What kind of stone?
Is it basically Sandstone / limestone? It was laid down under water and over time is compressed - the layers are caused by slight changes in the size of the sand grains and the direction of the water & currents. If mica gets in between the layers of sand there will be a tendency for the sandstone / limestone to sheer along the mica layer.
Around the edges just now - when water( & ice) get in it can spread into the whole slab. sandstone/limestone is quite soft. Slate is very hard (metamorphic (heat treated) fine clays but the mica between the slate make it split along the strata of the bedding layers.
Did a wee bit ( 2years) geology at Uni 50 years ago - hence a little knowledge but is a bit foggy - as is much of my knowledge these days.
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