On some beaches, in the highland area and relatively far from the water at low tide, you sometimes walk like cotton wool and submerge in the sand unusually. Although it is still often possible to see traces of a flood from the last flood, unlike the sea’s edge, there is no solid ground here.
You can see many small holes and real pimples on the smooth part of the beach. If you carefully remove a handful of sand from the ground without crushing it, then you hold a kind of sand foam; The structure is punctuated by large and small cavities.
If one observes the action over the course of several days, it becomes clear that tides are responsible for the ventilation. At high tide, a much larger area of the steep shore is gently washed by the water compared to the ebb and flow.
In the sands of the sea, which Linnaeus has not yet arranged according to their shapes
(George Christoph Lichtenberg, 1742-1799)
Sand is made of irregularly shaped grains that only touch in a few places. Between them they form a coherent capillary system. If water penetrates from above, its weight displaces the air it contains. It cannot move down because the water table has already filled the empty spaces there. So the height of the tide is pushing the air up in a wedge-shaped form (see illustration).
This puts the gas under pressure because the narrow capillaries oppose the flow. This is similar to how it feels with a disposable syringe when you squeeze out water with an air bubble inside. It is tightly compressed by the piston. When exposed to such pressure, fine streams of air flow together in the sand once they come in contact with each other. Finally it is discharged to the surface. On the one hand, they appear as holes in the floor, and on the other hand, small cones swell if the top layer contains airtight areas. Both structures form a layer of foamed sand and are exposed over a large area at low tide. If you then carefully cut the cones with a knife, you can convince yourself that there are already cavities under the blisters.
When the sea level rises, you can still sometimes see how regularly rising and falling waves flood holes and then release them again. You can see how the bubbles emerge from flooded holes directly behind the ascending water front, only a few centimeters deep.