The Grand Canyon's walls reach about 5,000 feet (about one mile) below the rim to the river. Each rock layer, except for a couple exceptions, represents a period when a particular environment prevailed. For example, the Kaibab Limestone, the uppermost rock formation at the Grand Canyon, formed in shallow warm seas about 250 million years ago. The Coconino Sandstone, the third strata from the top, formed at a time when dune fields, like those found in the Sahara, stretched across the western United States. Below these upper layers, the strata become progressively older and reveal different chapters in our earth's history. The oldest rocks at the Grand Canyon also happen to be amongst the oldest rocks exposed on our planet. Almost a vertical mile below the rim lay what is referred to as basement rocks. These formations comprised of Granite and Schist formed over 1.8 billion years ago when tremendous forces created an ancient mountain range equivalent to the present-day Himalayas.
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