▁period ▁of ▁w ides p read ▁contin ental ▁den ud ation ▁that ▁ex hum ed ▁and ▁deeply ▁er oded ▁P rec amb rian ▁rocks ▁and ▁exposed ▁them ▁to ▁extensive ▁physical ▁and ▁chemical ▁weather ing ▁at ▁the ▁Earth ' s ▁surface . ▁As ▁a ▁result , ▁Pow ell ' s ▁Great ▁Un con form ity ▁is ▁unusual ▁in ▁its ▁ge ographic ▁extent ▁and ▁its ▁strat ig raph ic ▁significance . ▁ ▁The ▁length ▁of ▁time ▁represented ▁by ▁Pow ell ' s ▁Great ▁Un con form ity ▁var ies ▁along ▁its ▁length . ▁Within ▁the ▁Grand ▁C any on , ▁the ▁Great ▁Un con form ity ▁represents ▁a ▁period ▁of ▁about ▁ 1 7 5 ▁million ▁years ▁between ▁the ▁T onto ▁Group ▁and ▁the ▁young est ▁sub div ision , ▁the ▁Si xt ym ile ▁Form ation , ▁of ▁the ▁Grand ▁C any on ▁Super group . ▁At ▁the ▁base ▁of ▁the ▁Grand ▁C any on ▁Super group , ▁where ▁it ▁trunc ates ▁the ▁Bass ▁Form ation , ▁the ▁period ▁of ▁time ▁represented ▁by ▁this ▁angular ▁un con form ity ▁increases ▁to ▁about ▁ 7 2 5 ▁million ▁years . ▁Where ▁the ▁T onto ▁Group ▁over lies ▁the ▁V ish nu ▁Bas ement ▁Ro cks , ▁the ▁Great ▁Un con form ity ▁represents ▁a ▁period ▁as ▁much ▁as ▁ 1 . 2 ▁to ▁ 1 . 6 ▁billion ▁years . ▁( See ▁also ▁ge ological ▁times c ale .) ▁ ▁French man ▁Mountain , ▁Nev ada ▁A ▁prominent ▁expos ure ▁of ▁Pow ell ' s