Westerners learn that “Everything Decrepitates with Time”
Education begins quickly on the Westerners’ geology hike to two local sinkholes, led by member-geologist Paul Lindberg. In a gully en route to Devil’s Kitchen sinkhole just north of Sedona, Paul points out how the relatively soft Hermit Formation sandstone, upon which most of Sedona is built, breaks down during the weathering process. “Decrepitation is something we all can relate to!” We look towards Coffee Pot Rock, and then the cliffs to the north, as he points to the northwest-trending rock joints in the cliffs. Over time, we are told, those fractures helped nature sculpt the landscape’s spires and notches. We better understand those finger-like structures, seen also in the Cockscomb and the Mace on Cathedral Rock. Paul explains that starting 70 million years ago, the land rose and flexed. The joints were formed, with cracks extending down into the earth’s mantle, like the separation between the fingers. They are found throughout the Sedona area and “the feature is conspicuous in the walls of Devil’s Kitchen sinkhole.”
Fast forward to the 1880’s. In an account recorded in Those Early Days, published by the Sedona Westerners and the Sedona Historical Society (1968,) Albert Thomson reports that his parents “heard the crash when the sinkhole caved in… Dust filled the sky all day blocking the sunlight, like smoke from a fire” (p.67).
Looking into the cavity, we study the rounded weathered edges of the 1880 collapse of the south end, comparing that to the sharper corners of 1989 and 1995 north side breaks. Paul points out the south rim overhang, deeply undercut and concave, “just hanging together like a jigsaw puzzle.” He explains that sinkholes form and collapse upward in a bell shape, and indeed we see the curving fractures on the south surface, where there is a hollow below. We follow some fractures as far as 100 feet from the sinkhole’s edge. 1000 feet away stand houses! Open cracks on the west side have widened over several decades.
Paul states that sinkholes begin as water percolating down through the rock dissolves the limestone more than 600 feet below, forming large water-filled caves. Eventually a “roof” collapses as the cavity works its way upward toward the surface, a testament to the power of water. Thankfully we have the aquifer beneath us so we can live here in the desert.
At Marg’s Draw, we hike to another sinkhole, “The Devil’s Dining Room.” At our feet as we approach lie pieces of chert, many of which have been flaked, thousands of years prior, by human hands. There is a moment of silence while this sinks in. We learn that chert deposits are left as insoluble fragments eroded from Kaibab Limestone outcrops high on the plateau rim. It remains as softer sandstone washes away with each rainstorm.
Approaching the sinkhole lip, we watch for concentrations of coal-sized black basalt rocks appearing with greater frequency on the sandstone. Peering into the small opening into the 90-foot deep cavity that enlarges downward, we see a foot-wide vertical black basalt dike on the far wall. Lindberg explains that it once fed a lava flow that originated in the earth’s mantle 30 miles down, surfaced high overhead, and long-since eroded away, And there it lies hidden inside the sinkhole wall! Dikes extend as far away as House Mountain. Finding them helps him to decode the early landscape.
On the way back to the trailhead, we study sponge fossils in Kaibab Limestone boulders washed down by floods from the Moggollon Rim. As the limestone dissolves away, the mounded fossil outline of the sponge emerges. Paul reminds us that one of the fundamental rules of geology is that “rocks roll downhill.”
From sinkholes to chert, to early man making arrows, to sponge fossils and back up to the fingerlike formations of Coffee Pot, from rising lava to eroding dikes, we move forward and back through eons of time and up and down and across space. Everything is changing. Horizontally, vertically, everywhere and in every time, we witness in 3 D the miracle that is our planet.
Paul Lindberg deserves “kudos” (coups dos) for his vision and skill in conveying this; pats on the back hardly do justice to this day
The Westerners always welcome new members. If you are interested in joining the club, log onto our website, www.sedonawesterners.org., for membership information. You may also join at one of our monthly meetings. Our next one will be on Thursday, Jan. 12, beginning at 7p.m. at the Jewish Community of Sedona and the Verde Valley, 100 Meadowlark Drive, Sedona.