flatiron

A triangular slab of rock tilted steeply against a mountainside, its broad base at the bottom and its point at the top — resembling the shape of an old flat iron used for pressing clothes. Flatirons form when a resistant layer of rock (usually sandstone) is tilted steeply by tectonic uplift and then eroded into isolated triangular remnants. The Flatirons above Boulder, Colorado, are the iconic example.