In this image from NASA's Mars Reconnaissance Orbiter, dark branching features can be seen on the floor of the Antoniadi crater. Their shape resembles that of giant ferns. The dark channels are filled with a rocky material which is harder than the surrounding sediment. As the softer sediment around the channels eroded away, the harder, rocky material was left behind to form the raised, fern-shaped features.
NASA / JPL-Caltech / University of Arizona