Sedona, Arizona, is renowned for its striking red rock formations. This distinctive landscape, characterized by towering buttes and mesas, invites curiosity about their composition and the forces that shaped them. This article explores the types of rock prevalent in Sedona and the natural processes responsible for their vibrant color and iconic forms.
The Dominant Red Rocks: Sandstone
The primary rock type defining Sedona’s landscape is sandstone, a sedimentary rock formed from compacted sand grains. These sandstones were deposited over millions of years, creating layered formations that tell a story of ancient environments. The most prominent of these is the Schnebly Hill Formation, a dark red sandstone layer that is a major component of Sedona’s “Red Rocks.”
The Schnebly Hill Formation, ranging from 800 to 1000 feet thick, was laid down in flat, horizontal layers, interspersed with thin layers of limestone. This suggests a past environment influenced by both wind-blown sands and periodic incursions of shallow seas. Other notable sandstone layers in the region include the Coconino Sandstone, known for its lighter, often cream or golden color, and the Supai Group, which consists of interbedded sandstone, shale, and limestone. The Coconino Sandstone originated from vast, ancient wind-blown sand dunes.
The Vivid Red Hue: Iron Oxide’s Role
Sedona’s signature red color results from the presence of iron minerals within the sandstone, which have undergone a process called oxidation. These iron minerals, primarily hematite, react with oxygen and water, essentially “rusting” the rock. This process stains the otherwise lighter quartz sand grains, giving the rock its distinctive red, orange, and even brownish hues.
The intensity of the red color can vary depending on the concentration of iron oxide and the extent of its exposure to the elements. This oxidation often occurred when the sediments were exposed to oxygen in arid environments before or during their burial. While the Coconino Sandstone is typically pale, it can also be iron-stained and appear bright red in certain areas.
How Sedona’s Landscape Was Carved
The dramatic landforms of Sedona, including its buttes, mesas, and canyons, were sculpted by a combination of geological uplift and relentless erosion. Millions of years ago, the horizontal layers of sedimentary rock, including the sandstones and limestones, were uplifted as part of the Colorado Plateau. This uplift was partly driven by tectonic forces.
Following uplift, wind and water became powerful agents of erosion, selectively wearing away the softer rock layers while leaving the more resistant formations standing. This process, known as differential erosion, explains why some layers form sheer cliffs and others create gentler slopes. This interplay of geological forces carved the iconic red rock formations that define the breathtaking landscape of Sedona today.