A series of Questions for our Geologists.

For one thing if a flat segment of sediment is folded up drastically and then worn down there will be a pretty strong indication of this in the remaining "cores". The two sides of the loop of sediment would remain separated by where the rest of the mountain was.

Absolutely true. The link that you provided from AIG acknowledges that the sand found out west comes from the East Coast. When Lake Missoula produced the scablands of the Columbia Basin it pretty rapidly wore through basalt bedrock.It seems to me that we need to know several things.

• how big was the original structure?
• what was it made of?
• what methods could erode such material?
• where was the eroded material deposited?

For example. Erosion from a river generally produces a fan shape outflow. On the otherhand, a flood generally distributes material fairly uniformly over the whole area flooded.

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Aslan is not a Tame Lion

Great, do we see that in this segment? Does that explain the twisting parallel lines we see there?

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Aslan is not a Tame Lion

The Arbuckle Mountains between Ardmore and Sulphur, Oklahoma are a fantastic example of a fold with the middle eroded out. At the center of the structure you have Cambrian (or Precambrian?) stromatolites and such, and as you travel north or south you cross outcrops of practically every oil-producing formation in the state. Th fully "reconstructed" fold would supposedly be nine miles high, but of course it actually eroded as it bent up.http://www.geog.unt.edu/...frame_files/FIELDTRIPS/ok/ok.html

In general, the assymetry of the folding shows movement from the east toward the west. This conforms to the direction of thrusting that we see in the same rocks. That means that the eastern margin of the NA continent was likely overriden by crustal rocks from the east, in this case the African continent. Fortunately, we have fragments of this overriding continent in the Blue Ridge and its equivalent crystalline rocks.Crystalline rocks of the Precambrian form the basement rocks for the Paleozoic rocks. These are exposed in places like the White Mountains and other areas, possibly as referred to by other posters. The fact that these rocks are exposed at the surface and the sedimentary rocks uplifted around thme suggests that there was a mountain range.The metamorphic minerals are part of the crystalline complex, but also occur in the sedimentary sequence. This means that they have been buried much deeper than they are today. The assemblages are also zoned so that we can generally see more deeply into the mountain range by using the higher temperature minerals. In fact, as a curiosity, there is a mineral assemblage known as blueschists in parts of the greater Appalachian orogney that suggest deep burial but low temperatures. This is consistent with being rapidly overidden by a large, package of supracrustal rocks. And no, I wouldn't expect your layman to recognize the metamorphic assemblages without some
help.Because of differential erosion of multiple orogenic events, the picture of the entire orogen can be quite complex even though a single area can be explained rather easily. The linear ridges you see throughout the Valley and Ridge Province is the effect of eroding upturned sedimentary rocks that have been folded along hinges that run roughly parallel to the continental margin. The reason the seem to swirl around is that the folds do not have horizontal hinges. In other words, the folds have swales and culminations that stand lower and higher than other parts of the folds. That is why we see all of the Z patterns in the northern part of the province.

Coragyps,Good link. In fact, it mentions blueschists in the Arbuckle Mountains. The only problem is that the guide is wrong. They are high-pressure and low-temperature indicators, not the other way around. I'm certain it's just a typo.Nevertheless, this guide shows a feature that can only be described as the core of an old mountain range that has been deeply eroded. It is an uplift with the oldest rocks in the center and younger rocks as you traverse outward from the core. This is the classical way of identifying anticlines (fold crests) as opposed to synclines (the fold troughs).

I actually understood some of that, but much of it pointed to other areas than those under discussion right now.If we look at the area between Birmingham, AL and Knoxville, TN we see particular structures. I'd like to concentrate in that area before moving north.Specifically, on the line running NW from Birmingham, (those with Google Earth can find this structure as follows. Look for the non-color area east of Birmingham. Pick the top quadrant of the non-color area. From a midpoint of that quadrant go due west.) I see an ARK. It is a boat shaped formation with the bow pointed NW. The top of this structure is nearly level. The sides are very steep and it seems to rise 600 to 800 feet above the surrounding area.What is the origin and method that produced such a structure.

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Aslan is not a Tame Lion

Don't see it. Try lat/long.

try roughly 33 56' N, 86 15' W

Ah. It's an erosional feature at the edge of the Appalachian Plateau. It looks like a small plateau or mesa made up of the flay-lying Pottsville Formation. YOu can see the flat layering in the Google-Earth depiction. This is a common feature to the north and west of the Appalachian orogen. Deformation dies out west of the mountains and we get flat stratigraphy dissected by dendritic patterned drainages rather than the trellis pattern of the Valley and Ridge province. See if you can find your way through to the geological map of Alabama from this site:GSA/OGB

Okay, 'splain for me.How did it happen? Why is it flat on top? Was it raised up, or rest worn down?

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Aslan is not a Tame Lion

Erosion of flat-lying sediments. The topography reflects the layering. Probably of a resistant bed. Happens all the time. Well, it had to be raised above sea level first, but basically, it is an erosional feature.You are welcome...This message has been edited by edge, 02-05-2006 11:53 PM

okay. I think I am making progress.It was raised above sea level.So was it raised above the surrounding area, or was the surrounding area eroded away?

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Aslan is not a Tame Lion

The latter. But if you look to the NW, the plateau continues in that direction. Without the effects of erosion it would be part of that same plateau.

Anyone know where I can find references on the geology of the northern Andes? Specifically, there are some fascinating strata and foldings exposed in the highlands on the road between Quito and Tabacundo, Ecuador, and I can't find anything that explains what I'm looking at. Any help would be appreciated.