The geologic history of the Moon based upon the layering of impact ejecta and the resulting time sequence of events.
Present - 1.1 b.y.
Rayed fresh craters. We are currently in the Copernican period of lunar history. There are no definite mare lava flows as young, but small numbers of superposed impact craters suggest that a few lava deposits in northern Oceanus Procellarum are of Copernican age. The most significant geologic activity on the Moon during the Copernician period has been the continuing (but infrequent) impact cratering.
1.1 - 3.2 b.y.
Non-rayed fresh craters. This was the longest period in Lunar history. During this era late-stage volcanism filled low-lying regions in and around Mare Imbrium and Oceanus Procellarum. Ejecta and secondary craters from some Eratosthenian impact craters are flooded or embayed by mare lavas.
3.2 - 3.75 b.y.
Maria formation. The Upper Imbrian period was when most of the maria that we presently see on Moon formed - apparently radioactive heating was intense during this time.
3.75 - 3.85 b.y.
Basin formation. The short interval beginning with the Imbrium impact and ending with the Orientale impact is called the Lower Imbrian period. The formation of the Imbrium and Orientale basins provides the most important and widespread stratigraphic boundaries between the ancient, heavily cratered Moon and the more recent Moon dominated by lava flows and a great reduction in impact cratering.
3.85 - 3.92 b.y.
Multiring basins. The Nectarian period is named for the impact basin the contains the relatively small Mare Nectaris, which is just one of about ten multiring basins that formed in a brief interval. These basins are all heavily degraded by impacts.
3.92 - 4.5 b.y.
Intense cratering. The Pre-Nectarian period started with the intense cratering associated with the accretion of the Moon by mutual collisions of millions of smaller bodies. The fragments of ruined craters are the oldest and most difficult to recognize on the Moon.
- Space artist Don Davis' imaginative images prepared under the the direction of Don Wilhelms comparing the Moon's present nearside as it might have appeared in the Middle of the Imbrian Period, End of the Imbrian Period and Today are available in the Library of Congress' American Memory Map Collection.
A list of the major craters and basins of each stratigraphic age group.
- Wood, 2003, p. 53-55
- System/Approximate age: Current USGS stratigraphy as presented in Wilhelm's Geologic History of the Moon.
- 1962: EM Shoemaker & RJ Hackman: Stratigraphic basis for a lunar time scal in Z Kopal & ZK Mikhailov (editors) The Moon. The foundation of all geologic mapping of Moon and other planets and satellites in the solar system.
- 1964: JF McCauley: “The stratigraphy of the Mare Orientale region of the Moon” in Astrogeology Studies Annual program Report, Aug. 1962-July 1963. Flagstaff. First detailed mapping of a lunar basin made possible by Lunar Orbiter images.
- Lunar geologic timescale (Wikipedia)