Here's why:

* Erosion is the process of wearing away and transporting rock and soil.

* Deposition is the process of dropping off sediment in a new location.

Sea caves are formed when waves repeatedly crash against a cliff face. The force of the water, combined with the abrasive action of sand and other debris carried by the waves, gradually wears away the rock, creating a hollow space. This is a clear example of erosion.

While deposition might occur *within* a sea cave (like sand accumulating on the floor), the cave itself is a result of erosion.