FIG. 2.–CROSS-SECTION OF YARROW DAM.

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: