We may now inquire as to the necessary conditions for artesian wells. This may best be done by the aid of diagrams. Figure 20 illustrates a very common case where a porous layer, lying between impervious layers, passing under a valley, comes to the surface of the hills on each side where the water enters the porous layer. On sinking a well to the water-charged layer, the water rushes through the hole to a greater or less distance above the surface. In Figure 21 the porous and impervious layers are simply tilted, and the water under pressure rises through the free opening to the surface. Wells of this kind are also common in the Atlantic Coastal Plain of the United States. In another case, less comprehensible to the layman, the porous water-bearing stratum curves downward under a hill or mountain, water entering it where it is exposed on each side. Under such conditions a flowing artesian well cannot be drilled at or near the summit, but since the water is under pressure it will rise in the hole to a level approaching that of the lowest part of the outcrop of the porous layer on either side of the hill or mountain. This is essentially the condition of things toward the interior of Iowa, where water from the deeper wells rises 2,000 feet or more in the holes, but does not reach the surface. [Illustration: Fig. 21.--Section illustrating the thinning out of a porous water-bearing bed. A, inclosed between impervious beds B and C, thus furnishing the necessary conditions for an artesian fountain at D. (After U. S. Geological Survey.)] The drilling of deep wells, where records, including samples of rock materials brought up, have been kept, has been a great aid to the geologist in determining, or rendering more precise, the knowledge of not only the kinds of rocks underground, but also the thicknesses and structural relations of the formations.