Let's look at some more things that aren't flat.
Large paleovalleys carved into the underlying Redwall Limestone developed through dissolution i.e. karstification, and likely were enlarged by west-flowing streams. --- Timons and Karlstrom (eds.), Grand Canyon Geology, Geological Society of America, 2012.
Sink holes, caverns, and solution cracks common in upper parts of the Redwall limestone are in places partly or entirely filled with red mudstone accumulated during deposition of the overlying Supai formation. --- E. D. McKee, U.S. Geological Survey, "The Redwall Limestone", Ninth Field Conference of the New Mexico Geological Society
The top of the Mississippian Redwall limestone in the Grand Canyon area was subject to extensive karstification during a period of about 30 million years from the late Meramacian to early Morrowan time. This hiatus has recently been shown to be much shorter, possibly only 5 million years, in the western Grand Canyon where tidal and deltaic channels draining westward toward the retreating sea are eroded into the Redwall surface. These channels have average depths of about 107 m (350 ft). --- T. Troutman, University of Texas at Austin, "Genesis, Paleoenvironment, and Paleogeomorphology of the Mississippian Redwall Limestone Paleokarst, Hualapai Indian Reservation, Grand Canyon Area", Cave Research Foundation Newsletter vol. 29 no. 1, 2001.