complex process whereby freshly deposited loose grains of sediment are converted into rock. Lithification may occur at the time a sediment is deposited or later. cementation is one of the main processes involved, particularly for sandstones and conglomerates. In addition, reactions take place within a sediment between various minerals and between minerals and the fluids trapped in the pores; these reactions, collectively termed authigenesis, may form new minerals or add to others already present in the sediment. Minerals may be dissolved and redistributed into nodules and other concretions, and minerals in solution entering the sediment from another area may be deposited or may react with minerals already present. The sediment may be compacted by rearrangement of grains under pressure, reducing pore space and driving out interstitial liquid.