According to a new paper Mars was not a water world in the deep past but an ice world. Rivers and alluvial fans were most likely made under a covering of ices says a paper in Nature Geoscience. 

https://www.forbes.com/sites/jamiecartereurope/2020/08/04/was-mars-once-the-white-planet-ice-sheets-not-rivers-created-martian-valleys-say-scientists

Very interesting piece of news. Thank you. +1

Although, AFAIK, Mars' surface is very homogeneous, I wonder if one could detect signs of past transport phenomena, normally associated to glaciers, like erratics. Something like that would definitely clinch the case, IMO. Or maybe Mars' geology is just too boring to hope for that. Scratching patterns could be another sign. Maybe also difficult to see on Mars' surface, as Martian soil is generally soft and windswept.

Very nice, Moontanman... as you once proposed and I accepted as more logical.