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Natural Science Forum / Biology / Paleontology / May 2005Geology/paleontology fossil Question
Could anyone recommend a website that shows how strata build up over time, and how fossils that were way underground, originally under the ocean, end up on the surface, on top of a mountain? We find shells and trilobites here in Eastern Pennsylvania, and my kids want me to explain how they got up to the top of of the mountain, and when I started to explain, I realized I did not truly understand the mechanics of this, or the time scale. What confused me is that if we are finding fossils that are more or less 250 million years old, were there once other fossils on top of these, from a more recent time? What happened to the other things that I assume would have died and been fossilized on top of these shells for the next 250 million years? Are the more recent strata worn away? (Sorry if this is a silly question) Thanks so much. Debbie Schneider > Could anyone recommend a website that shows how strata build up over time, > and how fossils that were way underground, originally under the ocean, end > up on the surface, on top of a mountain? Try a google search on keywords "plate tectonics" and "orogeny". That might help. > We find shells and trilobites here in Eastern Pennsylvania, and my kids want > me to explain how they got up to the top of of the mountain, and when I [quoted text clipped - 4 lines] > million years old, were there once other fossils on top of these, from a > more recent time? Very likely. There were certainly other rocks on top of them. Whether those rocks contained fossils is another question. > What happened to the other things that I assume would > have died and been fossilized on top of these shells for the next 250 > million years? Are the more recent strata worn away? Yes. Two things have to happen for you to find a fossil. First, the area has to be a spot of sedimentary deposition for a while, in order to accumulate some sediments. Then it has to be a spot of erosion for a while, in order to expose your particular sediments at the surface. Most spots have undergone multiple cycles of deposition and erosion, and what you see exposed at the surface is the net sum of lots of these additions and subtractions. The reason the shells are at the top of a mountain is that the rocks they are in have been uplifted. Uplift results in rapid erosion. The Grand Tetons, a handy example are in the neighborhood of 10,000 feet high. They're fairly new mountains, but in order to reach that height today, they have suffered around 150,000 vertical feet of uplift and erosion; 10,000 feet is just the small amount you have left when you subtract the erosion from the uplift. And this is true with all mountains -- there's much more missing than is still there. The reason for the uplift is plate tectonics. > (Sorry if this is a silly question) > > Thanks so much. > Debbie Schneider > Could anyone recommend a website that shows how strata build up over time, > and how fossils that were way underground, originally under the ocean, end [quoted text clipped - 12 lines] > > (Sorry if this is a silly question) Dr Harshman has given a good basic explanation so I'll just add a few details. The Appalachian chain of mountains that runs along the eastern coast of North America is the result of the ancient collision of two tectonic plates which turned a shallow sea bed into towering mountains which over the millennia have eroded down to the low mountains which you see now. Ken first off sedementary rocks { rocks wich fossils are found} will only form under certian conditions. as an exapmle if there is a high amount of erosion there most likely not going to be much material if any at all deposited for rocks to be able to form. After the pennsylvanian peroid most of pennsylvania was rolling highlands and mountians meaning that there was very little rock deposited. Second as mountian grow they push up the upper most layers wich are almost always younger. As the mountian grows there is an increase in erosion. so that means as the top layers erode the older one are exposed. This means that most of the time the higher up you are in the mountians the older the rocks. A good example is Pikes peak in colorado. As you drive from the Kansas border you start by leaving the niobara chalk that date from about 80 million years ago. By the time you reach pikes peak the rock are precambrian in age dating from about 1.3 billion years ago. Most likely ther was younger rock in your area but these rock eroded away. A good text book called "The Earth Through Time" explains it well. I used this book when i took my courses in historical geology. I have have found it in several local librarys. so i'm sure it may be around by you. if not i think most good geology book should explian what i said in more detail. I hope this helps. good luck. >> Could anyone recommend a website that shows how strata build up over >> time, and how fossils that were way underground, originally under the [quoted text clipped - 20 lines] > which over the millennia have eroded down to the low mountains which you > see now. Is this quite true? I would have imagined that the uplift that led to the current mountains would have been fairly recent, i.e. within the last 50ma at most, whatever the age of the fold belts in question. Mountains tend not to last that long, as mountains. Is there a real geologist in the house? A quick search found these: http://csmres.jmu.edu/geollab/vageol/vahist/mtnmodel.html http://www.geol.umd.edu/~tholtz/G102/102ceno2.htm http://www.gps.caltech.edu/~rkopp/collegepapers/potomac/potomac.html The current mountains are attributed to differential hardness of exposed rocks causing uneven erosion, and isostatic uplift (several episodes, the most recent apparently of Miocene age). >>> Could anyone recommend a website that shows how strata build up over >>> time, and how fossils that were way underground, originally under the [quoted text clipped - 35 lines] > rocks causing uneven erosion, and