Gorge in northwestern Arizona, USA, containing the Colorado River. It is 350 km/217 mi long, 6–29 km/4–18 mi wide, and reaches depths of over 1.7 km/1.1 mi. The gorge cuts through a multicoloured series of rocks – mainly limestones, sandstones, and shales, and ranging in age from the Precambrian to the Cretaceous – and various harder strata stand out as steps on its slopes. It is one of the country's most popular national parks and around 5 million tourists visit it each year. Protected since 1893, and accessible by rail since 1901, the Grand Canyon was made a national monument in 1908, a national park in 1919, and a World Heritage Site in 1979. Most visitors approach the canyon via the more accessible South Rim; the North Rim is around 300–500 m/1,000–1,500 ft higher, and is closed during the winter. The national park has an area of 4,931 sq km/1,904 sq mi, and is bounded by Glen Canyon to the east and Lake Mead to the west. On its way through the canyon the Colorado River drops 670…