Sedimentology encompasses the study of modern sediments such as sand, silt and clay and the processes that result in their formation, transport, deposition and diagenesis. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures. Sedimentary rocks cover up to 75% of the Earth's surface, record much of the Earth's history and harbor the fossil record. Sedimentology is closely linked to stratigraphy the study of the physical and temporal relationships between rock layers or strata.