winds create desert surfaces
A soil observation from my recent trip to the Mojave desert
The desert is a dynamic place. Soil particles aren't prone to staying put. Wind and water create desert surfaces by lifting and moving soil, dust, and sand.
Wandering around Death Valley recently, I noticed small piles of sand gathering next to shrubs. Shrubs of Creosote Bush (Larrea tridentata) dot the valley floor in an almost consistent pattern that arises from the limited water resources of the desert. Adjacent to each shrub lies a miniature sand dune. Just one small hill of sand, nestled atop an otherwise flat surface covered with some scattered rocks. These little dunes were as consistent as the shrubs themselves.
Sand formations like these that are held in place by plants are called phytogenic dunes1 ("phyto" for plant, and "genic" for producing). These single dunes are sometimes called shadow dunes because they form consistently on one side of the plant depending on the direction of the prevailing wind. The shrubs are barriers that slow down sand grains flying through the air. The shrub acts both as a wind break and a mesh, where sand still moves through the branches and leaves, but slows down enough in the process to settle out of the wind and accumulate in a pile below.
Because wind moves through the shrub, the sand dunes will emerge on the other side following the direction of the prevailing direction. In this case, the winds were coming up from the south and the dunes formed on the north side of the shrubs. I love those little wind ripples that form on the surface of the sand pile. Wind ripples can appear within minutes2. Because they form perpendicular to the wind, they also serve as a marker of the dominant wind direction2.
We use the term eolian (also written aeolian3) to describe wind-driven processes in the Earth system. Soils derived from eolian parent materials are those that are forming in sediments that have been moved and deposited by wind4. Some of my favorite eolian soils near where I live are forming in old sand dunes along the coast.
The shadow dunes in the desert remind me of the snow fences I first encountered in Laramie Wyoming when I was a graduate student at Colorado State University. Driving along a highway known for its crosswinds, I kept noticing these incomplete fences near the roadside. They look like palettes made of large planks of wood that have been turned upright. The pieces are modular - sometimes it is a short fence with only a handful of pieces, while other fences stretch across longer distances. Until you know what they are for, they look like fences to nowhere that don't connect to anything on the other side.
These "snow fences" are there to help keep snow from accumulating on and near the highway to make traveling in windy winter conditions safer. The snow fence works much like the wind processes that create those little sand dunes. Wind blows snowflakes in a consistent direction. As snowy wind passes through the snow fence, it slows down and the flakes drop to the ground. Piles of snow accumulate on the opposite side of the fence from where the wind originated.
The shadow dunes and snow fences remind us that processes occurring in the Earth system are consistent, even in places that seem worlds apart. Wind is always with us in some form.
To spend a lot of time in the desert is to accept the wind as part of the landscape. This is harder than it sounds. I don't know anyone who enjoys experiencing strong winds and dust storms. Two days after I observed these shadow dunes, the winds came up over 50 mph for hours on end. We thought about turning back, going home, and cutting our trip short, but I am glad we stayed. There is no use fighting an essential truth of the desert - winds always return. But so too does the eventual calm, revealing new desert surfaces to explore.
Thank you for taking the time to care about soil today. If you read all the way to the end, reply to this email with a smiley face. It will make my day :).
Take care and stay curious,
Yamina
Footnotes
1 Cooke, R. U., Warren, A., & Goudie, A. S. (1993). Desert geomorphology. CRC Press.
2 Lancaster, N. (2009). Dune Morphology and Dynamics. In: Parsons, A.J., Abrahams, A.D. (eds) Geomorphology of Desert Environments. Springer, Dordrecht.
3 it is hard to keep track of all the vowels!
4 we also use the term "loess" to describe wind-blown silt, whereas eolian sand refers to wind-blown sand. Eolian used as an adjective is more inclusive of wind-driven processes, rather than focusing on a specific type of sediment.