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| A mixed forest of Ponderosa Pine, Gambel oak, pinyon pine, and juniper cover the slopes in Walnut Canyon. Three sedimentary rock formations crop out in the canyon. The oldest, Coconino Sandstone, forms the steep inner gorge of the canyon. This is overlain by the Toroweap Formation that underlies the forested slope midway up the canyon slopes. The upper third of the canyon walls consists of Kaibab Limestone. These rock formations accumulated in shallow seas that covered this region in the late Paleozoic Era (in the Early Permian Period, about 270 to 275 million years ago; Mathis and Bowman, 2005). Structural folding in the region during the Laramide Orogeny began about 70 million years ago at the end of the Cretaceous Period. Periods of uplift followed into the late Tertiary when regional uplift of the Colorado Plateau took place and volcanism associated with the San Francisco Volcanic Field began around 6 million years ago (NPS, 2008; Foos, 1999; Priest and others, 2001). Erosional downcutting of Walnut Canyon and other canyons occurred over the last several million years and is still proceeding. |
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