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Jon Burch Photography
Photograph - Digital Capture
The red rocks of Sedona show layers of time in the rock strata. This view is from one of the pull off's near Bell Rock in Oak Creek. Located just at the base of the Mogollon Rim, the red rocks form an escarpment that runs east-west through the middle of Arizona and defines the boundary between the Colorado Plateau to the north, and the Basin and Range to the south. The Mogollon Rim is about 200 miles long, and ranges between 2000 and 3000 feet in height. But in the Sedona region, erosion has gradually eaten away at the rim, moving it northward a distance of about four miles and leaving behind some of the most spectacular and picturesque canyons and buttes found anywhere in the world.
The deep red color for which Sedona is famous is due to the presence of hematite or iron oxide, staining the sandstone of Schnebly Hill and Hermit Shale layers. The steep terrain is due the top layers of the strata being composed of basalt and limestone, which are both harder than the underlying sandstone. Water running off the edge of the escarpment dissolves the lower layers, creating the steep cliffs. Eventually enough soft material is broken down and eroded, undercutting the cap layer, subsequently breaking it off in large slabs allowing it to fall into the canyons. This process exposes new softer material and the process repeats, with the cliff face about twenty feet further north than it was before.
The red rocks themselves were formed by a layer of rock known as the Schnebly Hill Formation. Schnebly Hill is a thick layer of red to orange-colored sandstone, and a member of the Supai Group, which was deposited during the Permian Period about 299 to 251 million years ago. Approximately 800 to 1000 feet thick, Schnebly Hill is the major component of Sedona's well known "Red Rocks" visible in the area.
The Schnebly Hill formation is comprised of the Sycamore Pass and Bell Rock members, which are separated by a ten to twelve foot thick layer of grey-colored limestone called the Fort Apache Member, formed during a major incursion of an ancient Sea. There are no fossils in the Schnebly Hill sandstones, and virtually none in the thin limestone layers.
Photograph copyright Jon Burch Photography
March 5th, 2013
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