Deposition is the process by which wind or water sediment settles out in layers. Furthermore, over time, the remains of living things may harden in this sediment and change into fossils. After sediment is deposited the processes of compaction and cementation change the fragments into sedimentary rock. Compaction. At first, the fragments of rock lie loosely together. But gradually, over many, many years these thick layers build up. This build up becomes heavy and presses down on the layers beneath it. Compaction is the pressing down of layers forcing the sediments to fit closer together. Over millions of years, this process can squeeze fragments tightly together. The layers often remain visible in the rock and may be used to identify sedimentary rock. Cementation