Changing Face of a Sand Dune
March 15, 2012
These two photos show the same sand dune in southern Death Valley, taken 15 years apart. Both photos are towards the north, and both were taken within 15 minutes of sunset. I shot the first image in mid-January, 1996 and the second in late November, 2011; the azimuth for sunset on these dates was nearly identical. Variations in shadow strengths therefore reflect changes in orientation of the dune rather than sun angle.
This dune is a “star dune”, named so because it has multiple ridges that lead towards its summit, an effect of wind that blows in multiple directions. For this reason, the dune is also fairly symmetrical, with its west side (left) only slightly steeper than the east. Without a single prevailing wind direction, star dunes migrate very slowly if at all, and typically grow upwards rather than laterally. Many star dunes reach heights of 200 meters or more; this particular one has a height of about 50 meters.
Even so, this dune has changed its shape over the last 15 years, likely as the result of wind that blew from east-to-west more than in the other directions. Most notably, the narrow swale between the high point on the dune and the small ridge in the middle ground widened considerably, so as to nearly isolate the ridge from the main dune. This widening appears to have occurred at the expense of the brightly lit western face of the main dune, which also looks to be deflected westward, given the decreased strength of its shadow. Additionally, the crest of the small ridge shows much more curvature than it did 15 years ago, in a sense that also suggests west-directed wind.
Photo details: 1996 photo - Canon F1, Kodachrome 25 film - 2011 photo: Canon 5D Mark II with EF100-400mm f/4.5-5.6L lens at 190mm.