However, because the Apollo Moon rocks all come from a relatively small region on the Moon, many scientists worried that the effect was really just a local pounding. Because lunar meteorites are a more random sampling of the Moon than the Apollo samples, the Lunar Cataclysm does indeed seem to have been a Moon-wide phenomenon. When you look through even a small telescope, though, craters seem to pop up everywhere. In fact, the lunar highlands are saturated with craters, which means that if a new crater were to form, it would have to wipe out other craters because there is no empty space left.
Craters come in all sizes, from micrometers to hundreds of kilometers in diameter.
In fact, the dark maria are circular because they fill in the largest lunar craters, the basins. The number of craters on a planetary surface is related to how frequently an impactor hits and the age of the surface. Older surfaces have more craters on them. At the beginning of the solar system, there was much more material available to be swept up than there is now, so the frequency of impacts might be expected to taper off with time. This would mean that very old surfaces have many more craters than young ones.