Sure, but does there need to have been much movement along the fault plane (if that's what it is) indicated by my yellow line? There's no visible overhang associated with it.

Even the movement along the red "fault" was only a short distance, as judged by the width of the eroded overhang, and mostly towards the camera, so I wouldn't expect it to break up the previous strata visibly much.

3D will help!

That was a very nice enhancement, fredk. It brought out a few things that were only subtly suggested in the raw imagery.

There is a whole lot of stuff going on here, and this kind of remote field geology is not easy to do. If we could be there in person to scramble around this promontory and see things with our own eyes, we would probably figure out 80-90% of the puzzle in 20-30 seconds.

We are seeing a whole mix of features created by different kinds of processes....sedimentary features, structural features, secondary geochemical features, and erosional features...to name a few. We need to concentrate on those that are most important. This bedrock is really busted up, and there is some noticeable downward creeping or settling of blocks. Concentrating on the internal sedimentary structures, the laminae and truncation surfaces, we need to keep in mind that these are the eroded and buried remnants of previously existing three dimensional bedforms (dunes and ripples and whatever). To properly analyze what we see, we need to consider all of the apparent dips and angles of the layers and laminae in three dimensions.

The view of Cape Desire from Cabo Corrientes so far leaves something to be desired, but remember that we have several views of its other side from across the Bay of Toil. I think some aspects are more convincing in the views from those earlier sols. Ooh! There is some color LBL stereo from the other side. I have no time to play with it tonight, though.

Faults with little movement sound reasonable to me. That would keep the strata in their original alignment, but might allow some sort of concretion process along the fault plane, creating a hardened layer (overhang) that could also be impermeable to subsequent saline solutions (tonal boundary).

So, we're seeing any number of faults in Victoria's walls. Here's the question -- how much of this faulting was caused by the impact event, and how much was already there when the impact occurred?

I think that the faulting that causes features like Guam (aka The Dock) was caused by the impact. Impacts can cause arcuate faulting in the surrounding rockbeds that is roughly concentric to the crater rim. These arcuate faults tend to form in lobes, rather than in perfectly concentric circles, probably controlled by pre-existing faulting and also by changes in the tensile strength of the rockbeds through which the seismic shock passes. (I'm very tempted to say that Guam defines where a cape and bay will eventually appear as Victoria continues to erode, and I'm also tempted to say that such impact-related faulting has created the cape-and-bay structure we see now.)

I'm also tempted to say that a majority of the faulting we're seeing in the walls themselves is also impact-related, being the result of the extensive fracturing that the rockbeds underwent during the impact process. But it's very possible that some of the faulting was controlled by existing faulting in the rockbeds. (Note that this means there is something about the rockbed target of the Victoria impact that created these cape-and-bay fault structures which doesn't occur around most Martian craters, since most martian craters don't exhibit the same cape-and-bay rim structures.)

The straight-line faulting we see in the east-southeast wall looks more like pre-existing faulting that is now controlling how Victoria's rim is eroding along that edge of the crater. I doubt that this particular straight-line fault was exposed by the original impact -- it's only becoming visible as the crater walls erode and slump back to it.

I think that a computer-generated model of the cracking patterns one could expect in the types of rockbeds we've observed at Meridiani from a Victoria-sized impact could shed some light on the various fracture planes and fault lines we're seeing in the walls, here...

-the other Doug

Just a couple of thoughts -
If this is indeed a hardened fault plane, perhaps analogous to the ones recently identified from HiRISE, is it the first one Opportunity has visited? Will they become more frequent as we get nearer to the HiRISE ones?

I don't think this particular one will be visible on the other side of Cape Desire because it would have reached the pre-impact surface somewhere in mid-cape.

Edit - Hi other doug we overlapped there. I think the Cape Desire fault (if it is one) and its hardening by concretion (if it was) would be pre-impact. If there were pre-existing hardened faults in the region then they would have been more reistant to fracture by the impact than the surrounding rock.

I just want to make sure that we're all using terms in the same way. As I understand it, a fracture is any break in the rock. In some cases there may have been motion along the fracture surface, in that case the fracture is called a fault. In other cases the rocks my have fractured without any displacement occurring along the fracture surface. Fractures without movement are usually called joints.

There are many ways to recognize faulting in rocks: offset beds across the fault, fracturing along the faulted surface, and secondary mineralization along the faulted surface are a few.

Certainly we are seeing lots of fractures in the walls of Victoria, but I would hesitate to conclude too quickly that many of them are faults.

