THE WORK OF GROUND WATER
Land waters. We have seen how large is the part that water plays at and near the surface of the land in the processes of weathering and in the slow movement of waste down all slopes to the stream ways. We now take up the work of water as it descends beneath the ground,—a corrosive agent still, and carrying in solution as its load the invisible waste of rocks derived from their soluble parts.
Land waters have their immediate source in the rainfall. By the heat of the sun water is evaporated from the reservoir of the ocean and from moist surfaces everywhere. Mingled as vapor with the air, it is carried by the winds over sea and land, and condensed it returns to the earth as rain or snow. That part of the rainfall which descends on the ocean does not concern us, but that which falls on the land accomplishes, as it returns to the sea, the most important work of all surface geological agencies.
The rainfall may be divided into three parts: the first dries up, being discharged into the air by evaporation either directly from the soil or through vegetation; the second runs off over the surface to flood the streams; the third soaks in the ground and is henceforth known as ground or underground water.
The descent of ground water. Seeping through the mantle of waste, ground water soaks into the pores and crevices of the underlying rock. All rocks of the upper crust of the earth are more or less porous, and all drink in water. Impervious rocks, such as granite, clay, and shale, have pores so minute that the water which they take in is held fast within them by capillary attraction, and none drains through. Pervious rocks, on the other hand, such as many sandstones, have pore spaces so large that water filters through them more or less freely. Besides its seepage through the pores of pervious rocks, water passes to lower levels through the joints and cracks by which all rocks, near the surface are broken.
Even the closest-grained granite has a pore space of 1 in 400, while sandstone may have a pore space of 1 in 4. Sand is so porous that it may absorb a third of its volume of water, and a loose loam even as much as one half.