Where possible, craters are classified using the five-grade system devised by Ralph Baldwin. The classes are as follows -

1. Young craters, from impacts after the formation of the Maria, with the steepest, sharpest walls.
2. Intermediate craters, from before the Maria - with walls showing slight degradation due to subsequent meteoric bombardment and subsidence.
3. Older craters - with outlines considerably more rounded and interrupted by later impacts.
4. Ancient craters, dating back almost to the Moon's formation - with highly broken and smoothed features.
5. Craters which have been partly obscured by flooding with lava at the time of the formation of the Maria.