Rock arches are rock formations formed by erosion in the shape of great arches or semi-circles. These formations are typically found in the badlands and deserts (where they are formed by wind erosion) or along the coast (where they are formed by water erosion). Those along the coast may collapse and form free-standing sea stacks. Fictional rock arches may form the entrance into a holy site, town, or other place. They frequently are used as terrain obstacles in a race course where skilled pilots or sailors must successfully navigate.