Were you ever down by the beach when the wind was blowing in from the
open sea? Did you see the white-caps? Did you see the surf as one great
green wave after the other came marching in, and fell over with a
magnificent roar on the beach, while the salt spray filled all the air
with briny fragrance? If you have seen these splendid sights, you
remember how the waves seemed to march along in a great procession
before the strong wind. Strange sights are these, yet the way in which
the wind starts the waves and keeps them moving is stranger still.
If you go to Sandy Hook, or to Rockaway, or Coney Island by boat, you
will see some of these things. The wind blows upon the water, and as it
moves very easily, a part of the water is pushed up into little heaps by
the wind. If the wind is light, these tiny heaps are small, and soon
fall down again. When the wind ruffles the water in this way, we call it
a ripple. When the wind blows stronger, it pushes up more water, and we
call these heaps and ridges waves. As the wind keeps on blowing in the
same direction, the heaps rise and fall quickly, and the waves appear to
move along over the surface of the water in a great procession. It is
really only an appearance. The water does not move along, but only up
and down, as the motion started by the wind passes over the surface.
However, for our purposes, it is enough to describe things just as they
look.
Under the waves the water is calm and still. The huge billows that roll
over the sea in storms are seldom much more than fifteen feet high, and
they pass over the surface without disturbing the water beyond the depth
of a few feet. Every wave has a top, a middle part, and a bottom, or
lower part. When a wave coming in from the sea approaches the shore, the
bottom of the wave strikes the land first. The sand catches and holds it
back and makes it go slower. The top of the wave, not feeling this
friction against the ground, rushes forward, leaving the lower part
behind. As the wave comes nearer to the beach, the bottom part is held
back more and more, and the whole wave tips over. It pitches forward as
if tripped up, and the top rushes onward swiftly, while the lower part
lags behind. The crest, or upper edge, rises higher, for there is no
room for it all to pass, and it lifts up as if trying to stand upright.
The air gets caught under the crest of the wave in front, and in a
moment the wave, unable to rise any higher, falls flat on its face upon
the sand. The air caught under it bursts out with a roaring sound, and
escapes through the water in a million white bubbles that make the water
look like milky foam.
The white-caps you see upon the open water are made in the same way. The
wind seems to be impatient that the waves move so slowly, and it knocks
their caps off, and the poor waves seem to get very mad about it, and to
grow quite white in the face. The top of the wave tries to rush ahead of
the lower part, and tumbles over in the foaming water-fall the sailors
call a white-cap.
When a wave reaches the shore, something very curious happens. The
bottom of the wave strikes the ground first. The wave drags over the
sand as it passes on toward the beach, and draws some of the loose sand
after it. First, the smaller and lighter grains are rolled along or
lifted up and carried a short distance by the wave. As the water grows
more shallow, the wave scrapes and drags over the sand, and the larger
grains and even small pebbles are rolled along after the lighter sand.
But the wave must go slower here, and thus it lets go its hold and drops
its load. When it has passed, the sand, that may have been level before,
is raised into a low ridge or windrow. The smaller and lighter grains,
being carried farthest, are dropped in one place, and the heavier grains
and small pebbles are dropped in another place.
The next wave may stir up and drag along more sand, and lay it down, all
sorted out, on the ridge. Other waves may follow, and do the same thing,
and so the heap begins to grow: the baby sand-bar has been born. It may
have been a mere trifle that started it just there--a crab or the bones
of a dead fish, some gravel dropped from a piece of melting ice, a stray
bit of sea-weed. No matter what it was, or how trifling the obstruction,
the loose sand rolled along by the wave caught just there, and was left
behind; the next wave left a little more, and each in turn added to the
heap.
Waves are very irregular in size, and perhaps some big fellow may lift
up more sand than he can carry, and may drop it all in one place. Then
for some time the weather may be pleasant, and the tiny ridge, perhaps
not a quarter of an inch high, and twenty feet wide, may rest awhile.
Then a storm comes, with large waves, and when they meet this slight
obstruction they go over it more slowly, and drop part of their loads
upon it. So it may grow very fast in a single day. In front, toward the
sea, the sand will be scooped out in long trenches, and behind it will
be a stretch of deeper and smoother water. After that every wave that
comes in stumbles and appears to trip just there, and there are
white-caps over that spot even in pleasant weather. When the smooth
swelling rollers are coming in from the sea, they appear to be angry
every time they strike their feet on the hidden bar, and they tumble
over with a roar, and show a white feather of foam in their caps.
The sand-bar, once started, never stops growing or changing. It grows
wider and higher, or it changes its shape, twisting about in the
strangest manner. Smaller bars spring up upon it, or disappear only to
grow up in another place. At last, some spring day, when the tides run
low, the bar appears above the water. Strange things have happened to
it. The fish have made it their home, lively crabs scamper about on the
wet sand, and thousands of clams find a snug resting-place there.
One day last summer I found one of these young sand-bars cast up by a
storm at the eastern point of Coney Island, near the inlet at the end of
the Marine Railway. It was so strange that I took a shot at it with my
camera, and here it is. It is a very small affair, and you may not be
able to find it next summer, for I dare say the next storm tore it all
to pieces, or carried it away and put it somewhere else.
[Illustration: FINGERS OF THE SEA.]
You see the long, low heap of sand thrown up by the waves. Beyond is the
sea looking toward Rockaway. Behind the bar is a long pool of still
water, and you can see how the waves, in pushing the sand forward, drove
it into the pool in long fingers, or capes. See the sea-weed and
rubbish thrown up by the surf. It is all sorted out, the larger pieces
at the top, and the smaller bits trailing along toward the pool. All the
light sand is arranged by itself next the pool of still water. This bar
was thrown up on top of the beach by a storm, yet it serves to show how
the sand-bars made under water look. Even on shore you can hear the bars
roaring and moaning all day and night, as the great work of the sea goes
on, never stopping, never hurrying, for centuries after centuries.