托福TPO1阅读文本及题目答案Part1.docx

托福TPO1阅读文本及题目答案Part1TPO1GroundwaterGroundwateristhewordusedtodescribewaterthatsaturatestheground,fillingalltheavailablespaces.Byfarthemostabundanttypeofgroundwaterismeteoricwater;thisisthegroundwaterthatcirculatesaspartofthewatercycle.Ordinarymeteoricwateriswaterthathassoakedintothegroundfromthesurface,fromprecipitation(rainandsnow)andfromlakesandstreams.Thereitremains,sometimesforlongperiods,beforeemergingatthesurfaceagain.Atfirstthoughtitseemsincrediblethattherecanbeenoughspaceinthe"solid"groundunderfoottoholdallthiswater.Thenecessaryspaceisthere,however,inmanyforms.Thecommonestspacesarethoseamongtheparticles-sandgrainsandtinypebbles-ofloose,unconsolidatedsandandgravel.Bedsofthismaterial,outofsightbeneaththesoil,arecommon.Theyarefoundwhereverfastriverscarryingloadsofcoarsesedimentonceflowed.Forexample,asthegreaticesheetsthatcoveredNorthAmericaduringthelasticeagesteadilymeltedaway,hugevolumesofwaterflowedfromthem.Thewaterwasalwaysladenwithpebbles,gravel,andsand,knownasglacialoutwash,thatwasdepositedastheflowsloweddown.Thesamethinghappenstothisday,thoughonasmallerscale,whereverasediment-ladenriverorstreamemergesfromamountainvalleyontorelativelyflatland,droppingitsloadasthecurrentslows:thewaterusuallyspreadsoutfanwise,depositingthesedimentintheformofasmooth,fan-shapedslope.Sedimentsarealsodroppedwhereariverslowsonenteringalakeorthesea,thedepositedsedimentsareonalakefloorortheseaflooratfirst,butwillbelocatedinlandatsomefuturedate,whenthesealevelfallsorthelandrises;suchbedsaresometimesthousandsofmetersthick.Inlowlandcountryalmostanyspotonthegroundmayoverliewhatwasoncethebedofariverthathassincebecomeburiedbysoil;iftheyarenowbelowthewater'suppersurface(thewatertable),thegravelsandsandsoftheformerriverbed,anditssandbars,willbesaturatedwithgroundwater.Somuchforunconsolidatedsediments.Consolidated(orcemented)sediments,too,containmillionsofminutewater-holdingpores.Thisisbecausethegapsamongtheoriginalgrainsareoftennottotallypluggedwithcementingchemicals;also,partsoftheorigina