The greatest concentration of natural stone arches in the US is found in this rugged 120sq mi park. An arch is a geological formation which resembles a bridge, formed by the erosion of a cliff by wind or water. More than 2 000 arches have been counted within this park's serpentine network of multicoloured canyons, distinguished by slender spires and improbably balanced rocks. Most of the sites are at their most beautiful in the evening, when the light turns the sandstone a reddish hue.