Flowing wells[121] depend upon certain relations of rock structure, water supply, and elevation. Generally speaking a flowing well is possible in any place underlain by any considerable bed of porous rock, if such rock outcrops at a sufficiently higher level in a region of adequate rainfall, and is covered by a layer or bed of impervious, or relatively impervious rock. This statement involves four conditions, all of which are illustrated by Fig. 199, where _a_ is the bed of porous rock. It is not necessary that the beds of rock form a structural basin, nor is it usually necessary to take account of the character of the rock beneath the porous bed which contains the water. The bed of porous rock is the “reservoir” of the flowing well. Formations of sand or sandstone, and of gravel or conglomerate, most commonly serve as the reservoirs. In order that it may contain abundant water it must have some thickness, and its outcropping edge must be so situated that the water may enter freely and be replenished, chiefly by rain, as the water flows out at the well. A relatively impervious layer of rock above the reservoir (_b_, Fig. 199) is most important; otherwise the water in the reservoir will leak out, and there will be little or no “head” at the well site. Thus if the rock overlying stratum _a_ (Fig. 199) were badly broken, the fractures extending up to the surface, the conditions would be unfavorable for flowing wells. Under such conditions, wells in the positions of those shown in Fig. 199 might get abundant water, but they would not be likely to flow. If the stratum next below the reservoir is not impervious, some lower one probably is. No layer of rock is more impervious than one which is full of water, and the substructure of any bed which might serve as a reservoir is usually full of water, even if the rock be porous. If the outcrop of the reservoir be notably above the site of the well, and if it be kept full by frequent rains, the “head” will be strong, though the water at the well will not rise to the level of the outcrop of the reservoir. Experience has shown that an allowance of about one foot per mile of subterranean flow should be made. Thus if the site of the well be 100 miles from the outcrop of the water-bearing stratum, and 200 feet below it, the water will rise something like 100 feet above the surface at the well. This rule is, however, not applicable everywhere. The failure of the water to rise to the level of its head is due to the adhesion and the friction of flow through the rock. The more porous the rock the less the reduction of head by friction. The height of the flow is also influenced by the number of wells drawing on the same reservoir, on the degree of imperviousness of the confining bed above, etc. Flowing wells, often relatively shallow, are frequently obtained from unconsolidated drift. Some such relations as suggested by Fig. 220 would afford the conditions for flowing wells in such a formation.