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Steep, grayish (sometimes draped in green shrubbery) walls of Precambrian-age (~ 1.8 billion year old) metamorphic and igneous rock form flank the blue-gray Gunnison River, which flows along the bottom at Black Canyon of the Gunnison National Park in western Colorado, USA. Above the canyon, outcrops of the Upper Jurassic (~150 million year old) Morrison Formation and Upper Cretaceous (~85 million year old) Mancos Shale form low, rounded hills beneath a blue sky dotted with white clouds.