re: Strange sand bank in front of Opportunity.
The faint hexagonal patterns look like soil creep is occurring due to differential thermal expansion of dark and light grains in the diurnal (or annual) temperature cycle.
(I don't mean to imply that dark or light grains expand more or faster because of their colour, but that they are different composition which influences both colour and physical properties)
I agree that there's a risk that this is quite a loose sand drift - a bit like quicksand on Earth, but hopefully not as bad...