At the Yarrow dam site it was necessary to go 97 ft. below the original surface to obtain a satisfactory formation or one that was impermeable. A central trench was excavated to bed rock, parallel to the axis of the dam, and filled with clay puddle to form a water-tight connection with the rock, and prevent the water in the reservoir from passing through the porous materials under the body of the embankment. This interesting dam will be more fully described later, when the different types of earth dams are discussed. CHAPTER III. _Outline Study of Soils. Puddle._ The following study of soils is merely suggestive and is here given to emphasize the importance of the subject, at the risk of being considered a digression. Soil formations are made in one of three ways: 1. By decomposition of exposed rocks. 2. By transportation or sedimentation of fine and coarse materials worn from rocks. 3. By transformation into humus of decayed organic matter. The transforming agencies by which soils succeed rocks in geological progression have been classified as follows: 1. Changes of temperature. 2. Water. 3. Air. 4. Organic life. _Heat_ and its counter agent frost are the most powerful forces in nature, their sensible physical effects being the expansion and contraction of matter. _Water_ has two modes of action, physical and chemical. This agent is the great destroyer of the important forces, cohesion and friction. _Cohesion_ is a force uniting particles of matter and resists their separation when the motion attempted is perpendicular to the plane of contact. _Friction_ is a force resisting the separation of surfaces when motion is attempted which produces sliding. The hydrostatic pressure and resultant effect upon submerged surfaces need to be kept constantly in mind. When the surface is impermeable the line of pressure is normal to its plane, but when once saturated there are also horizontal and vertical lines of pressure. Since the strength of an earth dam depends upon two factors, namely, its weight and frictional resistance to sliding, the effect of water upon different materials entering into an earth structure should be most carefully considered. This will therefore occupy a large place in these pages. An earth embankment founded upon rock may become saturated by water forced up into it from below through cracks and fissures, reducing its lower stratum to a state of muddy sludge, on which the upper part, however sound in itself, would slide. The best preliminary step to take in such a case is to intersect the whole site with wide, dry, stone drains, their depths varying according to the nature of the ground or rock. _Air_ contains two ingredients ever active in the process of decomposition, carbonic acid and oxygen. _Organic Life_ accomplishes its decomposing effect both by physical and chemical means. The effect of organic matter upon the mineral ingredients of the soil may be stated as follows: