The boulders are known to geologists as septite concretions. They are born out of the mudstone cliffs, some emerging quite high up, as the action of the weather erodes the soft material.
They were formed when this cliff was deep under the sea, as mud and calcite hardened around a central core, moulded to uniformity by the constant movement of sediment from molluscs and other crustaceans. The concrete mud cracks and secondary calcite seeps in. The different materials form veins through the mud.
The sediment was then lifted up in tectonic action, forming the low cliffs. No one knows how many more of these round boulders have yet to appear.