▁west ▁of ▁approximately ▁ 5 5 ▁deg . ▁Thus , ▁mapping ▁reli ability ▁decre ases ▁west ward . ▁ ▁M apping ▁methods ▁and ▁principles ▁are ▁adapted ▁from ▁those ▁developed ▁for ▁lun ar ▁phot oge olog ic ▁mapping ▁( Wil hel ms , ▁ 1 9 7 0 , ▁ 1 9 7 2 ; ▁Wil hel ms ▁and ▁McC au ley , ▁ 1 9 7 1 ). ▁Map ▁units ▁are ▁distinguished ▁on ▁the ▁basis ▁of ▁top ography , ▁texture , ▁and ▁al bed o ▁and ▁are ▁ranked ▁in ▁relative ▁age ▁on ▁the ▁basis ▁of ▁super position ▁and ▁tran section ▁relations , ▁density ▁of ▁super posed ▁cr aters , ▁and ▁sharp ness ▁of ▁top ography . ▁Because ▁of ▁the ▁lack ▁of ▁a ▁w ides p read , ▁easily ▁ident ifiable ▁strat ig raph ic ▁dat um ▁on ▁this ▁part ▁of ▁Mercur y , ▁a ▁morph olog ic ▁classification ▁of ▁cr ater ▁and ▁bas in ▁materials ▁was ▁the ▁basis ▁for ▁determ ining ▁relative ▁ages ▁of ▁many ▁materials . ▁A ▁phot om osa ic ▁map ▁of ▁the ▁best ▁available ▁photograph s ▁a ided ▁greatly ▁in ▁ge olog ic ▁interpretation ▁and ▁mapping . ▁ ▁Str at ig raph y ▁▁ ▁The ▁rock ▁units ▁are ▁sub div ided ▁into ▁three ▁major ▁groups : ▁pla ins ▁materials , ▁terra ▁materials , ▁and ▁cr ater ▁and ▁bas in ▁materials . ▁The ▁pla ins ▁and ▁smooth ▁terra ▁units ▁are ▁considered ▁to ▁be ▁vol can ic ▁in ▁part , ▁and ▁thus ▁may ▁have ▁a ▁different ▁origin ▁from ▁the ▁impact ▁bre c