Someofthemostbeautifulcavesareformedinglaciers(冰川).Streamsofmeltingiceandsnowtunnelthroughtheglaciersthesamewaythatwaterfromafaucetmeltsitswaythroughanicecube.Waterfromthesurfacedripsdownthroughcrackshollowingoutthetunnelsanddecoratingthecaveswithcrystalicicles.Thesmoothwallsandfloorsaresoglasslikethatpebblesfrozensixfeetdeepcaneasilybeseen.Crystal-cleariciclesdrapingfromtheceilingsflashbluegreenasthoughtheywerecarvedfrompreciousjewelsinsteadofice.?
AlthoughmostofthecaveiceintheUnitedStatesisfoundinlavecavesthereareanumberoflimestoneicecavesaswell.Somepeoplebelievethatthisicewasformedthousandsofyearsagowhentemperaturesweremuchlowerthantheyaretoday.Othersthinkthatthecaveicebrokeofffromtheancientglaciersastheyspreadoverthecountry.
Todaymanycavescientistshaveanotheridea.Theybelievethatcoldwatersinksdownthroughcracksintothesecavesuntiltemperatureischillyenoughtofreezethewaterthatseeps(渗入)in.Theicethatformskeepsthecavecoolandthathelpsbuildupstillmoreice.Manycavesbecomecoveredwithsomuchicethatnooneknowsjusthowthickitis.InsomesuchascrystalFallsCaveinIdahotherearefrozenriversandevenfrozenwaterfalls.NativeAmericansandearlysettlersusedtostorefoodintheseundergroundrefrigeratorsandchipoutblocksoficetomeltfordrinkingwater.?
1.Whatisthemainideaaboutthispassage?
A.Characteristicsofglaciers
B.Usesforicecaves
C.Theoriginofcaveice
D.Whereglacierscanbefound
2.Whatcausesicetobuildupincaves?
A.Riversandwaterfallswater
B.Iciclesaccumulateontheceilings
C.Caveicebreaksoffglaciers
D.Coldwaterseepsinandfreezes
3.Itcanbeinferredfromthepassagethattheearlysettlersappreciatedtheicecavesfortheir______.
A.practicalityB.beautifulinteriors