Millions of years are needed to provide enough compaction to solidify giant sand dunes. In the case of the ancient sand dunes, wind was the environmental force that caused sand particles to accumulate over such long periods. All sedimentary rocks are made of materials called sediments deposited by a particular force or process. Those sediment deposits then become cemented or compacted together over time, forming a solid rock. The process of sedimentary rock formation involves a variety of sediment types.
Some sediments are tiny grains of sand.
Others include broken pieces of rock rolling along the bottom of a stream, fragmented seashells tossed upon a shoreline, or simply layers of mud. Sediments may also include plant or animal remains. Based on their origin, sediments are categorized into three main types. The most familiar example of this process is the evaporation of seawater, which leaves salt behind. The process by which sediments become compacted and cemented together into a sedimentary rock.