Sedona Westerners in the Red Rock News

April 3, 2015

Black Rocks Also Tell A Story

 


by Ernie & Lynn Pratt

Amid the beauty and splendor of the red and white rocks gracing Sedona and the Village, are the generally overlooked black rocks that cap many of our mountains. When examining them more closely these volcanics have interest and a story that is worth telling.

During a recent tracker hike, a group of 12 Sedona Westerners were lead up the flank of House Mountain by Wayne Ranney to learn more about the geology of the black rocks. Ranney a geologist; trail guide and author became interested in the earth sciences while working at Phantom Ranch in the Grand Canyon. This interest developed into a much-varied career, but with a focus on the Verde Valley and the Colorado Plateau. As part of his Master’s thesis, Wayne completed a detailed map of the House Mountain area in 1988.


The House on House Mountain.

The hike was just over 6 miles up the Turkey Creek trail to the top of the crater achieving 800 feet in elevation. Ironically, the first part of the hike was across and up the familiar red rocks of the Schnebly Hill Formation, with breaks along the way to explain their depositional history. Wayne pointed out that some of the large open areas around the Turkey Creek tank were actually ancient dune fields, consisting of fine windblown sands shed from the nearby highlands.

At House Mountain itself, the Schnebly Hill Formation (which included the Fort Apache limestone Member) sandstone is capped by a grayish then reddish cinder zone and finally by black basaltic rocks. House Mountain is a shield volcano occupying a large part of Sedona’s back yard. It is typified by low-lying gentle slopes, formed by the flow of very fluid lavas that erupt in a circular pattern covering many miles.

The group of Sedona Westerners learned on this trip not all basalts were made equal. House Mountain had lava flows that were 15 to 13 million years old, the same age as all the volcanic rocks that have stood on the drive from Phoenix northward to this area. However, in contrast, the lavas topping Mount Wilson and northward up Oak Creek Canyon had been much younger, under 6 million years in age. As the hike progressed, it was further learned that the term basalt was actually a generalization. Closer study and mapping shows that the basalt field actually consisted of many different kinds of rock with long and complicated names that only a geologist could love. One of these, nepheline monzosyenite (an unusual black and white granite look-alike) briefly held our interest, given that it was sourced from the House Mountain magma chamber and consisted of a very special mineral assemblage. When the tracker group reached the saddle after a steep climb, they were able to look downward into the crater with its two resurgent domes and across to the “house” on House Mountain, a remnant lava flow on the very crest of the old rim. Early settlers thought it looked like a house in the distance.
All through the climb Wayne was bombarded (not by the volcano) but by the various trackers with lots of questions. The group had been asked to pay special attention to the loose rocks that lay along the pathway as they climbed. Those rocks would give up clues to the general stratigraphy they would encounter on the way up. In addition to the numerous red rocks and loose sand along the trail the tracker group encountered a bright white limestone that turned out to be the much younger Verde Formation.

Near the top of the mountain was a thick preserved outcrop of flat lying Verde Formation which when carefully mapped, showed that at one time the House Mountain volcano was underwater covered by the ancestral lake which once occupied the Verde Valley. The trackers had lunch discussing many things, including the merits of seating themselves on eroded generally much smoother red rocks versus the House Mountain cinder rocks that were definitely rougher and less comfortable. The had an enjoyable and educational day.

For those who would like to know more about Sedona’s local geology, Wayne Ranney has written two books specifically about the Sedona area: ‘Sedona Through Time” and “Ancient Landscapes of the Colorado Plateau” that are available at many local outlets.

If you are interested in joining the club, please go to our website at http://sedonawesterners.org or just come to one of our monthly meetings. The next one will be on Thursday, April 9, 2015 at 7 pm at the Sedona Elks Club, 110 Airport Road.