About 300 million years ago, rushing streams pulled gravel and boulders from the nearby Caledonia Mountains. When the material reached ground level, the water slowed down and the gravel sank.
Over millions of years, the pebbles, boulders, and finer materials compressed - got pushed together - and became hard, like cement. This kind of rock is called conglomerate. The cliffs and pillars here are made of conglomerate rock with layers of other rock - shale and sandstone. (See photo below.)
As more years passed, the entire region was lifted up and tilted, which caused the rocks to crack - or to fracture - and become slightly separated into blocks.
Then continental glaciers moved in. When they started melting, the water made the cracks bigger. Next, the sea helped isolate the blocky pillars. As the sea water kept coming in and out with the tides, the bases of the pillars got smaller and the first ``flowerpots'' were formed.
Some of the old flowerpots have toppled, but as waves wear away the base of the cliff (making sea caves), eventually the blocks of rock become separated, and new flowerpots form.