▁Great ▁Un con form ity ▁occurs ▁in ▁French man ▁Mountain ▁in ▁Nev ada . ▁French man ▁Mountain ▁ex poses ▁a ▁sequence ▁of ▁Ph an ero zo ic ▁str ata ▁equivalent ▁to ▁those ▁found ▁in ▁the ▁Grand ▁C any on . ▁At ▁the ▁base ▁of ▁this ▁sequence , ▁the ▁Great ▁Un con form ity , ▁with ▁the ▁T ape ats ▁Sand stone ▁of ▁the ▁T onto ▁Group ▁over lying ▁the ▁V ish nu ▁Bas ement ▁Ro cks , ▁is ▁well ▁exposed ▁in ▁a ▁manner ▁that ▁is ▁at yp ical ▁and ▁scientific ally ▁significant ▁in ▁its ▁combination ▁of ▁extent ▁and ▁access ibility . ▁This ▁expos ure ▁is ▁frequently ▁illustrated ▁in ▁popular ▁and ▁educational ▁publications , ▁and ▁is ▁often ▁part ▁of ▁ge ological ▁field tri ps . ▁There ▁is ▁a ▁gap ▁of ▁about ▁ 1 . 2 ▁billion ▁years ▁where ▁ 5 5 0 ▁million ▁year ▁old ▁str ata ▁of ▁the ▁T ape ats ▁Sand stone ▁rest s ▁on ▁ 1 . 7 ▁billion ▁( 1 7 0 0 ▁million ) ▁year ▁old ▁V ish nu ▁Bas ement ▁Ro cks . ▁ ▁As ▁a ▁w ides p read ▁phenomen on ▁The ▁term ▁" Gre at ▁Un con form ity " ▁has ▁also ▁been ▁used ▁to ▁refer ▁to ▁the ▁anom alous ▁concentration ▁of ▁un con form ities , ▁including ▁bas ement ▁non con form ites , ▁below ▁the ▁base ▁of ▁the ▁Camb rian . ▁Charles ▁Wal c ott ▁was ▁among ▁the ▁first ▁to ▁note ▁this ▁phenomen on , ▁remark ing ▁in ▁ 1 9 1 0 :