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.
Written by: Candy Hansen (audio: Tre Gibbs) (11 February 2015)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_039747_1090
Image: NASA/JPL/University of Arizona