These form in areas with moderate constant winds, and travel downwind while retaining their shape. Can also happen in snow, though those are generally less widely noticed -- and less permanent. Per Wikipedia:
As barchan dunes migrate, smaller dunes outpace larger dunes, bumping into the rear of the larger dune and eventually appear to punch through the large dune to appear on the other side. The process appears superficially similar to waves of light, sound, or water that pass directly through each other, but the detailed mechanism is very different.Which is, of course, fascinating. By way of remembering the pronunciation, also sometimes spelled barkhan. Adopted in 1888 from Russian, where it was adopted by naturalist Alexander von Middendorf from Kazakh (the deserts of Kazakhstan have many barchans), presumably from Turkic roots.
The barchans in the park walk across the desert in a westerly direction at a speed of about 17 meters per year.
(Apologies for skipping last week, but I was caught up in an out-of-town family emergency.)