This is covered by:

 Eduqas A,  OCR B, WJEC

Rock Pools (Tidal Pools)

Rock pools are an erosional feature found in the inter-tidal  zone; thats the area between hightide and lowtide.  They are formed by the abrasion of joints, fractures and weaker areas of horizontal rock surfaces that are exposed on the beach.

As the waves move over the rock surface it experiences grinding from the sand and pebbles picked up by the waves, and the softer parts of the rock erode faster than the harder parts. A pebble may become trapped in the small eroded pocket and be swirled around by wave action , grinding away at the edges of the hole, widening and deepening it.

The resulting holes are filled with sea water at high tide, and retain it as the tide goes out.  Each time the tide comes back in the pools are abraded a little more and re-filled with fresh sea water.