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,~ "".... :4~':' even a Iarge body of nonlayered rock that has sizable openings. An aquifer may ~ only a few feet thick, or trIlS or hundeds of fec=t. It may be jusI helow the surface, or hundreds of feet below, II may un- derlie a few acres or many square miles. 1be Dakota Sandstone in the West carries water OHT great distances, across !oe\'eral :States. ~Iany aquifers. however, are only local in extent. Cndemeatb the water-bearing rocks e\'erywhe-rt. al some deplh, are rocks that arc watertight. This depth may be a few hundred feel. or ten.~ of thousands of feet. The amount of water that a gi\'en rock can contain depends on the porosity of the rock-the ~paces bel ween the grains or the cracks that can fill with water. If the grains arc all about the same size, or wen sorted. as the geologists say, the spaces between them account for a large propor- tion of the \\'hole volume. This L" true of gravel and sand, Howt"vt"r, if the grains are poorly wrtt'd, that i<;, not all the same size, the spaco between the larger grains ""ill fill with small grains instead of water. Poorly sorted rocks do not hold as much wat('r as well-sortt"d rocks, The ~ra\'('1 in figure 5 is moderately wdl sorted. If water is to mo\"(' through the rock, the pores musl be connected onc 10 another. If the rock has a great many connected pore spaces, or which a large part arc sizable so that water can move freely through them. we say that the rock is pl'rmeab!e. Large amounts of water arc a\'ail- able to a wdl from saturated penn('able rocks. But if the porf$ or cracks are small. poorly con- nected, or n('ar!y lacking, tht" aquifer can yield only a small amount of water to a well. The poros-ity of different kinds of rock \'anes widely. In ~ome the porosity is les... than 1 pt'rcent: in others, lTlff.'tly unconsolidated rock such a.~ ~and and graHI. it may be as high as 30 or'W percent. :\ rock that will be a good source of water must contain either many pore space;, or many crJ.Cb. or both. .\ compact rock such as granite. almost without pore spac~, may be pemleahle if it con- tains enou~h sizable fr.u:tures. :"early all con- solidated rock formations are broken by cracks, called joints by .l!:eOlog-L'il'i (fi.c:, _'l), Tht'S(' joints arc cau~d hy the s.ame kind of strt"S"'t'5 in the earth's crll"t thaI CalL"(' earthqllak~. .\t first they arc just hairline cracks, hut they tend to open through th(' day-to-day action of rain. sun. and fJ'O<,!. The ice c~~als formed by water that freezes in rock (Te\;CCS will cau_~ the rocks to split open. Heating by the sun and cooling at night catL"C expansion and contraction that produce the same resuk Waler will enter the joints and grad- ually dis."Olve away the rock. enlarging the open- ings. If the joints intersect each other, waler can mo\'e from one to another, much as it 110\\'s through the water pipes in a municipal water sys-- t('m. Granite and ~late are 16.'" porous than sandstone, \\'hen ",'ater circulates in them, it d~ so through joint cracks. The water }ield 0/ wells drilled in thoc rocks depends on how many joints arc in!cn.ccted hy the well. and how wide the)' are. Water ",'ill mo\'e faster in certain kinds of rocks. A clayey silt ha\'ing only \"Cry tiny pores will not carry water \"Cry readily, but a coarse gravel will carry water freely and rapidly. Sandstone is a rock having nalural pore spaces through .....hich water will mo\'(' more easily than it will through tighter rock, such a~ granite. Some rocks are what we call ca\'crnous: they ha\'e hollowed-out open- ings in the-m. Some limestone is like this, and water often flows throu~h lime.'\lone at a faster rate than through other fomlations. Gra\'e! has numerou~ open spaces, \\'ater may travel through it at ratc-s of lem or hundreds of feet per day. In ..ill or fine S3.nd it may mo\'C only a few ilH:hes a day, Flow of streams is measured in feet pt'r ~col1d . movement of ground water is usually mea.~ured in feet per year. Ground \\'aler movt";<. through permeable rocks, and around and in between impcmll~able ones. Ju_~t like surfact" water. it takes the palh of least rcsi~tance. "\hhough it mo\'es so slowly, it may tranl for mil~ befon:: it emergC'S as a spring. or seep!' U!1.<;een into a stream. or is tapped by a well. Fifty milf:'" is not uncommon. I n the Dakota Sandstone beneath the north('rn Great Plains. wOlle( tra\-e1~ hundred~ of milC'S underground. Therc isn't nec~""'1.fily a relatiqn between the water-bcarin.e: capacit} of rocks and the depth at \~hich they art' found. A \'e~' den.'\(' granite may be found al the earth's <,urfa(c, as in At'\\" Eng- land. whiIe- a parow' sand:<tone may lie sc\'cral tholl"",'\nd fcel tx'low the surf act", a~ in Ihe Great Plains. HO\q'\'('r. on lhe a\'(~ra~e. porosit}, and permeability P-O\\' Ics.<; a<; depth incre3<.('s. Rocks Ihal yield rrcsh water han been found at deplhs of more than 6,000 feet (and ~ah~ water has come from oil \H~ll.s at depths of more than '20.0Cl0 )