Nearly two billion years of the Earth's geological history have been exposed as the Colorado River and its tributaries cut their channels through layer after layer of rock while the Colorado Plateau was uplifted.
While the specific geologic processes and timing that formed the Grand Canyon are the subject of debate by geologists, recent evidence suggests it was at least 17 million years ago. Since that time, the Colorado River continued to erode and form the canyon to its present-day configuration.
Before European immigration, the area was inhabited by Native Americans who built settlements within the canyon and its many caves. The Pueblo people considered the Grand Canyon (or Ongtupqa) a holy site and made pilgrimages to it. The first European known to have viewed the Grand Canyon was García López de Cárdenas from Spain, who arrived in 1540, but afterwards none visited for over two hundred years.
The park was founded as Grand Canyon National Monument in 1908 by President Theodore Roosevelt, and became a National Park in 1919. Today the park contains over 1.2 million acres (490,000 ha), slightly less than the entire state of Delaware, and sees about five million visitors a year.