Gneiss with boudinage structure near Lavertezzo, Ticino, Switzerland. The blocks were scoured and shifted during flood events. The rocks originally were sedimentary, then became metamorphic at great depth within the Earth's crust during the collision of continents, and were finally uplifted and exposed during the formation of the Alps. The dark, sausage-shaped structure in the foreground represents boudinage, structures formed by extension, where a rigid tabular block of dark amphibolite, is stretched and deformed amidst less competent surrounding material.