Elfrieda channelFrom Lincoln
Presumably older impact craters made pretty rays too, but those rays have faded with time. For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If they don't overlap, we can use crater counting to figure out which one is older and which one is younger. There were many impacts before Nectaris, in the pre-Nectarian period including 30 major impact basins , and there were many more that formed in the Nectarian period, the time between Nectaris and Imbrium.
The other way we use craters to age-date surfaces is simply to count the craters. Although no absolute methods were available to establish actual dates, Lyell needed to assign very old dates to the strata to make them consistent with the long eons of time that would be necessary to meet the new uniformitarianism theory developed by James Hutton and himself. The chronostratigraphic scale is an agreed convention, whereas its calibration to linear time is a matter for discovery or estimation. Relative ages are more accurate, among scientists anyway. The unconformity consists of many vertical tilted layers of grey shale overlaid by many layers of horizontal red sandstone.