The great cliffs of lower Zion Canyon consist of Navajo Sandstone. The cliffs form from the gradual erosion and undercutting of shale beds within the Kayenta Formation. When the shale erodes away beneath the sandstone cliffs great blocks of the overlying Navajo sandstone can splinter away along vertical fractures and collapse into the valley. This view is from an overlook near the South Entrance. The Virgin River runs along the edge of the cottonwood grove in the foreground.