13.5: Sand Dunes and Dune Fields

Forming Sand Dunes

Sand dunes are accumulations of sand; when they coalesce in large areas, these are called "dune fields". Such features often come to mind when people envision a desert environment.

An internal cross section of a sand dune shows a feature called cross bedding. As wind blows up the windward side of the dune, it carries sand to the dune crest depositing layers of sand parallel to the windward side (Figure \(\PageIndex{1}\)). The sand builds up the crest of the dune and pours over the top until the leeward (downwind or slip) face of the dune reaches the angle of repose, the maximum angle which will support the slip face. Dunes are unstable features and move as the sand erodes from the stoss side and continues to drop down the leeward side covering previous stoss and slip-face layers and creating the cross beds. Mostly, these are reworked over and over again, but occasionally, the features are preserved in a depression, then lithified.

Figure \(\PageIndex{1}\): An illustrated cross section of a sand dune shows internal sediment layers that form cross bedding. As the wind blows from right to left, sand grains are pushed up the gentle slope of the dune. They then fall down the leeward face, forming tilted layers called "cross beds". "Formation of cross-bedding" by NPS, is in the public domain.

Types of Sand Dunes

The size, character, and extent of dune fields depend on many variables that determine the supply and mobility of eolian sediments. Factors include source materials, wind direction and magnitude, precipitation patterns, and landscape configurations relating to sediment supply, migration paths, and depositional setting. During wet periods, vegetation cover stabilizes dunes, leads to soil formation, and limits eolian sediment supply from source regions (playas, washes, fans, etc.). Conversely, during arid periods these source regions are re-activated.

The linkage between climate and dune reactivation is not clearly resolved. When dry periods persist, the supply of sediment provided by streams may cease, and the readily wind-transportable material may diminish. As a result, sand dunes may erode away due to lack of the supply of new sand, and desert pavements may form. The probable most effective climate for sand dune field formation is a balance of episodic, intense storms (to generate and move sediments to eolian source areas) with intervening windy, long, hot, dry periods (to hinder soil development and plant growth).

The Algodones Dune field, also known as the Imperial Dunes, is located southeast of the Salton Sea. This dune field is located at the junction of Arizona, California, and Baja California del Norte in Mexico) and displays a variety of different dune types. It is approximately 72 kilometers long by 10 kilometers wide (45 miles by 6 miles) and extends along a northwest-southeast line that correlates to the prevailing northerly and westerly wind directions.

Distinctive dune forms are visible in a photo of the Algodones Dune field taken from space (Figure \(\PageIndex{2}\)). They include transverse megadunes and linear dunes. The transverse dunes, appearing much like a large tank tread along the long axis of the dune field, are the largest features. They were formed by strong and constant northwesterly winds. A fine pattern of smaller dunes has formed along the crests and windward (northwestern) sides of these large dunes and point to continuing sediment transport activity. Networks of small dunes along the western side of the dune field suggest that wind direction here is more variable, and perhaps influenced by the large transverse dunes themselves. Long, linear dunes extending northwest-southeast are visible along the western edge of the dune field.

The oldest of the dunes in Figure \(\PageIndex{2}\) are the large, compound crescent dunes in the middle of the field. Compound crescent dunes are crescent-shaped and have other dunes on top of them. The crests run diagonally, southwest to northeast. This orientation is an indication that the dunes formed earlier than others in the field, at a time when wind conditions were different. The compound crescent dunes in this dune field are between 15,000 and 18,000 years old and slowly creeping southeast.

The rest of the dunes in Figure \(\PageIndex{2}\) are younger. Most are simple crescent dunes (the most common type in the world) that get their crescent shape when winds blow consistently in the same direction. These dunes are migrating toward the east-southeast.

The last type of formation in Figure \(\PageIndex{2}\) are linear dunes that run in long lines down the western edge of the field.

The source of sediment in this dune field is Lake Cahuilla, a large freshwater lake that occasionally forms in southern California. The lake fills when the Colorado River Delta clogs with silt, diverting the river to the north into the Imperial Valley. It slowly disappears when the river returns to its former course. Waves on the shores of the lake deposit the sand that feeds this dune field.

Figure \(\PageIndex{2}\): A portion of the Algodones dune field with crescent and linear dunes indicated. "Algodones Dune Field" by M. House after work by NASA, is in the public domain / a derivative of the original work. Access a detailed description of this map.

Kelso Dunes and Devils Playground

The Kelso Dunes and Devils Playground is a large area of eolian (wind blown) sand deposits within the Mojave National Preserve, east of Barstow, California. The dune field is also the largest in the Mojave Desert region and is notable because of its “booming” sands. A booming sound is created when people run down these dunes, probably due to the rounded shape of the sand grains in these dunes (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): The Kelso dune field contains a high dune that stands out from the surrounding low-lying desert region. “Star Dune in the Kelso Dune Field” by USGS, is in the public domain.

Singing Sand Dunes, National Geographic

This short video discusses the origin of the "booming" sounds that originate in large dunes like those at Kelso Dunes.

Depositional features that can be found here include dunes (both actively-forming / migrating dunes and plant-stabilized dunes), sand sheets (sandy flats areas transitional between the source areas and the dunes), and sand ramps (sand build-ups on the flanks of the mountains). This dune field is located in the southeastern end of the greater Soda Lake-Kelso basin where the Granite Mountains and southern Providence Mountains form a barrier to prevailing winds. Wind entraining a flux of sand derived mostly from the Mojave River area at the western end of the basin is deposited near the rising mountain front of the Granite and Providence Mountains, causing the coarser fractions to accumulate.

During the Quaternary, the climate has changed. Wet regions have become drier and drier regions have become wetter. This change in climate has influenced the formation, stabilization, destruction, and re-activation of dune field systems through time in this region. The Kelso Dunes probably looked much different during the wetter periods of the Quaternary than they do today. In fact, field evidence suggests they may have been mostly or entirely stabilized by plant cover in the past.

Lake sediments constitute one of the major sources for eolian sediment, along with sediments derived from lower (distal) alluvial fans and ephemeral washes (such as Mojave River for the Kelso Dunes field). Investigations have demonstrated that at least two major pulses of dune emplacement occurred between 35,000 to 25,000 and 15,000 to 10,000 years before present, and a period of active dune reworking and construction has been ongoing for most of the past 4,000 years. These are times of dramatic increase in sediment supply from drying lakes and river input.

References

1. Dohrenwend, J. C. (2007). Development of pediment domes. In Geology and Mineral Resources of the East Mojave National Scenic Area, San Bernardino County, California (pp. 76-80). USGS.
2. Geology of Joshua Tree National Park | U.S. Geological Survey. (n.d.). USGS.gov. Retrieved September 3, 2023, from https://www.usgs.gov/geology-and-ecology-of-national-parks/geology-joshua-tree-national-park
3. Harden, D. R. (2004). California Geology. Pearson Prentice Hall.
4. Johnson, C., Affolter, M. D., Inkenbrandt, P., & Mosher, C. (2023). An Introduction to Geology. Salt Lake Community College. https://slcc.pressbooks.pub/introgeology/
5. Our Dynamic Desert. (2009, December 18). Our Dynamic Desert. Retrieved August 31, 2023, from https://pubs.usgs.gov/of/2004/1007/intro.html