An intriguing hypothesis... I do not see any apparent sign of underlying crater on the surface, but if the bottom of tracks is everwere made of heavy soil, it seems that there is a deep depression in front (rear) of Opportunity...

Hey, maybe is safer to avoid go to South in this point! They should move perpendicular to present direction, then try alternative ways... otherwise, even if they avoid the dune, small trip will result in another stall!
Even worse, increased sand depth could be a general characteristic of all terrain around Erebus (in fact, stall occurred in a transition region were terrain height slightly increase) and this could be the end of the mission!!!
I'm stongly hoping I'm wrong!!!
Other interpretations?

And there's more - look at the apparent change of the color of the soil just left of the line between the hard and soft soil. In the map that Pando made, it seems, that Oppy stuck in a much much brighter place than the soil covered before that, and that to the left of this bright patch there's no more such bright patches way down to Erebus (where those bright patches are rocks). So to me, the way to Erebeus doesn't look so bad.

Even if stall occurred on the bright patch, I think that real issue could the dark "new" terrain between Oppy and Erebus.
I just posted my consideration in the other thread "Free" (http://www.unmannedspaceflight.com/index.php?showtopic=1048&view=findpost&p=11986) and I reccommend you to read also related items in the "Opportunity Route Map" thread...

Yes, I saw them, but I had to go to work. Now I'm at work, so I'll have time to look to them :-) And the other thread is much more suitable for this discussion :-)