A dirt dam is not generally desirable, since in most cases the dam must also be used as a waste weir; that is, the freshets must run over the dam. This means that unless the crest of the dam is protected with timber or masonry the dam will be washed out; as happened, indeed, in the terrible flood at Johnstown, Pennsylvania, several years ago. If it is possible to carry the overflow water of the stream away in some other channel than over the dam, then a dirt dam is not objectionable, although always a dirt dam is best with a masonry core. A very good dam can be made by driving three-inch tongue-and-grooved planking tight together across a gulley and then filling in on each side so that the slope on each face is at least two feet horizontal for every foot in height. This last requirement means that if the dam is ten feet high, the width of the dam at the base shall be at least forty-five feet, the other five feet being required to give the proper thickness to the dam at the top. [Illustration: FIG. 39.--Concrete core in a dam.] In the second type of dam this central timber core is replaced with a thin wall of concrete as shown in Fig. 39, from six to twelve inches thick, sufficing to prevent small animals burrowing through the dam and at the same time to make the dam more nearly water-tight. Sometimes stone masonry is used, building a light wall to serve as the true dam, and then holding up this light wall with earth-filling on each side. If neither plank, stone, nor concrete can be used, the central core is made of the best earth available, a mixture of clay and sand preferably, and special pains are taken in the building to have this mixture well rammed and compacted. The writer has recently heard of a dam on a small stream being made by the continual dumping of field stone from the farm into the brook at a certain definite place. This stone, of course, assumed a slope at each side and settled in place from year to year as the dam grew. The mud and silt of the stream filled up the holes between the stones, so that the dam was finally practically water-tight. This made a cheap construction and had the additional value of serving to use up stones from the fields. It was necessary, since the spring floods poured over the top of this dam, to protect the top stones, and a plank crest was put on, merely to keep the dam from being washed away.