▁the ▁species , ▁delay ▁between ▁expos ure ▁and ▁colon ization , ▁varying ▁growth ▁rates ▁from ▁region ▁to ▁region ▁as ▁well ▁as ▁the ▁fact ▁that ▁growth ▁rates ▁are ▁not ▁always ▁constant ▁over ▁time , ▁dependence ▁of ▁the ▁rate ▁of ▁growth ▁upon ▁substr ate ▁texture ▁and ▁composition , ▁climate , ▁and ▁determ ining ▁which ▁l ichen ▁is ▁the ▁largest . ▁ ▁Method s ▁ ▁Several ▁methods ▁exist ▁for ▁d ating ▁surfaces ▁with ▁help ▁of ▁l ichen ometry ; ▁the ▁most ▁simple ▁re lies ▁on ▁a ▁single ▁largest ▁l ichen ▁while ▁other ▁methods ▁use ▁more . ▁There ▁are ▁also ▁differences ▁in ▁the ▁way ▁the ▁l ichen ▁is ▁measured ; ▁while ▁some ▁suggest ▁that ▁the ▁largest ▁diameter ▁should ▁be ▁measured , ▁other ▁scient ists ▁prefer ▁the ▁diameter ▁of ▁the ▁largest ▁ins cribed ▁circle . ▁A ▁problem ▁in ▁d ating ▁l ich ens ▁is ▁the ▁fact ▁that ▁several ▁th al li ▁can ▁f use ▁together , ▁making ▁several ▁minor ▁l ich ens ▁appears ▁as ▁a ▁larger ▁one ▁of ▁older ▁age . ▁L ichen omet rist ▁Tom ▁Brad well ▁has ▁listed ▁the ▁following ▁five ▁method ▁families ▁as ▁the ▁principal ▁ones ▁into ▁which ▁most ▁other ▁methods ▁can ▁be ▁class ified : ▁L arg est ▁l ichen ▁( LL ): ▁When ▁the ▁single ▁largest ▁l ichen ▁of ▁a ▁species ▁is ▁used ▁it ▁means ▁that ▁the ▁l ichen ▁that ▁is ▁oldest ▁or ▁grows ▁in ▁most ▁favor able ▁conditions ▁is ▁used ▁to ▁date ▁the ▁minimum ▁age ▁of ▁the ▁exposed ▁surface . ▁This ▁was ▁the ▁original ▁l ichen ometric ▁from ▁which ▁others ▁then ▁developed