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Some geological materials (like solid rock) are incredibly tough, but others (like piles of volcanic ash) are quite soft. Some materials are soft enough that they can be eroded by the wind alone and yield landscapes that look like what we see in this HiRISE image.

The long straight ridges seen here are called yardangs and they form on Mars (and Earth) when the wind strips away the inter-ridge material. This process is greatly aided when the wind is also blowing sand along. The sand grains do an effective job at stripping away loose material: these ridges are literally being sandblasted.

Yardangs are useful features to recognize because the tell us the direction the wind is blowing in. They take a long time to form so this direction is the dominant wind orientation averaged over a long period of time (which might be quite different that the winds on Mars today). These yardangs also tell us that the surface here is made up of loose weak material and this information, in conjunction with other data, can tell us what the material is composed of and what the history of this particular site on Mars has been.

The University of Arizona, Tucson, operates HiRISE, which was built by Ball Aerospace & Technologies Corp., Boulder, Colorado. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Reconnaissance Orbiter Project and Mars Science Laboratory Project for NASA's Science Mission Directorate, Washington.