Here's one of the Twin Rocks up close - now an arch after perhaps millions of years of oceanic wear and tear. Geologically speaking, twin sea stacks like these were usually connected at one point, and usually part of some larger basalt structure before that, which has been worn away. There was likely enough erosion to make an arch out of the larger structure, which eventually crumbled away to create two such sea stacks.

One day, there will probably be three stacks when this arch crumbles.