▁indicated ▁by ▁progress ively ▁fewer ▁cr aters ▁in ▁each ▁success ively ▁younger ▁cr ater ▁class . ▁Appro xim ately ▁half ▁of ▁the ▁mapped ▁area ▁contains ▁a ▁high ▁density ▁of ▁cr aters ▁and ▁mult iring ed ▁bas ins ▁formed ▁by ▁the ▁inten se ▁early ▁bomb ard ment . ▁It ▁is ▁doubt ful ▁that ▁any ▁primary ▁cr ust al ▁material ▁has ▁been ▁preserved ▁without ▁bre c ci ation ▁and ▁red ist ribution ▁by ▁repeated ▁impact s . ▁The ▁present ▁cr ater ▁population ▁represents ▁only ▁the ▁cr aters ▁surv iving ▁at ▁the ▁end ▁of ▁the ▁stage ▁of ▁highest ▁meteor oid ▁flux . ▁As ▁the ▁impact ▁flux ▁decre ased , ▁cr ater ed ▁pla ins ▁materials ▁of ▁possible ▁vol can ic ▁origin ▁were ▁depos ited ▁in ▁broad , ▁low - lying ▁areas , ▁flo oding , ▁emb ay ing , ▁or ▁partially ▁b ury ing ▁pre ex isting ▁cr aters . ▁The ▁young est ▁mult iring ed ▁bas ins ▁( R en oir , ▁Rod in , ▁and ▁the ▁un named ▁bas in ▁at ▁ 1 5 ° ▁S ., ▁ 1 5 ° ) ▁formed ▁near ▁the ▁end ▁of ▁this ▁stage ▁( about ▁c 3 ▁time ), ▁as ▁did ▁the ▁Cal oris ▁Bas in ▁on ▁the ▁opposite ▁side ▁of ▁the ▁planet ▁( Mc C au ley ▁and ▁others , ▁ 1 9 8 1 ; ▁Sch ab er ▁and Mc C au ley , ▁ 1 9 8 0 ). ▁C rat ers ▁that ▁formed ▁still ▁later ▁during ▁the ▁period ▁of ▁low ▁impact ▁rates ▁are ▁well