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These boulders varied in size. This one is at least 2.5 m in diameter. It would require a 3 step ladder to stand on its top. Many were broken off at the top, leaving a flat space to sit on. Most had varying coloured lichens growing on the red rock.
Wikipedia describes these as:
"The most striking features of this landscape are the round reddish boulders. These are sandstone concretions and at up to 2.5 m in diameter, they are among the largest in the world. The boulders were formed in prehistoric seas as layers of sand, calcite and iron oxide collected around a nucleus formed by shells, leaves or bones. The concretions grew larger as the circulating waters deposited more layers. The reddish colour comes from iron oxide."