Aerial photograph of dykes in rocks of the Val Forno, Grison Alps, Switzerland. Rock breaks during orogeny due to the enormous forces which act on the hard layers and rock masses. Cracks open several hundreds and thousand of meters underground. Hot fluids circulating in the depths of the Earth fill the cracks and crystallise when the rock formation is lifted during the orogeny and cools down. During erosion, the cracks appear as bright dykes in the dark grey and brown rock.