Gullied Trough in Noachis Terra (extra-detail mgnf - false colors)

The majority of the gullies on both sides of the trough appear to originate at a boulder-rich layer seen in this subimage.
The layer appears dark on the sunlit slope because the boulders sticking out from the slopes cast shadows. If these gullies formed by water from the subsurface, then it is possible that this layer is a permeable layer that conducted water to the surface.
The layer is deteriorating and traveling down slope in the form of boulders.
These boulders can clearly be seen in the alcoves of the gullies on both sides of the trough.

Note that the alternating stripes on the lower right (Dx) side of the image are an artifact from camera noise. They are not real features.