<P> 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 . Major exceptions include the Permian Coconino Sandstone, which contains abundant geological evidence of aeolian sand dune deposition . Several parts of the Supai Group also were deposited in non--marine environments . </P> <P> 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 5--10 thousand feet (1,500 to 3,000 m) of uplift of the Colorado Plateau, starting about 65 million years ago (during the Laramide Orogeny). 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). </P> <P> Weather conditions during the ice ages also increased the amount of water in the Colorado River drainage system . The ancestral Colorado River responded by cutting its channel faster and deeper . </P> <P> The base level and course of the Colorado River (or its ancestral equivalent) changed 5.3 million years ago when the Gulf of California opened and lowered the river's base level (its lowest point). This increased the rate of erosion and cut nearly all of the Grand Canyon's current depth by 1.2 million years ago . The terraced walls of the canyon were created by differential erosion . </P>

Map showing the location of the grand canyon