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Tension gashes

Quartz-filled tension gashes are common features associated with semi-brittle deformation in the upper to-mid crust and you can find them in mountain areas across the UK. The ones show here are on the North Cornwall coast near Bude and are associated with the Variscan mountain building event (~290 million years ago) which produced the Pennines and Southern Uplands of Scotland. The compressive stresses during mountain building associated with collision of tectonic plates can cause fractures to open in brittle rocks, aligned parallel to the direction of compression. These are tension gashes. You often find groups of them them diagonally ligned up (en echelon) and kinked into "s" or "z" shapes. In this case the stresses were shear stresses and tension gashes continue grow parallel to the maximum compressive stress but once they form they are sheard and so rotate. Tension gashes are very good indicators of the directions of palaeostresses during crustal deformation events, and being filled by quartz, they often weather proud to provide grippy patches on otherwise slippery rocks.