The two main processes that cause sediment to be lithified into solid sedimentary rock are:

1. Compaction: As layers of sediment accumulate, the weight of the overlying layers presses down on the lower layers. This pressure squeezes out water and air from the spaces between the sediment grains, causing them to pack more tightly together.

2. Cementation: Dissolved minerals in groundwater precipitate out between the sediment grains. These minerals act as a glue, binding the grains together and forming a solid rock. Common cementing minerals include calcite, silica, and iron oxides.