Thanks for clarifying the terminology. As far as the Cape Desire feature is concerned - in that case fredk was proposing a genuine fault with movement to explain the overhang. My modified suggestion is a cemented joint or fracture, because I think that accounts better for the sharp tonal contrast across it. It also happens to be 'the latest fashion' What do you think?

Fredk has made some interesting observations, and he might be right in his conclusions. Just for the sake of argument, I'll propose another hypothesis. Perhaps the lighter toned layer represents the deposition of a different dune system. There may have been some wind erosion after the lower, darker layer was deposited; then new sediment was deposited on top. The lighter shade of the upper layer might be because the size of the grains are different or the composition might be a bit different. Perhaps the upper layer contains a greater admixture of salt as part of the original deposit. The overhang might be due to some differential weathering of the two units; or a fracture (without movement) may have occurred along the contact between the two due to the difference in grain-size or composition of the two units. It's just another hypothesis. I could be very, very wrong.

A fault contact between the two layers is a possibility, but we really need to be able to examine the contact much more closely and in more places before we can come to any conclusions. In the distance it appears as if the lighter toned bed displays some broad undulations in it's position on the crater wall. Wierd.

It could be that we ARE seeing lots of faults in the sides of the crater, I think we just need to be careful about how we identify them. I think dvandorn is correct. There should be faulting associated with the impact. It's reasonable to expect that there would have been some movement along the rim as the margins of the crater slipped towards the center.

We may be looking at a crater, but I suspect it's really just an exposure. I don't expect to see many impact -related features in the strata, because those have gone. Anyhow, this has been (still is) a really interesting discussion. A purely depositional explanation for the Cape Desire diagonal feature still seems unlikely to me. I hope we hear something soon from the MER team. Thanks all.

Well, when you start getting into that it becomes mostly a semantic discussion. The hole/exposure is here only because there was an impact crater, regardless of it's current condition. The depth of this hole and its circular geometry pretty much dictate an impact crater origin for such a feature in this region. But, I know what you are saying.

I wanted to comment on a couple of other things. I'm in Gray's camp with regard to the overhang being a differential erosion feature. I also agree with him regarding cautious use of the F-word. A while back that meant festoons, which we are still on the lookout for, but today I'm talking about faults. I think there is ample reason to suspect that some minor faulting might exist here, but I'm not convinced, yet.

We have the suggestive shape of the dock/Guam feature mirroring the adjacent cape and bay outline. I will not say that it is not the result of the bedrock in front of it slumping into the crater void along a normal fault, but I think there could be other explanations for it and what appears to be its subdued extension to the east. The ejecta layer does not appear to be entirely homogeneous.

This whole cape and bay thing has had me really scratching my head for a long time. I have some real problems with the Edgett hypothesis now that we've observed a nice sampling of the rim. In some of the images I have noticed a crude organization within the ejecta layer. I am seeing discontinuous "strata" within it that sometimes seem to erode differentially. I am wondering if the dock might simply be an eroded remnant of such a layer.

What a fortunate rover we have here, to have found its way to so many fascinating places. I think I have asked this question previously, but I don't think anyone has replied. Who is having fun?

Just to clarify - I was not questioning the impact origin, just emphasising the consensus view that it didn't directly create the cliff surfaces we're examining now. As to what did, what's wrong with the undermining and collapse idea? It would certainly create plenty of randomly oriented vertical fractures, unlike the impact event itself which presumably created conical and radial ones. Fun indeed

I haven't discarded the concept. I'm just not as convinced as I once was.

As for the fractures, there are so many reasons to expect separate populations of pre-impact, impact, and post-impact fracturing that I think they may be difficult to distinguish in all but the most obvious cases. Everywhere Opportunity roamed since day one, we saw fractured bedrock. It seems a safe bet to assume the impact slammed into previously fractured rock. The dehydration of the ubiquitous sulfates and the attendant shrinkage over eons seems to be a good explanation for that.

I was guessing that the impact created a whole swarm of fractures that would appear to be randomly oriented in close-up views. Then there comes a long history of the cliffs sagging into the hole under the forces of gravity and erosion.

Thanks for answering the question. It seemed that you were.

Long baseline view now available for the feature in question on Cape Desire, sols 1091 and 1095 pancam L2:
Click to view attachment

That's very nice! Hopefully we'll get the one to the left of that as well.

Cosmicrocker I think you're dead right to emphasise the complexity of the situation. It's all too easy to forget what a huge timespan is available for successive processes.