Have you ever heard of the Huaynaputina eruption of 1600 AD, which may have had global consequences? Well if you're ever driving in the Panamerican Highway between the Arequipa and Moquegua departments in southern Peru, you'll be constantly reminded of it. That happened to me last year when during a trip I noticed these gray dunes, some even barchan-shaped, resting on the flanks of rock formations, mostly on one side of the highway. I knew that wasn't common sand, and later I found out it was ash from the Huaynaputina. Sadly, I didn't take any photos (now I wonder why...) but Google Earth provides a great view.

I really like the 1st image because you can actually see the ash getting blown by the wind from NNE to SSW, almost painting the landscape. The 2nd image is a zoomed view of the highway, where the ash gets trapped in the rock flanks facing north, developing dunes. The 3rd image is a regional view with the Huaynaputina volcano on the NE corner, and the area I highlighted to the SW.

Check it out on Google Maps.