托福TPO1阅读Passage3原文文本+题目+答案解析.pdf

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Groundwater

Groundwateristhewordusedtodescribewaterthatsaturatestheground,filling
alltheavailablespaces.Byfarthemostabundanttypeofgroundwaterismeteoric
water;thisisthegroundwaterthatcirculatesaspartofthewatercycle.Ordinary
meteoricwateriswaterthathassoakedintothegroundfromthesurface,from
precipitation(rainandsnow)andfromlakesandstreams.Thereitremains,sometimes
forlongperiods,beforeemergingatthesurfaceagain.Atfirstthoughtitseems
incrediblethattherecanbeenoughspaceinthe“solid”groundunderfoottohold
allthiswater.

Thenecessaryspaceisthere,however,inmanyforms.Thecommonestspacesare
thoseamongtheparticles—sandgrainsandtinypebbles—ofloose,unconsolidated
sandandgravel.Bedsofthismaterial,outofsightbeneaththesoil,arecommon.
Theyarefoundwhereverfastriverscarryingloadsofcoarsesedimentonceflowed.
Forexample,asthegreaticesheetsthatcoveredNorthAmericaduringthelastice
agesteadilymeltedaway,hugevolumesofwaterflowedfromthem.Thewaterwasalways
ladenwithpebbles,gravel,andsand,knownasglacialoutwash,thatwasdeposited
astheflowsloweddown.

Thesamethinghappenstothisday,thoughonasmallerscale,whereverasediment-
ladenriverorstreamemergesfromamountainvalleyontorelativelyflatland,
droppingitsloadasthecurrentslows:thewaterusuallyspreadsoutfanwise,
depositingthesedimentintheformofasmooth,fan-shapedslope.Sedimentsarealso
droppedwhereariverslowsonenteringalakeorthesea,thedepositedsediments
areonalakefloorortheseaflooratfirst,butwillbelocatedinlandatsomefuture
date,whenthesealevelfallsorthelandrises;suchbedsaresometimesthousands
ofmetersthick.

Inlowlandcountryalmostanyspotonthegroundmayoverliewhatwasoncethe
bedofariverthathassincebecomeburiedbysoil;iftheyarenowbelowthewater’s
uppersurface(thewatertable),thegravelsandsandsoftheformerriverbed,and
itssandbars,willbesaturatedwithgroundwater.

Somuchforunconsolidatedsediments.Consolidated(orcemented)sediments,too,
containmillionsofminutewater-holdingpores.Thisisbecauseth