Slate belts are usually found in mountainous areas where geological forces have deformed and shortened the earth’s crust. Rocks respond differently to these compressive forces; some such as quartzite respond by folding while other weaker rocks, such as mudstone, develop a cleavage perpendicular to the direction of maximum stress. Under the microscope, it can be seen that this cleavage consists of individual grains of quartz, which have been flattened, separated by platey minerals (phyllosilicates) white mica and chlorite. It is these zones of platey minerals, called cleavage domains, which enable the rock to be split into slabs suitable as roofing slates.
The thickness of roofing slates varies from 3mm to over 15mm depending on the spacing of the cleavage domains present. Shape too affects the ability to cleave the rock; the straighter and more continuous the cleavage domains, the more easily the rock is split Other factors such as the proportion of rock occupied by the cleavage domains also , affects the ability to cleave into slates.