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boulder balanced on a pinnacle rock, another boulder, or in some other precarious position. Some perched rocks form in place, as where rainwash (and in some cases wind) has removed fine material from around the boulder. Others may be transported by tectonic forces (involved in deformation of the Earth's crust) or by ice (such as erratics, or glacier transports) and let down to an unsettled position. Perched rocks commonly have a hard capping, such as ferruginous duricrust, and they often show crumbling or exudation around their sides.