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Defined by a very strong dimensional-preferred orientation of clay minerals (phillosilicates) in a very clay-rich sedimentary rock. This very closely spaced cleavage is a characteristic of slates. The cleavage is essentially continuous as viewed at a hand sample scale. It forms at relatively low metamorphic grades due to (i) rotation of phyllosilicates; (ii) alignment of newly grown phyllosilicates and (iii) very slight recrystallization. The photograph if of slaty cleavage in Ordovician slates from Rhosneigr, Anglesey, north Wales, the very locality that led to research on dilation computation using reduction spots by Dennis Wood. The cleavage is not very strong as this area is on the flank of the Caledonian orogenic belt and therefore the imprint of Caledonian orogenic movement