▁or ▁used ▁as ▁reference . ▁Despite ▁re lying ▁upon ▁a ▁single ▁l ichen ▁this ▁technique ▁is ▁pra ised ▁for ▁its ▁simplicity ▁and ▁allows ▁obtain ing ▁an ▁image ▁of ▁the ▁age ▁of ▁rock ▁expos ure ▁while ▁still ▁in ▁the ▁field . ▁L arg est ▁five ▁l ich ens ▁( 5 LL ): ▁This ▁method ▁is ▁a ▁development ▁of ▁the ▁LL ▁and ▁was ▁developed ▁in ▁the ▁ 1 9 7 0 s ▁to ▁avoid ▁reli ance ▁on ▁one ▁single ▁potentially ▁anom alous ▁l ichen . ▁It ▁has ▁been ▁proved ▁that ▁neither ▁accuracy ▁nor ▁precision ▁impro ves ▁significantly ▁by ▁having ▁more ▁than ▁five ▁l ich ens . ▁F ixed - area ▁largest ▁l ichen ▁( F ALL ): ▁This ▁technique ▁was ▁initially ▁spe cially ▁designed ▁for ▁d ating ▁rock falls ▁and ▁tal us ▁con es ▁with ▁no ▁uniform ▁age ▁of ▁de position . ▁The ▁largest ▁th all us ▁in ▁a ▁unit ▁area ▁is ▁measured . ▁The ▁sample ▁areas ▁are ▁usually ▁b ould ers ▁with ▁surfaces ▁of ▁about ▁ 1   m ². ▁Size - fre quency ▁approach ▁( SF ): ▁The ▁analysis ▁of ▁size ▁and ▁frequency ▁of ▁l ich ens ▁was ▁initially ▁done ▁in ▁order ▁to ▁study ▁l ichen ▁populations ▁and ▁pre ex isting ▁th al li ▁growing ▁on ▁surfaces , ▁but ▁has ▁since ▁been ▁used ▁as ▁an ▁effective ▁absolute ▁and ▁relative ▁d ating ▁method . ▁L ichen ▁cover ▁approach ▁( LC ): ▁This ▁method ▁works ▁with ▁the ▁prem ise ▁of ▁that ▁the ▁area ▁covered ▁by ▁a ▁single ▁species ▁will ▁increase ▁over ▁time , ▁and ▁by