I see something interesting here. It looks like the river bed is free of craters. Its as if it was formed after the cratering period.

These are essentially raster scan renderings of a 1-D 2 bit signal. As folks have noted the authors chose 3500 at the raster line size. They also noted that no matter what the raster line size is, patterns appear.

I believe that the source of the patterns are long repeated sequences. A sequence that repeats after a constant number of base-pairs will appear as a diagnonal line. This is independent of the raster line size, however the slope of the line will differ dependent on raster line size. Arcs and circles represent sequences that repeat after a variable number of base-pairs which varies by some polynomial.