Stack – An isolated pillar of rock left when the top of an arch has collapsed. Over time further erosion reduces the stack to a smaller, lower stump

A stack of money looks like a coastal stack.

The best way to learn what an arch is is to see how an arch develops.

A stack looks like a tall stone tower separated from the main coastline. It can take thousands or even millions of years for a stack to form, but the process is always as illustrated above.

See hydraulic power for details of how hydraulic action has the power to break through rocks.