One of the great things about living in Southern Utah is the abundance of different climates within a small geographic area. When temperatures rise into the 90s and 100s in the low-elevation valleys, we can be in cool alpine meadows at 10,500′ in less than an hour. When snow, slush, and mud cover the trails in winter, vast portions of the Mojave and Great Basin Deserts are within a day’s drive. One of these desert areas is Valley of Fire State Park in southern Nevada, not far from I-15 between St. George and Las Vegas.
Perhaps not surprisingly, upon arrival at Valley of Fire one is greeted with an array of whimsically sculpted red rock formations. Now red rocks are hardly unique in this part of the country, and the crimson cliffs here are no more notable than those found anywhere else in Utah or Arizona. But head into the interior of the park and you soon realize the allure of the Valley of Fire. After cresting the red cliffs, the hues begin to multiply exponentially and before long you are surrounded by just about every color of sandstone imaginable.
To put it bluntly, the colors at Valley of Fire are simply ridiculous…and attributable to its unique geologic location. The rocks here are mostly equivalent to those found throughout southwestern Utah and the Colorado Plateau. The Aztec Sandstone, the dominant rock unit exposed in the park, is the equivalent of the Navajo Sandstone that makes up the cliffs of Zion National Park. Geologists just assign it a different name when it appears in Nevada and the Great Basin. Perhaps the distinct name is appropriate though, given that the sandstone seems to take on a life of its own here.
Valley of Fire State Park lies within the Basin and Range province, a vast region covering Nevada and portions of half a dozen other western states where the Earth’s crust is being slowly but violently stretched apart. As the writer John McPhee once noted, so much stretching has occurred here that 20 million years ago, Salt Lake City and Reno would have been more than 60 miles closer together. Faults are abundant in this land, and fluids associated with some of these faults have at various times leached iron compounds from the originally all-red sandstone, causing some layers to become bright white, and re-deposited them in other layers, leading to the wide variety of colors.
Some of the most impressive colors are found just to the west of the “Fire Wave” feature near the northern terminus of the park’s scenic drive:
While there are numerous hiking trails, there is also lots of off-trail terrain to explore. Some of the most spectacular scenery can be found by parking at one of the numerous pull offs and just wandering out into the rock wonderland. One particular geologic feature of note is what are known as “shear-enhanced compaction bands,” thin brittle fins of rock that rise almost vertically out of the ground and often run continuously for dozens to hundreds of yards. At first glance, these features look like mineral veins, but upon closer examination they are composed of the same material as the surrounding sandstone, but are obviously slightly harder than the host rock. In many places there are two perpendicular sets of the bands, forming a checkerboard like pattern superimposed on the sandstone.
The bands are the result not of stretching, but of compressional forces that predate the formation of the Basin and Range. Stresses associated with an earlier mountain building episode (known as the Sevier orogeny) created these funky bands by essentially “squeezing” together (and even breaking) the sand grains that make up the rock, eliminating much of the empty space between the grains and forming a miniature layer of tougher, harder, and more compact sandstone that is slightly more resistant to weathering and erosion. As a result, the bands tend to just out from the surrounding slickrock by several inches, and even several feet in some locations. For such a seemingly obscure feature, many papers have been written about these compaction bands (and similar ones in a few other locations in the region). However my understanding of the structural processes behind their formation is limited and the most recent articles about them appear to be behind a paywall. If anyone reading this has more insight into these things, I would love to hear from you.
As mentioned before, these bands are quite thin, in most less than a centimeter thick and thus, sadly, quite brittle. They are easily broken by an errant boot step so if you find yourself among them, tread carefully so that future visitors will be able to experience this unique and colorful landscape.
Southern Utah is a mecca for tourists from around the world, and most of that blame can be placed on the shoulders of a single layer of rock: the Navajo Sandstone. Quite possibly one of the most famous geological formations in the world, the Navajo Sandstone is responsible for the soaring cliffs of Zion National Park, the monoclines of Capital Reef, and the undulating, swirling, entrancing patterns of the The Wave in Arizona and Grand-Staircase Escalante National Monument in Utah. The Navajo Sandstone also rears its beautiful head in lesser known gems, such as Snow Canyon State Park just a few minutes northwest of St. George, Utah.
Snow Canyon is actually several canyons in one, all cut into the Navajo Sandstone. The original Snow Canyon existed up until about one million years ago, when it was rudely filled in by a series of basaltic lava flows originating from the northeast. Water, being the couch potato that it is, doesn’t like to carve through hard volcanic rock, so the stream that had excavated Snow Canyon promptly jumped ship to find some more Navajo Sandstone, and thus began establishing a new canyon slightly to the west. The stream went about its business carving Snow Canyon #2 until about 10,000-20,000 years ago, when it was thwarted by yet another lava flow. True to history, the stream changed course a second time, and is now busily carving Snow Canyon #3 even further to the west. The result is a multi-tiered canyon, with the remnants of the canyon-filling lava flows forming the tread of each step.
The Navajo Sandstone itself is a colossal formation, several thousand feet thick in places, representing the lithified remains of a large Jurassic sand dune sea (known as an erg), likely analogous to the modern day Sahara desert. If you think Southern Utah is hot and dry today, imagine being there 180 million years ago when the climate was hot and hyper-arid. Add some dinosaurs and you’ve got yourself a fun day in the Jurassic desert. Over time, mineral-rich fluids percolated through the sand, depositing mineral cement in between the sand grains, binding them together into stone. The Navajo Sandstone is known for its spectacular aeolian (fancy geology-speak for “wind-blown”) cross-bedding, inclined layers that form when winds blow sand up the shallow face of a dune, only to have it tumble down the steep slip face on the other side.
