HiRISE: Boulders in gully alcoves

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…[More at link]