Green schist

Typically deep ocean sediments are fine grained oozes – mud in other words – destined to become mudstone or shale, but when trapped in a subduction zone and subjected to much higher pressures they become metamorphosed to schist. The lowest grade of metamorphism is green schist, where clay mineral are recrystallised to chlorite and any bedding is lost, but the rock develops a foliation [“schistosity”] which is often highly contorted.

Several rock units on Anglesey are of green schist facies but the New Harbour beds are by far the most contorted. Seen in many places on Ynys Gybi [Holy Island] perhaps the best is where they take their name from, the area where the Breakwater joins to the land.

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