Скачать или смотреть Crenulation cleavage in schist on the Mamores

The Dalradian (around 700 Ma old) schists in the Mamores saw a lot of deformation during the Caledonian (450 Ma ago) mountain-building event. First, they were buried and compressed which caused the platy mica minerals to grow to quite a large size and to align perpendicular to the main compressive stress. Then the stress regime changed and the platy foliation in the rock was tightly folded to start to develop a new orientation of cleavage. This form of secondary cleavage produced by folding of primary foliation is called crenulation cleavage and is an important indicator of changing stress orientations. When the process is complete it can be hard to identify, requiring microscope analysis - but here the crenulated folding is very clear.