Driving across the southwestern United States, one could be forgiven for thinking that all deserts are the same. However, differences in elevation, temperature, topography, and precipitation make them distinct in ways that are often hard to comprehend from a fast-moving car.
For all their differences though, both the subtle and the striking, the Mojave and Sonoran Deserts share one thing in common; the most visible symbol of each is a large, majestic, and photogenic plant perfectly suited for the harsh conditions in which it evolved to inhabit.
For the Mojave Desert, that plant is the Joshua tree (Yucca brevifolia). Not actually a tree but rather, as the scientific name betrays, a species of Yucca, early settlers bestowed the befuddling name upon this plant after noticing that the contorted arms resembled the prophet Joshua raising his arms to the sky in prayer.
Today, the Joshua tree is considered an indicator species for the Mojave Desert, as many other inhabitants of the Mojave (two-legged, four-legged, and winged alike) depend on it for survival. The Joshua tree grows through portions of western Arizona, southeastern California, and southern Nevada, but some of the largest and healthiest stands are protected within the boundaries of Joshua Tree National Park and Mojave National Preserve.
For the Sonoran Desert, the symbolic plant is the saguaro cactus (Carnegiea gigantea). Despite the nearly ubiquitous use of the saguaro as a symbol of the American Southwest, this excruciatingly slow growing cactus actually only grows in a small portion of the Sonoran Desert extending from extreme northwestern Mexico into south-central Arizona.
Like the Joshua tree in the Mojave, the saguaro is an integral part of the Sonoran Desert ecosystem. Birds such as the Gila woodpecker nest within the flesh of the cactus while the fruits and flowers provide a source of food for many other species, including humans. When the end finally comes for a saguaro (which can take well over 100 years), the flesh rots away to reveal an internal structure consisting of a series of wooden ribs, which often remain standing long after the saguaro dies:
Both the saguaro and Joshua tree face serious threats; in the long term from a climate that may change faster than they are able to migrate, and in the short term from a loss of habitat due to rapidly ballooning human populations in the desert regions that these giants inhabit.
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…