Editing Lunar geologic timescale (section)

===Eratosthenian===
{{main|Eratosthenian}}
The base of the [[Eratosthenian]] period is defined by the time at which craters on a geological unit of a certain size D<sub>L</sub> have been almost obliterated by erosional processes. The principal erosional agent on the Moon is impact cratering itself, though seismic modification could play a minor role as well. The absolute age of this boundary is not well defined, but is commonly quoted as being near 3.2 Ga. The younger boundary of this period is defined based on the recognition that freshly excavated materials on the lunar surface are generally bright and that they become darker over time as a result of [[space weathering]] processes. Operationally, this period was originally defined as the time at which impact craters lost their bright [[ray system]]s. This definition, however, has recently been subjected to some criticism as some crater rays are bright for compositional reasons that are unrelated to the amount of space weathering they have incurred. In particular, if the ejecta from a crater formed in the highlands (which is composed of bright anorthositic materials) is deposited on the low albedo mare, it will remain bright even after being space weathered.