At low tide this small tidepool is a world apart.
The attached organisms have the small, sandy basin to themselves again. The sprinkling of shells on the rock are exposed to air between tides.
This jointed granite is on the famous Pink Granite Coast of northern Brittany. Sometimes the joints in the rock form regular rectangular shapes like this tidepool, but most of the time they are more abstract.
The coarse sand at the bottom of the tidepool settled out of the wave wash. The fine white sand lining the sides may have blown in at low tide to coat the sides as the water receded, like bathtub rings.
This tidepool is shaped like a window and gives us a view into a small world where these organisms spend their entire lives.
The gently sloping Breton shore exposes vast stretches of this granite at low tide. There is a nearly endless variety of sizes, shapes, and depths of tide pools.
Then as the tide flow returns each of these separate worlds rejoins the Atlantic Ocean and become just an irregularity on the bottom.