the friction makes would cause the surf
waves which roll in over a continental shelf to be very low, probably
in height less than half that which they now attain. In fact, however,
there is an influence at work to increase the height of the waves at
the expense of its width. Noting that the friction rapidly increases
with the shallowing, it is easy to see that this resistance is
greatest on the advancing front of the wave, and least on its seaward
side. The result is that the front moves more slowly than the rear, so
that the wave is forced to gain in height; but for the fact that the
total friction which the wave encounters takes away most of its
impetus, we might have combers a hundred feet high rolling upon the
shelving shores which almost everywhere face the seas.

As the wave shortens its width and gains in relative height, though
not in actual elevation, another action is introduced which has
momentous consequences. The water in the bottom of the wave is greatly
retarded in its ongoing by its friction over the sea floor, while the
upper part of the surge is much less affected in this way. The result
is that at a certain point in the advance, the place of which is
determined by the depth, the size, and the speed of the undulation,
the front swiftly steepens until it is vertical, and the top shoots
forward to a point where it is no longer supported by underlying
water, when it plunges down in what is called the surf or breaker. In
this part of the wave's work the application of the energy which it
transmits differs strikingly from the work previously done. Before the
wave breaks, the only geological task which it accomplishes is
effected by forcing materials up the slope, in which movement they are
slightly ground over each other until they come within the battering
zone of the shore, where they may be further divided by the action of
the mill which is there in operation.

When the wave breaks on the shore it operates in the following manner:
First, the overturning of its crest sends a great mass of water, it
may be from the height of ten or more feet, down upon the shore. Thus
falling water has not only the force due to its drop from the summit
of the wave, but it has a share of the impulse due to the velocity
with which the surge moved against the shore. It acts, in a word, like
a hammer swung down by a strong arm, where the blow represents not
only the force with which the weight would fall of itself, but the