A soil of the past, that is, a fossil soil. Paleosols are most easily recognised when they are buried by sediments. They also include surface profiles that are thought to have formed under very different conditions from those now prevailing, such as the deeply weathered tropical soils of Tertiary geological age that are widely exposed in desert regions of Africa and Australia. Such profiles are generally known as relict paleosols. Those that can be shown to have been buried and then uncovered by erosion are known as exhumed paleosols. The main problem in defining the term paleosol comes not so much from complications such as these arising from its fossil nature, but from defining what is meant by soil, a term that has very different meanings for agronomists, engineers, geologists, and soil scientists. Considering research on soils of Antarctica and Mars and on paleosols in a variety of rocks ranging back to 3.5 × 10⁹ years old, soil can be considered distinct from sediment in that it forms in place, but soil need not necessarily include traces of life. At its most general level, soil is material forming the surface of a planet or similar body and altered in place from its parent material by physical, chemical, or biological processes.