The great depth of the Grand Canyon and especially the height of its strata (most of which formed below sea level) was caused by 5,000 to 10,000 feet (1500 to 3000 m) uplift of the Colorado Plateau. This started about 65 million years ago during the Laramide period of orogeny (mountain-building). This uplift steepened the gradient of the Colorado River and its tributaries, which increased their speed and their ability to cut through rock.