Gullies are often found on steep slopes. In the winter, this area is covered with a layer of carbon dioxide ice (dry ice). In the spring, when the ice warms up and transitions to gas, it dislodges material on the slope, forming a gully. In general, this process works best on fine material, leaving behind large boulders. These boulders can be seen collected in the gully alcoves. Occasionally, boulders slide or roll downhill, like those sprinkled downslope in this image. This set of gullies is found at -71 degrees latitude in the Southern hemisphere. http://photojournal.jpl.nasa.gov/catalog/PIA19295