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 consist of Kaibab Limestone. These rock formations accumulated in shallow seas that covered this region in the late Paleozoic Era (in the Late Pennsylvanian and Early Permian periods, between about 300 to 250 million years ago. The uplift of the Colorado Plateau began about 65 million years ago at the beginning of the Tertiary Period. Periods of uplift followed into the late Tertiary when volcanism associated with the San Francisco Volcanic Field began around 6 million years ago. Erosional downcutting of Walnut Canyon and other canyons occurred over the last several million years and is still proceeding.