This image from NASA Mars Reconnaissance Orbiter shows several seemingly active gullies and their associated fans near the Argyre region. As gullies form on slopes, sediment can become deposited to form lobe-shaped fans. Gullies can form with flowing water, but don't have to: sometimes they can be formed by the dry flow of sand and dust grains. This HiRISE image shows several seemingly active gullies and their associated fans. These particular gullies have many fine channels that are resolved nicely with a high resolution image. http://photojournal.jpl.nasa.gov/catalog/PIA19286