A especially peculiar property of the Navajo Sandstone is the presence of occasional beds containing abundant spherical concretions of sand held together by the iron oxide minerals goethite and hematite (see photo at top of page). Commonly known as “moqui marbles,” these small spherules are slightly harder than the rest of the sandstone, so as the rock weathers away, the concretions are left behind to accumulate in large quantities on the surface of the rock. Moqui marbles can be found in many locations throughout Utah. And on Mars. The discovery of nearly identical hematite concretions by the Opportunity rover was some of the first definitive evidence that liquid water once flowed on the red planet, since the formation of the marbles requires groundwater to dissolve, and then re-precipitate iron minerals in the subsurface. If you are intrigued by my incredibly vague and simplistic description, you can find much, much more on the moqui marbles and their mode of formation here. If not, you are hereby forgiven and are welcome to enjoy the final photo without guilt:
Continuing on with our recent geological theme here at Pyroclastic Pixels (you’d almost think I was a geology grad student or something…), today we are going to take a look at one of the most picturesque geological curiosities you’ll ever find: honeycomb weathering, also frequently referred to as “tafoni”. Those two terms aren’t really exactly quite completely equivalent but we’re not going to journey down the nit-picky fork in the road today. Honeycomb weathering is pretty cool. About the only thing that would make it better is if the holes were actually filled with honey. That joke sounded way better in my head than it looks on the screen.
Specific geographic and geologic conditions are needed for honeycomb weathering to develop, yet these conditions can be satisfied in a variety of places, from the arid deserts of the American Southwest, to the storm-battered shores of the Pacific Ocean. Here in northwestern Washington State, honeycomb weathering occurs along the coast, along and just above the high tide mark, in areas where a rock unit known as the Chuckanut Formation is present. The pictures on this page were taken at Teddy Bear Cove, just south of Bellingham, WA, which has some of the most spectacular examples I’ve ever seen. The Chuckanut Formation, or “the Nut” as I like to call it when I’m feeling lazy, is a thick series of sandstones, conglomerates, and occasional coal seams that were deposited about 60 million years ago when NW Washington occupied a large basin at the foot of an ancient mountain range that occupied more or less the same space that the Modern Cascades now occupy.
There is a good reason that sandstone is one of the rock types most susceptible to this type of weathering. Sandstone is essentially composed of countless tiny, sand-sized particles of various minerals (mostly quartz and feldspar in the case of “the Nut”) which are held together by some sort of substance, known as cement, that “glues” them all together into a solid mass. In most sandstones, this substance is either calcium carbonate (CaCO3) or silica dioxide (SiO2), also known as quartz. Honeycomb weathering forms when salt-laden sea spray lands on the sandstone. As the salty sea water evaporates, tiny salt crystals form on the surface of the rock. The growth of these salt crystals on the surface of the rock physically separates the sand particles from the cement. Over time (a long time…), this creates a small depression in the rock. Once a small indentation forms, a positive feedback effect is created; the hole has a greater surface area than a flat surface and thus more rock is exposed to incoming sea spray. Sand grains are thus separated from the cement at a faster rate, thereby enlarging the hole. In some locations, you can actually see little piles of sand grains in the cavities, grains that were once part of the rock but have now been forcibly removed by the salt. I’ve found that this is most prevalent in areas just above the high tide line where wave action can’t wash the sand grains back out to sea.
But Zach, you say…how then does honeycomb weathering form in places like the desert Southwest where the closest thing to sea spray you’re going to find is mule deer pee? Ah…well I’m glad you asked. We often observe honeycomb weathering in sandstone in places such as Southern Utah that are far away from the sea. I had some difficulty finding a halfway decent picture of desert honeycomb weathering from my archives, but I was able to find one that I took in 2008 in Capitol Reef National Park (see below). If you want to see a lot better examples, just do a Google image search for “Utah tafoni”. While the exact cause may vary, and the individual pits tend to be larger, the process involved is essentially the same. We still need to find some way to separate our sand grains from the cement. Many washes in the southwest are dry for most of the year but are very rich in dissolved salts when they do flood. In desert environments, it’s no surprise then that we tend to find honeycomb weathering predominantly along dry stream beds and canyons. When a flood comes through, even though the water may not be as saline as the ocean, it is still salty enough to form small salt crystals when it evaporates, which it invariably does. In other locations, slightly acidic groundwater percolating through rocks can actually chemically dissolve calcium carbonate cement, leaving the sand grains with nothing to cling to.
Hard as it might be for you to believe, this has been only a cursory explanation of the honeycomb weathering formation process. If your brain hasn’t begun to resemble honeycomb weathering by now and you are interested in the gritty details (perhaps you arrived here in the process of researching a paper or maybe you’re a geology nerd like me and just like knowing about such things), an excellent academic paper on the formation of honeycomb weathering can be found here. Regardless, your next step should be to pull out a geologic map, find the closest beach with some sandstone, pull your boots on and go find yourself some honeycomb weathering! Or you could always just look at the rest of these pictures I suppose…