【英语阅读理解,StandingontherimoftheGrandCanyon,gazingacrossthisgiantwoundintheEarth’ssurface,avisitormightassumethatthecanyonhadbeencausedbysomeancientconvulsion.Infact,theeventsthatproducedthecanyon,farfrom】

　　英语阅读理解,

　　StandingontherimoftheGrandCanyon,gazingacrossthisgiantwoundintheEarth’ssurface,avisitormightassumethatthecanyonhadbeencausedbysomeancientconvulsion.Infact,theeventsthatproducedthecanyon,farfrombeingsuddenandcataclysmic,simplyadduptotheslowandorderlyprocessoferosion.

　　ManymillionsofyearsagotheColoradoPlateauintheGrandCanyonareacontained10,000morefeetofrockthanitdoestodayandwasrelativelylevel.Theadditionalmaterialconsistedofsome14layeredformationsofrock.IntheGrandCanyonregiontheselayerswerelargelywornawayoverthecourseofmillionsofyears.

　　Approximately65millionyearsagotheplateau’sflatsurfaceintheGrandCanyonareabulgedupwardfrominternalpressure;geologistsrefertothisbulgingactionasupwarping;itwasfollowedbyageneralelevationofthewholeColoradoPlateau,aprocessthatisstillgoingon.Astheplateaugraduallyrose,shallowriversthatmeanderedacrossitbegantorunmoreswiftlyandcutmoredefinitecourses.Oneoftheserivers,locatedeastoftheupwarp,wastheancestoroftheColorado.AnotherriversystemcalledtheHualapai,flowingwestoftheupwarp,extendeditselfeastwardbycuttingbackintotheupwarp;iteventuallyconnectedwiththeancientColoradoandcaptureditswaters.Thenewriverthenbegantocarveoutthe277-mile-longtrenchthateventuallybecametheGrandCanyon.Geologistsestimatethatthisinitialcuttingactionbegannoearlierthan10millionyearsago.

　　Sincethen,thecanyonforminghasbeencumulative.Tothecorrosiveforceoftheriveritselfhavebeenaddedotherfactors.Heatandcold,rainandsnow,alongwiththevaryingresistanceoftherocks,increasetheopportunitiesforerosion.Thecanyonwallscrumble;theriveracquiresacuttingtool,tonsofdebris;rainfallrunningoffthehighplateaucreatesfeederstreamsthatcarvesidecanyons.Pushingslowlybackwardintotheplateau,thesidecanyonsexposenewrocks,andthepatternoferosioncontinues.

　　34.Whatdoesthepassagemainlydiscuss?

　　(A)Patternsoferosionindifferentmountainranges.

　　(B)ForcesthatmadetheGrandCanyon.

　　(C)TheincreasingpollutionoftheColoradoRiver.

　　(D)Thesuddena