Those "gullies" look like chemical stains or we...

Those "gullies" look like chemical stains or weathering.

You can see horixotnal bedding planes cross the vertical dark features almost undisturbed, so they are not an erosioanl feature like a gully, they are chemical weathering. Some fluid is weeping from the top of burns cliff and altering the rocks underneath it.

PS, where the heck is HORTON!