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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
building and volcanic activity. At the same time, external processes continually break rock apart and move the debris to lower elevations, as shown in Figure 1. The breaking down and changing of rocks at or near Earthâs surface is called weathering. Weathering is a basic part of the rock cycle and a key process in the Earth system. There are two types of weatheringâmechanical and chemical. Though these processes are different, they are at. work at the same time. Figure 1 Weathering Ice, rain, and wind are slowly breaking down the rock in this mountain.