[X] My Assistant

[aldo12xu]

Well, it's literally been months in the making -- on and off, whenever I've had some spare time -- but I finally finished a compilation on the geology of the different rock types that were observed by Opportunity on it's trek from Eagle to Erebus craters. In a way, it was good that Opportunity hadn't moved very far since last November. This way my update is still up to date

So if you have some spare time, have a look. There's a lot of info there, which I hope will serve as a useful guide once we get to see some deeper layers at Victoria.

The link to the write up is on my homepage.

Cheers,
Aldo.

[dvandorn]

Remember, when speaking of the general slope over distance and how that might affect what we're rolling over stratigraphically, that the surface may not have always reposed at the same slope that we see today.

As Aldo captured so well, this area seems to have seen repeated episodes of deposition, each involving long periods with high water tables and then undergoing evaporation and salt cementation.

When supported by a water table, such a landfill assumes a pretty flat surface. The deposited material fills in the uneven terrain that underlies it -- and on Mars after the LHB, most of the terrain was pretty uneven.

As the water table recedes, the loosely consolidated sand and cemented sandstone tends to contract and slump a bit. This allows subtle surface expressions of underlying terrain. Of course, buried terrain closest to the surface is expressed the most.

So, it's possible that ridges and depressions expressed faintly in the current topography reflect larger and more impressive terrain that lies beneath. In this case, except for minor amounts of deflation and additional deposition that may have occurred over the millennia, the overall surface can be considered a single, relatively flat unit that is draped over expressions of underlying topography.

Thus, when taking into account deflation and deposition that has occurred slowly since the great dry-out, any given stratigraphic level on the surface is likely to be within a few feet of the level at any other given point.

Or, in more basic terms -- take a lasagna and plop it down on top of a big meatball. The lasagna will have a bulge in its top (an expression of the underlying meatball), and as you traverse along the top of the lasagna you may find local variations in the thickness of the cheese and sauce layers atop the highest noodle layer. But you're ultimately traversing the same stratigraphic unit that you would have seen had the lasagna never been dropped onto the meatball.

-the other Doug