Page:Popular Science Monthly Volume 23.djvu/554

538 fully twice as great as the landward slope, and possesses some decidedly interesting characteristics. In the first place, it is singularly regular, varying but a very few degrees throughout its entire length; secondly, it is almost exactly the angle that such a mass of pebbles

would take if uninfluenced by the action of the waves. Constant washing has adjusted it precisely as if the pile had been left to its own conditions of equilibrium.

The general angle of the upper terrace is 29°. It has undoubtedly been lessened by travel, and by the fact that it is out of reach of any but the higher waves, and the constant tendency, therefore, is toward undermining it. This terrace is composed of rather large stones, quite unmixed with sand or gravel.

The second terrace is steeper, 32° nearly. It is exposed to the high tides, and its surface is made up of rather small stones, distributed with great regularity, still unmixed with gravel, but compacted by the waves to a remarkable degree of solidity.

The lower terrace has nearly the same angle as the middle one, but its composition is very different. The pebbles are of small size, mixed with much coarse gravel and sand, and pounded by the constant washing of every tide into a hard, smooth, regular slope that effectually resists any attempt at undermining.

The general angle of the wall as a whole is within a few minutes of 30°, as nearly as could be determined. Owing to the rough instrument employed—a simple arrangement of plumb-line and semicircular protractor the angles could only be measured to the nearest half degree, but the number of measurements taken leaves little doubt as to the accuracy of the result.

The size of the stones in some parts of the formation may give an idea of the tremendous force of the rushing waves that produced it. Stones of from twenty to thirty pounds in weight are common along the crest of the wall, fifteen feet at least above high-water mark. And not only this, but stones of the same size have been thrown completely over the wall into the road, more than a hundred feet from the