I made a 3-D GIF animation with R1/L7 (both IR) and it looks cool, but as usualy I can't post it here so if someone else wants to do that you can see that the dark vertical features are not carved gullies. Either some dark sandy material is flowing from higer up and leaving these trails, or it is a flouid causing chemical alteration and staining. If it is sand, it must be sticky or very heavy because otherwise the windo would blow it away quickly. There isn't much on that clif for the sand to stick too. I'm voting for fluid and cehmical alteration.
This might be a similar mechanism to gullies we have seen from orbit, although those only occur at high latitudes. IT might be that only much smaller gullies form at mid latitudes, the mechanism is stronger and forms larger gullies at higher latitudes.
