Sedona is located just at the base of the Mogollon Rim, 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. 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 (iron oxide, otherwise known as rust) that stains the sandstone of the Schnebly Hill and Hermit Shale layers. The steepness of the terrain is due the fact that the top layers of the strata are composed of basalt and limestone, which are harder than the underlying sandstone. Water running off the edge of the escarpment eats away at the lower layers, creating the shear cliffs. Eventually enough soft material is weatherd away that it undercuts the cap layer, which subsequently breaks off in large slabs and falls into the canyons. This exposes new soft material and the process starts again, with the cliff face now twenty-odd feet further north than it was before.

Figure 1 illustrates the many layers that underly the Mogollon Rim. Not all of these layers are present in Sedona: Most of the geological features in that region range as high as the Toroweap Sandstone (and in some cases, the Kaibab Limestone), and as low as the Hermit Shale layer. Some locations in and around Sedona contain sinkholes, which are presumably due to disolution of the thick Red Wall Limestone layer, causing the higher Supai Sandstone and Hermit Shale to collapse into the void.

Basalt Cap

Much of the Colorado Plateau in the vicintity of Sedona, Flagstaff, and the Grand Canyon is topped with basalt (lava) from volcanos such as the House Mountain Shield volcano in the Sedona village of Oak Creek. This layer (technicaly a series of layers, as it was formed during multiple eruptions) is 6 to 15 million years old, and can be up to 400 feet thick in some areas. It is no longer present in and around Sedona. There are no fossils in this layer.

Rim Gravels

This layer is formed by the erosion of additional layers that once topped the region. It is 15 to 20 million years old, and can be as thick as 300 feet. It is not present in the Sedona region.

Moenkopi Formation

This layer consists of Sandstone, siltstone and mudstone deposited in what were once tidal flat areas. It is about 20 million years old, and up to 400 feet thick. It is not present in Sedona.

Kaibab Limestone

This strata is a hard, erosion resistant limestone that is cream or grayish white in color, formed 240 to 260 million years ago from the remains of aquatic sponges. This is the highest layer present with any regularity in the Sedona area, and can be up to 300 feet thick, although sometimes even this is completely worn away. It is the same layer as found at the south rim of the Grand Canyon, and contains fossils of brachiopods, mollusks, and fish.

Toroweap Formation

The Toroweap is mixture of rock varieties, including sandstone, limestone, gypsum, mudstone and dolmite. 200 to 300 feet thick in most places, the various sub-layers were formed through the periodic advancment and retreat of a shallow sea 262 million years ago. The limestone layers represent times when the sea was relatively deep, while the sandstone and mudstone represent times when the region was either above sea level or covered with shallow water. In some places (though not near Sedona) the Toroweap contains salt and gypsum deposits, which are formed by the evaporation of salt water. Fossils in the Toroweap are similar to, though somewhat less abundant than, those found in the Kaibab, and include shellfish, brachiopods, crinoids, snails, scaphopods and clams.

The Toroweap is similar in color to the Kaibab limestone above it, though it is usually a little darker (yellow to gray). It tends to erode into an angled slope, in contrast to both the Kaibab (above) and Coconino (below), which frequently form near-vertical cliffs. This slope provides a foothold for pines and manzanita, and the layer can sometimes be identified from a distance by the "green line" that runs horizontally along the sides of some the mesas (see the photos above).

Coconino Sandstone

The Coconino is a 500 to 1000 foot thick layer of cream or golden-colored sanstone, formed after the final retreat of the Pedregosa Sea 265 million years ago. The layer consists of cross-bedded, wind-blown sandstone - essentially petrified sand dunes, some up to fifty feet in length - indicating that this layer formed on dry land. It is actually divided into two sub-layers, imaginitively named the "Upper Coconino" and the "Lower Coconino". The Upper Coconino makes up 20% to 40% of the total, and is separated from the lower by a prominent line. There are no fossils in this layer.

Schnebly Hill Sandstone

The Schnebly Hill Formation is a dark red sanstone, 800 to 1000 feet thick, that is the major component of the "Red Rocks" of Sedona. Unlike the Coconino, the Schnebly Hill sandstone is layed down in flat-bedded horizontal layers, interspersed with multiple thin white layers of limestone conglomerate. Both the flat strata and the limestone are indicators that the layer formed under water. However, microscopic examination of the quartz grains that make up the sanstone show that they are frosted and worn and of consistent size, clear indicators of wind-blown origin. The conclusion is that the region was subject to periodic incursions of the sea, which reworked wind-blown sand dunes into horizontal layers. Further to the northwest this layer is crossbedded similar to the Coconino, indicating that that area was not submerged.

The deep red color of the sandstone is due to hemetite (iron-oxide) which stains the normally white quartz sanstone. The Schnebly Hill Formation is comprised of the Sycamore Pass (top) and Bell Rock (bottom) 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 the Pedregosa Sea. There are no fossils in the Schnebly Hill sanstones, and virtually none in the thin limestone layers.

Hermit Shale

Though often referred to as shale, the Hermit formation actually consists of various soft, easily-eroded sedimentary rocks such as siltstone, mudstone and sandstone as well as shale. Formed 270 million years ago, it is generally a dark rust color due to iron-oxide staining, and is about 300 feet thick in the Sedona area. Near the top of the layer are desication mudcracks, some tens of feet deep, that are filled with Coconino Sanstone. This indicates that the sands of the Coconino covered the Hermit while the latter was still substantially saturated with water. Susequent drying caused the cracks to open, allowing the Coconino sands to filter down from above.

The city of Sedona is built near the top of the Hermit Formation. This layer contains fossils of land plants, such as ferns and conifers, with occasional fossilized insects, worm burrows, and invertebrate tracks.

Supai Sandstone

The Supai Sandstone--or Supai "Group"--was formed between 280 and 300 million years ago, and is between 200 and 600 feet thick. It consists predominantly of shale and sandstone with some limestone mixed in. Subdivisions include:

• Esplanade Formation (sandstone)
• Wescogame Formation (sandstone)
• Manakacha Formation. Cross-stratified quartz and sandstone)
• Watahomigi Formation (limestone over sandstone, and mudstone)

In the Grand Canyon region the Supai Group contains fossils of reptiles. Marine fossils predominate further west, where it passed below sea level. Fossil content in the Sedona region is unknown.

Redwall Limestone

Formed between 300 and 340 million years ago, the Redwall Limestone is the lowest strata visible in the Sedona region. It is dark brown in color, up to 300 to 600 feet thick, and can contain fossils of brachiopods, clams, snails, fish, and trilobites.

In the Grand Canyon region the Redwall Limestone is known to contain several extensive cave complexes. In the Sedona region no caves are known (or at least no caves are publicly acknowledged). However, there are several locations near Sedona in which sinkholes have formed in the overlying Schnebly Hill and Hermit strata. One example is the huge Devil's Kitchen sinkhole, which collapsed in the late 1800s and is almost 200 feet deep. Since the Schnebly Hill/Hermit formations consist primarily of insoluble sandstone, it is unlikely that substantial voids exist in those layers. However, all of the sinkholes occur along faults in the Schnebly Hill/Hermit formations which channel water and allow it to percolate down to the Redwall. The sinkholes are therefore likely due to large solution caves formed over millions of years in the Redwall, which have grown too large to support the overlying sandstone.

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