Summary of Lunar Stratigraphic Units

Most rocks on Earth are relatively young - less than a couple billion (10⁹) years or so, with very few rocks surviving that are older than about 3.5 billion years (3.5 ga). Conversely, most rocks on the moon are old, with almost none younger than 3 ga. This is primarily a result of the difference in size between the earth and the moon, both generate heat internally by the decay of radioactive isotopes, but since the moon is smaller it has a larger surface area to volume ratio, and looses this heat more readily than the earth. Thus the Lunar crust froze to a substantial depth early in its history, while the earth still has a thin crust and ongoing plate tectonic activity remodeling the surface. The geologic history of the moon thus primarily reflects events in its early history at around 3 ga to 4 ga. This discovery was one of the great triumphs of the Apollo program.

The major stratigraphic units of the moon (from youngest to oldest) are:

• Copernican - 1.2 ga to present - marked by fresh and rayed craters such as Copernicus
• Eratosthenian - 3.2 to 1.2 ga - includes both slightly degraded craters such as Eratosthenes and some maria
• Imbrian - 3.85 to 3.2 ga - Most maria, some Basins, degraded craters.
• Late Imbrian - Greatest period of volcanic activity, formation of most maria.
• Early Imbrian - Impacts forming Imbrian and Orientale basins, significant impact cratering
• Nectarian - Multiple large ringed basin forming impacts
• Pre-Nectarian - Formation of the lunar crust, very heavy impact cratering, including basin formation

The basic geologic history described by these stratigraphic units is 1) early solidification of an anorthositic crust, 2) massive impacts forming large ringed basins, 3) continually lessening impacts and volcanic eruption of large basalt flows flooding basin floors and forming the maria, 4) not much of consequences since about 3 ga, just occasional large impacts and a steady rain of very small impacts forming the regolith

In older literature and the USGS geologic quadrangles of the moon an earlier set of names are used:

• Copernican - Fresh, rayed craters
• Eratosthenian - Degraded craters
• Procellarian - Maria
• Imbrian - Ringed Basins
• Pre-Imbrian - Highlands (Terrae)

Major units recognized on the Geologic Map of the Cleomedes Quadrangle of the Moon (I-107) are:

• Copernican
• Eratosthenian
• Imbrian or Eratosthenian undifferentiated
• Imbrian
• Pre-Imbrian