From the World Heritage inscription:
Carved out by the Colorado River, the Grand Canyon (nearly 1,500 m deep) is the most spectacular gorge in the world. Located in the state of Arizona, it cuts across the Grand Canyon National Park. Its horizontal strata retrace the geological history of the past 2 billion years. There are also prehistoric traces of human adaptation to a particularly harsh environment.
The Grand Canyon is one of the greatest natural attractions on Earth. The colors of the rocks change color through the course of the day, culminating in a spectacular sunset.