediments.

Compaction is the rearrangement of sedimentary particles to reduce pore space and squeeze out pore water. Unlithified sediments generally contain some excess space between grains. This is especially true in the case of very fine-grained sediments such as clay and mud. Coarser grained sediments such as sand and gravels are heavy enough to settle with a minimum of pore space. Over time, as sediments accumulate in basins, the thickness and weight of the overlying sediments increases. The pressure on the buried sediments causes all the grains to compress together as tightly as possible. Excess interstitial water is also forced out and the sediments are now...