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Red Rocks
Geologist Wayne Ranney stands on the trail heading to Wilson Mountain near Sedona. Ranney has reissued a popular guide describing how the area's red rocks, canyons and mesas formed. (Cyndy Cole/Arizona Daily Sun)

At the overlook to Sedona's airport, most visitors head toward a ledge that gives views of the city and its red backdrop of rock fins and canyons.

Geologist Wayne Ranney walks the opposite direction, heading for some spherical rocks by the side of the road.

You don't see them if you aren't really looking, but what is now the top of the mesa and the foundation for the airport used to be a stream, and those used to be river rocks.

"They're landing on a surface that used to be the floor of Oak Creek," Ranney said of the airplanes at the airport.

He discovered this while mapping the topography surrounding Sedona for a master's thesis at Northern Arizona University, when he found the slope of the mesa had about the same incline as surrounding Oak Creek, before the creek cut downward.

In the new illustrated book "Sedona Through Time," Ranney explains why the rocks are red, how some of Sedona's rock layers line up with the Grand Canyon's, and what things probably looked like when North and South America, Western Europe and Africa formed one mass of land, called Pangaea.

An update of his previous guide, this one has new illustrations, storytelling and geology driving tours that cover Airport Mesa, House Mountain Volcano, Bell Rock, the Midgley Bridge area, West Fork and Oak Creek Canyon Vista.

The text is accessible and broken up often with colorful side notes and graphics showing what the land may have looked like over time.

Take Ranney's description of when Sedona's geologic story began -- about 316 million years ago, when the Southwest was closer to an inland sea and nearer the equator, and many of today's geologic features had yet to form.

"Not even geologists truly comprehend the vast length of time that geology represents, but a few analogies may help," he writes. "... It takes about 10 years for 316 seconds to pass. That many minutes would take us back to the time when prehistoric farmers were growing corn in the Verde Valley (about AD 1410). Another way to understand how incredibly large these numbers are, is to imagine tiny sand grains stretched out in a row. Each sand grain is only 1/25 of an inch in length, but 316 million of them laid end to end in a straight line would stretch almost 200 miles."

Ranney traces the rock formations that came long ago from sand dunes, filling depressions of the earth's crust, and old volcanic eruptions.

He explains each rock layer, in the book, describing what features are found in each.

At one point, he imagines seeing the changes from above, in a spacecraft, over 46 million years.

"At first, the view out our window shows the muddy deltas, sluggish rivers and low-lying dune fields of the Supai Group as it buries the tropical marine deposits of the Redwall Limestone," he writes. "To our right, we see South America approaching from the south and colliding with North America. The Ancestral Rockies are thrust up to the northeast of the Sedona area and a river system flows out of them delivering the pebbles, sand and mud of the Hermit Formation. Through time, these river systems dry up and are choked off as wind-driven sand is carried down from the north along the west coast of Pangaea."

Just upstream from the Junipine Resort in Oak Creek Canyon, Ranney looks at a ribbon of rock on the east side of the road, commonly called the Junipine Dike.

About 8 million years ago, magma of perhaps 2000 degrees Fahrenheit pushed its way through the Coconino Sandstone, splitting the rock apart, Ranney says, pointing out the baked sandstone on either side.

A similar line of red, burned soil can be seen along the roadside on the switchbacks heading down Oak Creek Canyon, where a volcanic eruption burned over the dirt and vegetation.

As part of the driving tour in the book, Ranney points out how the west side Oak Creek Canyon is 1,000 feet higher than the east, the fault dividing them, and the lava flow that covered the east side of the canyon 6 million to 8 million years ago.

As to the question of why the rocks are red, there's a thin layer of iron-oxide coating each grain of sand in the sandstone. Though it is a very small amount, it oxidizes when exposed to groundwater.

"The isolated, iron-rich grains act like little packets of red dye within the sandstone that can coat the otherwise white sand grains in the groundwater environment," he writes, with the rocks turning red before they emerge from underground.

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