I've been musing over a high rez image of Escher lately. (get it here) Like an Escher print, this puzzle may be more simple than first appears.

This is the current line o' thought:

The shapes suggest that the original sedimentary rock formed in a lake or shallow sea - which then dried and solidified - before undergoing a second wet episode and yet another drying out.

And large timescales may have been involved. The latter drying episode must have come after the impact that formed the Endurance crater, Grotzinger says.

From New Scientist: Water coursed through Martian hills

I present a different idea, one that still involves multiple wet dry cycles but not as much time between them.

In the picture of Escher, it is clear to see that bedding is dipping in this rock while the polygonal joint set appears to be vertical. That's the crux of the puzzle: if the beds were originally horizontal, they must have been tilted before the imposition of the polygonal fracture set. But what if that bedding was not orginally horizontal? In otherwords: the scoured top of a crossbed set.

In arid environments here on earth (which is not Mars*), this is how cross-bed sets form: There is a dune field. At the base of the dune field is a water** table. It holds together the lowest part of the dune field while the upper parts are free to migrate with the wind. If the water table rises, more of the dunes are protected. If it falls, the dune field is eroded more deeply (scoured). Does that make sense, or should I draw a picture?

Wait, I don't have to. Went off to google and found this: this, which I'd do a direct link to, but it's a pdf. The picture is from the White Sands National Park in New Mexico showing a scoured surface of cross-bedding. Pretty cool.

(The link is from chapter 8 of Geologic Overview of White Sands National Monument by S.G. Fryberger. I've only just discovered it, so I've not read it all, but it looks pretty good; especially chapters 6, 7 and 8. I've been looking for a good overview of this area, as I have believed for some time that it is a possible analog to what Oppy is checking out. At least it might some insight into the structures we are seeing. )

So, perhaps Escher was not tilted before descication polygons formed.

Just something to think 'bout.

~jms~

*offical geologic arm-waving disclaimer
** Second official geologic arm-waving disclaimer: I'm not saying the fluid on Mars was di-hydrogen oxide.