The first stage of erosion is the formation of gullies where the cliff is weakest, leaving spurs where the rock is more durable.
Sometimes the gullies continue as caves at the rear, and if the roof of the cave falls in, it can form a blow-hole where spray can shoot out in bad weather.
More often, caves form in the side of the spurs.
These caves soon break through to form arches.
Finally, the top of the arch falls, leaving a stack
The photos are from Selwicks Bay or Green Stack, both at Flamborough Head
the process is illustrated below
click for a bigger picture