SOIL PHYSICS 453 



see that wells, which often are merely holes in the ground, will 

 also be filled. But where does the water come from which 

 fills our wells partly full even after months of hot, dry weather? 

 There are weeks, even months, at a time, in some sections, 

 when but little rain falls and this merely wets the surface of the 

 soil, which is soon dried by evaporation. Evidently the ground 

 must in some way serve as a great reservoir storing a supply of 

 water which we are able to draw upon as needed. That this 

 must be so is made more evident by the fact that a good, 

 "never failing " well pumped empty at a certain time is found 

 soon to contain as much water as it did before the pumping 

 took place. 



530. The Earth a Great Sponge. The fact is that the earth 

 is riot unlike a great sponge. We have all seen a pail of water 

 quickly disappear when poured upon the dry earth. We have 

 all seen several inches of rain fall within a few hours, and still 

 it all disappeared nearly as fast as it fell if the earth was very 

 dry when it began to rain. Over most of the earth's surface, 

 the earth's crust is composed chiefly of porous soil. This 

 porous soil holds water much as a sponge does. 



531. Ground-water. The earth's crust is not composed of 

 the same material at all depths as that at its surface. Through 

 the upper Mississippi valley, for instance, we often find 2 or 3 

 ft. of black soil at the surface. Beneath this there may lie 

 6 or 8 ft. of yellow clay. Then, perhaps, is found a 2- or 3- 

 ft. vein of sand and gravel. Next, possibly, lies 10 ft. of blue 

 clay. This may rest upon a foot of gravel. Beneath this 

 gravel may lie a thick bed of almost impervious clay called 

 hardpan. And so on down through bedrock we find layer 

 upon layer of different substances. Each layer differs from 

 the one above it not only in material of which it is composed, 

 but also, and more important for our present purposes, in the 

 ease with which water can pass through it, or its POROSITY. 



No matter how many different layers, or STRATA, of material 

 there may be, or of what material those layers may be com- 

 posed, in time, water finds its way down through into bedrock. 



