What Do We Walk On? Ask a Geologist!

February 16, 2018


By John Losse

Dr. Ken Bork recently served as a guest speaker for the Sedona Westerners hiking club and provided a presentation on the geology of the Sedona area. Dr. Bork is a retired geology professor from Ohio.

What's a guy to do when he finds himself in Sedona after a career in Ohio as a paleontologist and geologist, teaching and directing students in the fascinating natural history of Ohio's 350 million-year-old rocks?  For Dr. Ken Bork the answer was easy -- immerse himself in the paleontology and geology of our wonderful red rocks and spread the word to anyone who cares to listen.

At a recent monthly meeting, the Sedona Westerners were entertained and enlightened by Dr. Bork in a lively talk and slide presentation entitled, "What the Westerners Walk On."  It turns out that the answers are both simple and complicated, and a lot depends on where a hiker is hiking. 

At the top of Wilson Mountain, for example, you'll be hiking on layers of basalt -- volcanic flows that occurred recently in geological time and form a cap protecting what is underneath it, except when it doesn't!  For example, Oak Creek Canyon resulted when a fault caused the east side to slip as much as a thousand feet below the west side. This means the rock layers don't match up, and the cracking caused by the fault provided a path for water to do its relentless work; thus, creating the lovely canyon we see today.

At the mouth of Oak Creek Canyon lies Sedona and its beautiful red rocks.  Dr. Bork explained that most of what we see, walk on, and build our houses on, is flat-lying shale. Sandstones create our scenic cliffs.  These rocks were deposited from 285 to 250 million years ago.  The sandstones of the Schnebly Hill Formation consist of quartz sand grains derived from ancient granite.  Why are they red? It only takes a small amount of iron oxide in the sand to do the trick.  In other words, it's a form of rust. 

But, it is not all red.  As our speaker explained, the whitish/pinkish layers we see at the top of Thunder Mountain or, looking east from Sedona, to Mund's Mountain, are Coconino Sandstone.  When we hike the Grand Canyon, we pass through the same Coconino Sandstone in the Canyon's upper layers. Sometimes the sandstone layers display striking internal cross-bedding.  This means the strata are actually compacted sand dunes formed by prevailing arid winds operating over a long period of time (and remember, there was plenty of time).

It is not all sandstone.  An important member of the Schnebly Hill Formation is a layer of Fort Apache Limestone, which demonstrates the presence of an ancient sea here at one time.

By the way, if you are hiking along a trail on the Kaibab Limestone, at the very top of our local sequence of sedimentary rock, and the rocks seem particularly unkind, cutting into your boots, they are probably chert. According to our speaker, chert is actually a fine-grained variety of quartz, which is hard and can be quite sharp.  Sharp edges can be formed when it is properly worked, so the ancient peoples of our area used it for making points, scrapers, knives and anything else requiring a sharp edge.

Have you ever wondered why, when you are hiking, nature occasionally provides a nice, long, flat shelf to speed you on your way?  Dr. Bork explained that this is because the Hermit Shale tends to erode into flat-lying "benches," which are an ideal place for building homes and communities.

These are a few of the features of our wonderful landscape that Dr. Ken Bork helped us to understand.  As he pointed out, we in Sedona live and hike in a place which is right at the edge of the vast Colorado Plateau, with a beautiful canyon to help expose its many layers, and we even have volcanoes and copper and gold deposits not far away -- plenty of material for another talk!

If you are interested in joining the club, please visit the Sedona Westerners website at www.sedonawesterners.org/membership.  You are invited to our next monthly meeting at 7 p.m., Thursday, March 8, at the Sedona Methodist Church, 110 Indian Cliffs Road.  Sedona Westerners, written this week by John Losse, appears every Friday in the Sedona Red Rock News.

 

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