Pyramids and Polygons – Why are the Rocks White?
January 06, 2023
By Ernie Pratt
Our Red Rocks offer beauty and interesting sights in all shapes and sizes. Here, ancient mud beds dried, cracked, and filled in with other minerals to form unique polygons.
On a sunny but somewhat cool day the Sedona Westerners Rustlers group met at the Posse Grounds in Sedona to prepare and hike the nearby Pyramid while seeking polygons.
Due to the size of our group, we quickly loaded into car pools to accommodate traveling and the limited parking at the trailhead.
On the trail we formed into two groups of hikers lead by Ernie Pratt and Jon Petrescu, with sweeping help from Jim Kemper and Dick Williams. This hike starts with an almost unpleasant, steep, wake up ascent that is somewhat tempered by spectacular views of Cathedral, the airport and a distant Thunder Mountain.
Non-stop geological observations punctuated with rudimentary archaeology typified the first part of this hike. We were rewarded with our first geological lecture that had an upside of allowing us to catch our breath before we proceeded onward. We learned the Pyramid is capped by a resistive limestone called the Fort Apache Formation, which slows erosion resulting in the pyramid shape.
On our descent we found our first set of polygons, which were purported to represent an area of regional drying of the near shore red sandstone sediments causing deep desiccation cracks. They were subsequently filled with white sands blown in from the regional dunes to the northeast. The polygonal patterns this phenomena produce are much repeated in nature, found in the northern tundra, mud flats and salt pans.
With the main climbing over, we walked along the very scenic path above Red Rock loop Road. Looking southward we could see the House Mountain shield volcano filling the horizon. Blessedly, it began to warm up, outer layers came off, and we enjoyed wonderful views of Red Rock State Park and the surrounding community.
Some small flowers remained in bloom, helping us on our way. Our leader tried to demonstrate to us the difference between rocks from a nearby sabkha bed and the surrounding red rocks, but the attempt failed as none of us were strong enough to break the rocks as instructed. We will just take his word for it.
At our lunch spot we sat upon more polygons, while entertained by yet more tidbits of geology. Looking up towards Scheurman Mountain, we were sitting in the shadow of some “wannabe” columnar basalts. The leader magically produced a handful of “vortex balls” for our further discussion and entertainment.
As we returned to the parking lot, we briefly looked at a wonderful, in your face view of Cathedral and discussed Paul Lindberg’s interpretation of how this unique, iconic landmark came to be formed and how it is now is just a small part of what was once a significant volcano. The actual mound that is thought to have once surrounded Cathedral was probably thousands of feet higher before wind, water, and time have carved it into the oft-photographed masterpiece it is today.
Further down the trail, we looked up into the cliffs to see many cliff swallow nests. Hidden in the overhangs, you wonder how many trips from Oak Creek and other water sources it takes for these birds to build their cliffside condominiums. They are well camouflaged to thwart intruders.
With our minds filled with pyramids and polygons, our boots and pants dusted with red rock, surrounded by joints, faults, concretions, and walking through towering red sandstone walls, there was a voice from the back of the group, completely off topic, asking why some bands of rocks were white?
So once again, we gathered around our resident geologist to hear an explanation of porosity, permeability, flushing, and regional water flow, all ancient happenings from the past. In reality, the concept is really quite simple. Some rocks are more porous than others, and iron, which causes Sedona’s Red Rocks, is flushed out of those rocks by water flow…thereby leaving them white. The mystery was solved.
With a collective sigh of relief that there was no geological exam at the end of this hike, we returned to the trailhead with a sense of well-being, having experienced a beautiful day, wonderful views, and great company all while covering an appreciable distance. Keep on moving and learning!
The Sedona Westerners always welcome new members, and we have hikes multiple days of the week for all abilities. If you are interested in joining the club, please visit our website at sedonawesterners.org. You will find an interesting history, the whole season’s list of planned hikes, and a handy membership link. It only takes five minutes to sign and start your new adventures here in the Red Rocks.