. ▁Both ▁instances ▁are ▁exception al ▁examples ▁of ▁where ▁the ▁contacts ▁between ▁sed iment ary ▁str ata ▁and ▁either ▁sed iment ary ▁or ▁cry st all ine ▁str ata ▁of ▁greatly ▁different ▁ages , ▁orig ins , ▁and ▁structure ▁represent ▁periods ▁of ▁ge olog ic ▁time ▁sufficiently ▁long ▁to ▁raise ▁great ▁mountains ▁and ▁then ▁er ode ▁them ▁away . ▁ ▁Background ▁Un con form ities ▁tend ▁to ▁reflect ▁long - term ▁changes ▁in ▁the ▁pattern ▁of ▁the ▁accum ulation ▁of ▁sed iment ary ▁or ▁ igne ous ▁str ata ▁in ▁low - lying ▁areas ▁( o ften ▁ocean ▁bas ins , ▁such ▁as ▁the ▁G ulf ▁of ▁Mexico ▁or ▁the ▁North ▁Sea , ▁but ▁also ▁Bang l adesh ▁and ▁much ▁of ▁Brazil ), ▁then ▁being ▁u pl ift ed ▁and ▁er oded ▁( such ▁as ▁the ▁on going ▁H imal ay an ▁or ogen y , ▁the ▁older ▁Lar am ide ▁or ogen y ▁of ▁the ▁Rock y ▁Mountains , ▁or ▁much ▁older ▁App al ach ian ▁( Al leg han ian ) ▁and ▁O u ach ita ▁or ogen ies ), ▁then ▁subsequently ▁subs iding , ▁eventually ▁to ▁be ▁buried ▁under ▁younger ▁sed iments . ▁The ▁interven ing ▁periods ▁of ▁t ect onic ▁u pl ift ▁are ▁generally ▁periods ▁of ▁mountain ▁building , ▁often ▁due ▁to ▁the ▁collision ▁of ▁t ect onic ▁pl ates . ▁The ▁" gre at " ▁un con form ities ▁of ▁regional ▁or ▁contin ental ▁scale ▁( in ▁both ▁ge ography ▁and ▁chron ology ) ▁are ▁associated ▁with