It's been claimed that there was once water flowing down the alluvial fan in Gale Crater and one might ask what was its source? The image above shows an Google Mars elevation profile for a path through the channel at the crater rim and down the alluvial fan. The area just to the north of the crater wall seems to have been filled in with material. Going farther north uphill one comes to what appears to be an eroded crater. Could this be a volcanic crater that has collapsed? Water is not necessary to explain the filled in material. It could have been blown in over a very long period of time. But could this collapsed crater or Mt Sharp have been a mud volcano? An underground source of churning mud might produce rounded stones like those from a rock tumbler and require less water to do so. A subsequent eruption could have deposited the material on the alluvial fan. We need to keep and open mind when it comes to an alien planet and perhaps the rock tumbler would be a good test that the proponents of flowing water on Mars would have to pass to prove their case. Appearances can be deceiving at times.