The ancestral Colorado River responded by cutting its channel faster and deeper. Wetter conditions during ice ages also increased the amount of water in the Colorado River drainage system. The result of all this erosion is one of the most complete geologic columns on the planet. The Colorado River basin (of which the Grand Canyon is a part) has developed in the past 40 million years and the Grand Canyon itself is probably less than five to six million years old (with most of the downcutting occurring in the last two million years). This uplift has steepened the stream gradient of the Colorado River and its tributaries, which in turn has increased their speed and thus their ability to cut through rock (see the elevation summary of the Colorado River for present conditions). The great depth of the Grand Canyon and especially the height of its strata (most of which formed below sea level) can be attributed to 5000 to 10,000 feet (1500 to 3000 m) of uplift of the Colorado Plateaus starting about 65 million years ago (during the Laramide Orogeny). Major exceptions include the Cococino Sandstone which was laid down as sand dunes in a desert and several parts of the Supai formation. Many of the formations were deposited in warm shallow seas, near-shore environments (such as beaches), and swamps as the seashore repeatedly advanced and retreated over the edge of a proto- North America. The major geologic exposures in Grand Canyon range in age from the 2000 million year old Vishnu Schist at the bottom of the Inner Gorge to the 230 million year old Kaibab Limestone on the Rim. There is no authoritative theory on the formation of Grand Canyon. According to Geologist Wayne Ranney: "To date, geologists have been unable to determine the canyon's precise age and what specific processes were at work in carving it". Geologists continue to debate ideas about the formation of Grand Canyon. The details of the canyon's formation are still highly controversial.
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