Monument Valley, Arizona | The Canyon De Chelly sandstone makes up most of the monuments in Monument Valley National Monument. The underlying Organ Rock shale gives away to erosion more easily than its overlying sandstone.
D.L. Baars, 1973, Permianland: The rocks of Monument Valley, in New Mexico Geological Society 24th Annual Fall Field Conference Guidebook. https://nmgs.nmt.edu/publications/guidebooks/downloads/24/24_p0068_p0071...