In addition to the water held in the loose rocks and soils near the earth's surface, large quantities occur in definite layers (usually strata) of porous rocks which very commonly extend at various angles, hundreds or even thousands of feet into the earth. A very fine illustration of this principle is the case of the Dakota sandstone formation of Nebraska. Almost anywhere across the State a well drilled through a bed of clay and into the porous sandstone layer encounters water. (Figure 19.) Another principle is also well illustrated, namely, that water in such a porous layer may actually travel hundreds of miles, water obtained from a well sunk to the Dakota sandstone having actually traveled under the surface of the State all the way from the eastern face of the Rocky Mountains, where rain and melting snow entered the upturned and exposed porous rock layer. Another good example is Iowa, where certain porous rock layers outcropping in the northwestern and northeastern corners of that and adjacent States gradually bend down under the State, reaching the greatest depths (up to 3,000 feet) far in the interior. From wells 3,000 feet deep near Boone, Iowa, it is, therefore, a fact that some of the water pumped out of the earth actually traveled underground all the way from beyond the corners of the State. This sort of travel of underground water is common in many parts of the world. It should be clearly understood that such water does not flow freely as in a pipe along subterranean passageways, but rather it slowly works its way between the grains of porous rock. Where such water moves distinctly downward, and the porous layer has both above and below it an impervious rock layer like shale or clay, it gradually gets under greater and greater pressure. In some cases such pressure has actually been found by deep drilling to be equivalent to that of a column of water several thousand feet high. The rate of motion of water in porous underground rock layers is very slow, data from various sources indicating a rate of speed of not more than one-fifth of a mile a year in coarse porous sandstone, while in many rocks it cannot be more than ten to fifty feet per year. [Illustration: Fig. 19.--Vertical (structure) section from the Rocky Mountains to Omaha, Nebraska, illustrating a widespread underground porous rock layer (the Dakota sandstone) charged with water under pressure, the clay formation acting as a cap rock. (After Darton, U. S. Geological Survey.)]