The first eye-catching difference between the two valleys is the dark coloration of Tithonium Chasma. At closer look, dark dunes can be spotted in the upper portion of the image (northwest). Due to the closeness to the Tharsis volcanic region, much of the rocks present in this area are composed of layer after layer of lava flows and volcanic ashes that may be the source of the dark colored dune-forming sands. Another very interesting feature are the two light-toned mounds, one just facing the dark dune field and cut by the image border and the other in the middle of this part of Thithonium Chasma. They measure more than 3000 m in height and display a surface that was intensely eroded by wind. These erosional features are called yardangs and indicate that the material forming these dome shaped mounds is much weaker and easily erodible compared to the surrounding rocks. The floor between the two light toned deposits shows peculiar little knobs, presumably made out of the same material like the mounds. Spectral investigations indicate important concentrations of water-bearing sulphate minerals in these deposits. Many scientists believe that these deposits developed by evaporation at a time the chasmata were filled with liquid water, this formation scenario however is still strongly debated in the community.