Good points Nick, and they prompted some thoughts of my own.

1. "Lack of Breccia"

This is interesting. Looking earlier than the questions of whether Mars was wet 4 billion years ago, I think there is a fascinating story here about the very early formation of Mars.

-- We don't we see breccias like we do on the Moon

-- We don't we see primoridal crystalline rock like we do on the Moon (admittedly, we haven't looked in the highlands yet)

-- We do see lots of heavily cratered surfaces, however

Does this imply that due to not having a gigantic impact, Mars didn't have a "magma ocean" the way the Moon (and presumably Earth) did, and so no early plagioclase rich crust formed?

Was Mars pumping out so much basalt early on that it had formed a complete basaltic crust (or buried something older) while the heavy bombardment was still taking place? The Moon, after all, only put out cosmetic amounts of basalt (the maria), and most of that came *after* the heavy bombardment was over.

Even so, why no breccias made out of basalt? Would we find them if we looked at the mountainous terrain around Hellas, or the bottom layers in Valles Marineris?

Anyway, just tossing out some uneducated thoughts. The basic point is that the fact that Mars has very un-Moon like rocks yet Moon-like ancient cratered surfaces is telling us something interesting about the first few hundred million years of Mars history, but what I don't know.

2. "Gentle upwarp of Gusev materials"

Seems possible. My Pacific Northwest predjudice makes me think of the folds and ridges you find here and there in the flood basalts of the Columbia Plateau. Mostly they are flat but here and there compressional forces have brought up a gentle ridge a few hundred feet high.

That being said, it also seems possible that the hills are remanent of an older, higher terrain. Either an erosional remnant of something that sat above the basalt we're driving on now, or hills that pre-date the basalt and were never buried by it. I still see a slight curve that makes me wonder if it's part of an old crater rim, but I can't say I've looked at it very closely.

Why so sure it is just uplifed basalt, Nick? I'm not arguing for one of the other interpretations, just wondering what your reasoning is.

3. "Top of sequence at Eagle"

Good points Nick and JMS. All those blueberries out there certainly suggest that a lot of stuff has already eroded off the top.

The only thing I can think of is that Squyres, et al., are concluding that we are looking at the top of the sequence due to the elevated bromine levels, which is one of the last things you see in the evaporitic sequence according to the table that JMS helpfully posted. Last things with Earth ocean water that is, and with some prettys imple assumptions about how the evaporation proceeded (like, one big slug of water evaporating w/o intermittent replenishment). Seems pretty flimsy if that's it.