The lunar geological timescale (or selenological timescale) divides the history of Earth's Moon into five generally recognized periods: the Copernican, Eratosthenian, Imbrian (Late and Early epochs), Nectarian, and Pre-Nectarian. The boundaries of this time scale are related to large impact events that have modified the lunar surface, changes in crater form that occur through time, and the size-frequency distribution of craters superposed on geological units. The absolute ages for these periods have been constrained by radiometric dating of samples obtained from the lunar surface. However, there is still much debate concerning the ages of certain key events, because correlating lunar regolith samples with geological units on the Moon is difficult, and most lunar radiometric ages have been highly affected by an intense history of bombardment.Lunar stratigraphy The primary geological processes that have modified the lunar surface are impact cratering and volcanism, and by using standard stratigraphic principles (such as the law of superposition) it is possible to order these geological events in time. At one time, it was thought that the mare basalts might represent a single stratigraphic unit with a unique age, but it is now recognized that mare volcanism was an ongoing process, beginning as early as 4.2 Ga and continuing to perhaps as late as 1.2 Ga (1 Ga = 1 billion years ago). Impact events are by far the most useful for defining a lunar stratigraphy as they are numerous and form in a geological instant. The continued effects of impact cratering over long periods of time modify the morphology of lunar landforms in a quantitative way, and the state of erosion of a landform can also be used to assign a relative age. The lunar geological time scale has been divided into five periods (Pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican) with one of these (the Imbrian) being subdivided into two epochs. These divisions of geological time are based on the recognition of convenient geomorphological markers, and as such, they should not be taken to imply that any fundamental changes in geological processes have occurred at these boundaries. The Moon is unique in the solar system in that it is the only body (other than the Earth) for which we possess rock samples with a known geological context. By correlating the ages of samples obtained from the Apollo missions to known geological units, it has been possible to assign absolute ages to some of these geological periods. The timeline below represents one such attempt, but it is important to note (as is discussed below) that some of the ages are either uncertain, or disputed. In many lunar highland regions, it is not possible to distinguish between Nectarian and Pre-Nectarian materials, and these deposits are sometimes labeled as just Pre-Imbrian.The Pre-Nectarian period is defined from the point at which the lunar crust formed, to the time of the Nectaris impact event. Nectaris is a multi-ring impact basin that formed on the near side of the Moon, and its ejecta blanket serves as a useful stratigraphic marker. 30 impact basins from this period are recognized, the oldest of which is the South Pole–Aitken basin. This geological period has been informally subdivided into the Cryptic and Basin Groups 1-9, but these divisions are not used on any geological maps.