On some beaches, in a high area and relatively far from the water at low tide, you sometimes walk on cotton wool and sink unusually deep in the sand. Although flood marks can often be seen from the last flood, unlike the seashore, there is no solid ground here.
On the soft part of the beach you can see many small holes and real pustules. If you carefully remove a handful of sand from the ground without crushing it, you catch a kind of sand foam; The structure combines with large and small cavities.
If one observes the action for several days, it becomes clear that tides are responsible for ventilation. At high tide, a larger area of a sloping beach is washed with water than at low tide.
In the sand of the sea, according to which shape no Linnaeus has yet arranged
(George Christoph Lichtenberg, 1742–1799)
The sand consists of irregularly shaped grains that touch only in some places. In the middle they form a coherent capillary system. If water enters from above, its weight displaces the air it contains. It cannot go downwards because the water table has already filled the empty spaces. So the rising tide moves the air upwards in the shape of a wedge (see illustration).
This keeps the gas under pressure as the narrow capillary opposes the flow. This is similar to what you feel with a disposable syringe when you squeeze water from the air bubble in it. It is greatly compressed by the piston. When exposed to such pressure, currents of fine air flow together in the sand as soon as they come in contact with each other. They eventually discharge to the surface. On the one hand, they appear as holes in the floor, on the other, small cone bulges if the upper layer contains airtight zones. Both structures shape the foam sand layer image and are exposed to a large area at low tide. If you then carefully cut the cone with a knife, you can convince yourself that there are actually cavities under the flakes.
When sea level rises, you can sometimes see how the rhythmically rising and falling waves fill the holes and then leave them again. You can see how bubbles come out of the flood hole directly in front of the rising water, where it is only a few centimeters deep.
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