How is a stump formed?

A large crack in the cliff is created due to hydraulic action, which is when water gets into the cracks in cliffs, and the air pressure widens the crack. It's then widened into a cave by hydraulic action and abrasion. The cave continues to become larger until it breaks through the headland to make a natural arch. The arch is eroded and collapses, leaving a tall rock called a stack. The stack is eroded and turns into a stump.