re: Ground Truth
Very nicely stated.
And I agree about the pale rocks, though perhaps we'll have a clue once Opportunity has analysed its local set of pale rocks and we think about their relationship to the dark sand locally - this is quite a common association across Mars - pale rocks surrounded by drifts of unusually dark dust. White Rock is just the same...
(But that doesn't neccesarily mean it's made of just the same stuff...)