Lunar geologic timescale

A chronology of the Moon’s geological history divided into intervals defined mainly by major impact basins and the rock units associated with them.

Lunar stratigraphy: The study and ordering of rock layers and geological units on the Moon. The timescale organizes lunar units into a relative sequence.
Imbrium basin: A vast impact basin on the Moon’s near side, surrounded by prominent rings and ejecta deposits. Its ejecta helped define the Imbrian System and obscured older terrain.
Pre-Nectarian: The earliest lunar geologic time system, preceding formation of the Nectaris basin. It covers the oldest preserved lunar crust and the earliest basin record.
William K. Hartmann: An American planetary scientist known for work on lunar cratering and the history of the Solar System. His lunar studies helped develop interpretations of crater chronology.
Lunar impact basins: Large lunar craters formed by asteroid or comet impacts, often surrounded by rings and ejecta deposits. Basin formation and preservation provide the chronology’s principal boundaries.
Orientale basin: A large, relatively well-preserved impact basin on the Moon’s western limb. Its clear basin structure offers a reference for recognizing impact-basin deposits.
Nectarian: A lunar geologic time system associated with the formation of the Nectaris basin and related deposits. Its defining basin gives the timescale one of its earliest named boundaries.
Don E. Wilhelms: An American geologist who mapped lunar geology and helped establish its stratigraphic framework. His mapping and synthesis shaped the modern sequence of lunar systems.
Lunar crater dating: A method for estimating the relative ages of lunar surfaces from the number and size of their impact craters. Crater counts help assign ages to surfaces whose rocks lack direct radiometric dates.
Lunar mare: A broad, dark plain on the Moon, mostly formed by basaltic lava filling impact basins. Mare basalts record volcanic activity during later parts of lunar history.

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