The Imbrium system - the second of the Moon's four geological phases - began 3.9 billion years ago with the formation of the Imbrium Basin after which it is named, and ended 3.6 billion years ago.

The Apennines, together with several other of the Moon's most spectacular mountain ranges, are formed of layered breccias thrown up by that same impact, which may have been caused by a decaying lunar satellite.

Photo: Richard Evans. The full size original of this and other lunar images, with additional narratives, may be seen at his Lunar Homepage.