Thanks for the link, LJ! Good info.

Let the arm-waving begin! (bold face added)

"These are much bigger than anything we've seen before of layered rocks on Mars," Squyres said during a mission briefing. "That is going to tell us much of what is going to happen at Meridiani Planum."While there is ample suggestion of crossbedding in the cliff and a healthy scattering of hematite-rich spherules across the crater, mission scientists aren't sure what Burns Cliff may tell them about the history of Martian water.Squyres said Opportunity's remote sensing instruments indicate the cliff is mostly basalt, a type of rock that makes up most of the seafloor on Earth. But whether that means the rock is solely sandstone or was once a Martian beach or sand dune remains to be determined, he added.

Mars rover prepares for a cliffhanger

Basalt sand beaches? Aloha Hawaii ! Can't think of any basalt dunes off the top o' my head, but I'm sure they exist, somewhere. Is there a known terrestrial sabkha developed on a mafic bedrock? I'll look into it.