Evidence that the Great Unconformity did not Form Before the Strata above it

Actually, you are correct. "Original Horizontality" is really an approximation. There are sediments that can be deposited at an angle such as alluvial fans, or or sand dunes, etc. I never know how deep to get into these discussions with laymen.

Okay, so where is the shearing texture between the two rock types? That would be evidence. Actually, that happens all the time as we have been discussing.And we have the evidence of sedimentary textures at the contact rather than shearing.And I'm really having a hard time figuring out your kinematics here on the shearing. How do you get such rough surfaces in all directions if you are calling the contact a shear zone? Which way are the rocks moving and why does the deformation not extend into the overlying sequence?

Exactly -- just in the last bit I have found (as I knew I would) reference after reference and picture after picture of situations (mud flows, flaser beds, etc) that form nonhorizontal deposits in higher energy environments.What's crazy it that when it suit her, Faith responds that "the most basic law of Geology" says that deposits must be horizontal. Next she tries to insert Walther's law all over the (inappropriate) place which of course when it IS invoked properly, isn't leaving purely horizontal deposits.But hey, I'll be waiting for her references for this "most basic law of Geology".JB

I don't think we want to get into all of the ramifications of Walther's Law here. It really is not as intuitive as Faith thinks.ABE: I guess if there's one message that I'd like to get across, it's that things are usually more complicated than you think. Edited by edge, : No reason given.

Agreed. The devil is in the details.JB

You are right that I don't have much information on this locale, but I can tell the deposition was not continuous because there is sandstone laying on top of gneiss, it could not be continuously deposited. Gneiss is metamorphic rock, sandstone is sedimentary rock. Deposition could not be continuous. I drew a yellow line on the top of the tan member. The grey member is above the line. Notice how the tan member pinches off against the red line? It does not reappear on the right side of the photo. The "arrows" point to where the tan member should be.If it was deposited on a flat surface, then why does the tan member not extend all the way across the photo? Now this is where I cannot figure out what you could possibly be thinking. So let me see if I understand your idea...> Deposition begins somewhere below the road.
> Deposition is continuous until the strata are deposited up to the level of the upper part of the photo.
> Tectonic forces cause uplift
> All the material below the red line is metamorphosed into gneiss
> The material above the red line remains sandstone (not metamorphic)
> The contact between the metamorphic rocks and the sandstone has fooled geologists for a hundred years who have thought it was an erosional surface.Is that about it?HBD

Sediment certainly can drape over a structure. It does not defy gravity, in fact it is gravity that causes the phenomenon. This is known as the angle of repose. This image shows the relationship between gravity, shear strength and shear stress. Gravity (working on the shape of the particle) determines the shear strength. The angle determines the shear stress. When the shear stress exceeds the shear strength, the material will slide done the slope. Figure A in the image above is what you are expecting that horizontally deposited sediment would look like in a basin. But if sediment was falling on the whole area equally, it would be falling on the slopes as well. Because of the angle of repose, it would not slide down the slope until the angle was steep enough to overcome the shear strength. So what you would actually get is something closer to figure B.Figure C is an illustration of how it would look if the sediment source was coming from the right. More sediment would be deposited closer to the source and again, it would remain on the slope until the shear stress exceeded the shear strength.The same principal applies to mounded features, just in reverse. (ignore the 1st figure) So, the Principle of Horizontality is a basic principal not a universal law.

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Thus the Principle of Original Horizontality is widely, but not universally, applicable in the study of sedimentology, stratigraphy and structural geology.

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Similarly, sediments may drape over a pre-existing inclined surface: these sediments are usually deposited conformably to the pre-existing surface. Also sedimentary beds may pinch out along strike, implying that slight angles existed during their deposition.

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You can easily test this with some colored sand and a fish tank.HBD

Oh OK.I am not really sure if metamorphic rock is supposed to be original created rock or what. If that is the case, then why make a big deal out of an unconformity? If not, then it metamorphosed during deformation, which doesn't really make sense. So I am not sure about the gneiss??? I was just considering everything above it, assuming the gneiss was already there. Yeah, it doesn't look like a whole lot of activity at this site... pretty quiet actually. Maybe, but it extends pretty high up. It looks to me more like the center section has been smashed and broken up somewhat, but it is hard to tell from the photo.The more interesting stuff is on the cross section.HBD

But there is no real disruption of bedding above the high point of the gneiss. You can follow it (the bedding) all the way across the photograph without interruption. I know this is no hindrance for Faith, but it's hard for me to see any such uplift.But then she's never given us an kinematic model for this deformation, has she? Uplift of the gneiss core would not do anything to the unconformity but offset it. I don't see it shearing nor do I see it turning from depositional to erosion (or whatever the heck she's talking about). The could be something going on in the gneiss, but, once again, it does not project into the overlying sandstone.

Or a snowstorm. JBEdited by ThinAirDesigns, : No reason given.

Yeah, and that's why I thought this was such an interesting example. What looks to me like uplift was not projected all the way up to the upper layers. The lower layers follow the yellow line but the upper ones don't.Then I saw that center section below the label. The cracks in the rest of the wall run vertical and horizontal - at right angles. But in that center section they are also at 45 degrees. And I thought... that explains it. Of course, that section could just be an artifact of the road cut. When I was a kid we drove through the Appalachians and for the longest time I couldn't figure out what those long, straight "cracks" were in the rock faces - all fairly equally spaced. I thought it was the strangest geological feature. Turns out they were the holes the construction crews drilled to drop in dynamite. HBD

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Then I saw that center section below the label. The cracks in the rest of the wall run vertical and horizontal - at right angles. But in that center section they are also at 45 degrees. And I thought... that explains it.

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Yes, there is a concentration of fractures at that point. But they do not really have any effect on bedding. They are probably some kind of response to uplift of the region and perhaps irregularities in the basement surface. And we can't really rule out human artifacts without being there.

Perhaps you have a larger version of the picture available than I do, because I can't seem to find the 45 degree cracks you refer to, but for sure the vertical striations regularly occurring across the face are cut drilling artifacts.JB

I'm sensing a degree of bogusity creeping into this discussion.1) I point out that there is a large vertical exaggeration in that diagram (it even says "vertical scale greatly exaggerated" in the lower left corner). The diagrams vertical dimension is about 14,000 feet, while the (unstated) horizontal dimension is probably several hundreds of miles.2) The basin is NOT a big (14,000 foot) depression waiting to be filled. Rather, it is a shallow depression which is further subsiding as deposition happens. I would guess that at any point in time, the slope of the deposition surface is actually quite close to horizontal.3) In the Silurian and Devonian units, the "horizontal" lines of the block pattern are not necessarily representing the bedding planes, especially at the tops of the units. To interpret those lines as bedding would imply that the tops of the Silurian and Devonian units are angular unconformities, when in reality (I strongly suspect) they are conformable contacts (gradational contacts?).Bottom line - I think the bedding of the various units were originally essentially horizontal. Deviations from horizontal are results of post sedimentation deformation. Actually, both sedimentation and deformation are simultaneous processes.MooseEdited by Minnemooseus, : Change subtitle.

Principle of original horizontality - WikipediaThe definition seems to have been compromised since Steno formulated it, which I think is mostly due to forgetting that it refers to the formation of strata and not sand dunes. In answer tio which I'd point out all the nice neat sandstone strata such as the Coconino which are surprisingly straight and flat and horizontal on top and bottom although the Coconino in particular shows a duney-like orientation of sand grains.But I think I'll probably have more to say about this later since I see a lot of silliness about "draped" sediments up ahead..Edited by Faith, : No reason given.