Ballachulish slate was quarried for its clean cleavage which made it a good roofing material as it can be split into thin sheets. Like most slate it isn't known for its great strength or resistance to wear and tear, so why does Ballachulish Slate form the clean arete of Schoolhouse Ridge?
You might expect slate to form low-lying ground rather than rugged ridgelines, and most slate regions, such as the Scottish borders around Moffatt or mid-Wales, have gently rolling hills. Blencathra and Skiddaw in the Lake District are another pair of anomalies - formed of Skiddaw Slate.
The cause of these slate mountains is the same in each case - contact metamorphism. In each case, there is a granite intrusion lying just below the surface. The heat given off by the cooling granite was sufficient to transform the weak platy clay and mica minerals into stronger high-temperature minerals. In addition to the heat, fluids released during crystallisation enhance reaction rates, and granitic magmas are particularly fluid rich, meaning that you get large halos of metamorphism (or aureoles) around granites.
See also:
Thermal metamorphism of Pyrite, North Ballachulish.
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Contact metamorphism and deformation at Syob Ban.
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Thernal metamorphism in xenolith on Am Bodach
