The third type of dam is entirely of concrete or stone masonry, concrete to-day being preferable because more likely to be water-tight. The problem with a concrete dam is to get a foundation such that the impounded water will not leak out under the dam, imperiling the very existence of it. The ideal foundation, of course, is rock, and in a great many locations can be found in the small gulleys where the limestone and shale peculiar to this region will answer as well as more solid rock for dams not more than ten feet high; but with gravel banks on the sides or with soft sandy bottom, or where the clay soil becomes saturated with water at times, the gulley offers great difficulties for the construction of a dam. It will be wise, under such conditions, to carry a cut-off wall, not necessarily more than twelve inches thick, well into the bank, that is, about ten feet on each side, and under the dam this cut-off wall ought to go down until it reaches another stratum of sand or clay or rock. This cut-off wall, then, surrounding the main dam, shuts off the leakage, and the dam itself can be built without danger of undermining. In many large dams this cut-off wall is carried down more than a hundred feet, especially where the depth of water behind the dam is great. For small dams, a row of plank driven down behind a timber sill across and in the bed of the stream will often be sufficient. [Illustration: FIG. 40.--Section of a flood dam.] The cross section of the main dam, in cases where flood water in the spring runs over the dam, should be such that the bottom thickness is about one half the height, and Fig. 40 (after Wegman) shows a suitable cross-section of a dam ten feet high. Figure 41 (after Wegman) shows a cross-section intended to carry the water over the dam, especially in times of flood, without danger of erosion. Sometimes, in a narrow gorge with rock sides, it is possible to save masonry by building the dam in the form of an arch upstream, the resistance to the force of the water being then furnished by the abutment action of the rock sides, instead of by the weight of the dam, as in ordinary construction. For a dam ten feet high, the necessary thickness of the curved dam would probably not be more than twelve inches, while the ordinary gravity dam would be three or four feet thick. The workmanship on the former, however, must be of a very superior order. [Illustration: FIG. 41.--Section of a flood dam.]