This map is certainly very interesting, but it suffers as a visualization. The topographical map is nice, due to its simplicity and well-chosen color scale, but the main geological map certainly has some problems.

The biggest problem is that they are choosing two represent two different dimensions of data(age/time period and terrain type), but are using seemingly random variations of hue to show these variations. Additionally, with so many distinct hues, it is nearly impossible to tell the difference between some areas on the map (for example, Hesperian basin unit and Hesperian polar look very similar).

A more effective way to visualize this data would be to apply a separate hue to the 8 categories of terrain unit, since these are categorical data points. Then, visualize the geological age on a brightness scale, since this is more of a quantitative measurement (obviously the current map also show the particular time period, we'll just ignore that for now). I think this would provide a much clearer view of the data, though you would lose a small bit of granularity.