isostatic uplift (several episodes, > the most recent apparently of Miocene age). Well my geology education is over 15 years old but at the time I was told that the original cause of the mountains was the formation of pangea 480 mya. Checking the web the USGS site here agrees with that: http://wrgis.wr.usgs.gov/docs/parks/province/appalach.html A little more research indicates that a later uplift is responsible for much of the present shape of the range. So I guess the correct answer is somewhere between our two positions. Ken >>>> Could anyone recommend a website that shows how strata build up over >>>> time, and how fossils that were way underground, originally under [quoted text clipped - 44 lines] > much of the present shape of the range. So I guess the correct answer is > somewhere between our two positions. You have to differentiate between the topographic mountains and the structural features. All the fold belts and such are ancient. But the topographic relief we currently observe is new. The same is true for many other mountain ranges: the relief -- the "mountainness" of them -- is fairly recent, or they wouldn't be mountains at present. Relief just doesn't last that long. Erosion removes it unless there's some counteracting uplift. >>>>> Could anyone recommend a website that shows how strata build up over time, >>>>> and how fossils that were way underground, originally under the ocean, end [quoted text clipped - 50 lines] > doesn't last that long. Erosion removes it unless there's some counteracting > uplift. That interpretation is controversial and not accepted by all. The most prominant proponent of this hypothesis is Dr. David Prowell, of the U.S.G.S., in Atlanta. It is an intriguing idea, but until more data is collected, I think the verdict is still out on that one. >>>>>>Could anyone recommend a website that shows how strata build up over time, >>>>>>and how fossils that were way underground, originally under the ocean, end [quoted text clipped - 55 lines] > Atlanta. It is an intriguing idea, but until more data is collected, I think > the verdict is still out on that one. What are the alternative explanations? I don't mean the explanations for the complicated geology of the area; I mean explanations for the current topographic relief. >>> Could anyone recommend a website that shows how strata build up over time, >>> and how fossils that were way underground, originally under the ocean, end [quoted text clipped - 32 lines] > causing uneven erosion, and isostatic uplift (several episodes, the most > recent apparently of Miocene age). http://academic.emporia.edu/aberjame/struc_geo/appalach/appalach.htm The Appalachian Mountains represent a series of Paleozoic orogenies along the eastern margin of North America. The Appalachians extend from Alabama through Maine in the United States, and continue across the southeastern provinces of Canada to Newfoundland. More than 10,000 m of Paleozoic sedimentary and volcanic strata are contained in the exposed portions of the Appalachian Mountains, about 10 times the thickness of equivalent strata in the mid-continent region. The tectonic history of the Appalachian Mountains is divided traditionally into three main orogenic phases during the Paleozoic. Each phase begins with accumulation of thick marine sediments and volcanic deposits, culminates with deformation and uplift of mountains, and finally ends with tectonic quiessence. A particular consequence of orogeny is production of sediment in great volumes as uplifted mountains undergo rapid erosion. Thus, each phase is marked by buildup of a so-called "delta" filling shallow seas on the continental side of the orogeny. These deltas are actually clastic fans that include sediments deposited in many environments--terrestrial, coastal, near-shore, and off-shore settings. Much of what is known about the timing, location, and magnitude of mountain uplift is deciphered from the record of sediment accumulation. Taconic Orogeny: -- First important tectonic activity took place during the Ordovician. Thrusting and folding was mainly in the northern portion. Uplifted mountains shed sediment to the west, and the "Queenstown delta" built up near Albany, New York. Acadian Orogeny: -- Major orogeny of the northern Appalachians occurred in the Devonian, centered in New England and southern New York. Strong folding, thrusting, metamorphism and granite intrusion took place. Sediments accumulated in the "Catskill delta" across southern New York and northern Pennsylvania. Allegheny Orogeny: -- Major orogeny of the southern Appalachians culminated in the Pennsylvanian. Thrusting, folding, metamorphism and intrusion happened from Pennsylvania southward to Alabama. A clastic "delta" spread over western Pennsylvania, West Virginia, Kentucky and Tennessee. All three orogenies are interpreted in terms of collisions during closing of the Iapetus Ocean between North America, Europe and Africa (Gondwana). Seafloor spreading, subduction-zone volcanism, and accretionary wedges were all part of a long, drawn-out sequence of events. Both continental and oceanic terranes were involved with collisions at different times and places. The Taconic Orogeny represented terrane collisions with North America. The Acadian Orogeny took place between North America and Europe, and is contemporaneous with Caledonian Orogeny of the British Isles, Greenland, and Scandinavia. Finally the Allegheny Orogeny involved the collision between Africa (a portion of Gondwana) and the North American-European continent. The ultimate result was joining of several large land masses to create Pangaea by the end of the Paleozoic. You heard it from a geologist. I hope this helps. >>>>Could anyone recommend a website that shows how strata build up over time, >>>>and how fossils that were way underground, originally under the ocean, end [quoted text clipped - 81 lines] > > You heard it from a geologist. I hope this helps. Not really, since the reference makes no attempt to explain why the mountains are currently mountains. The Canadian Shield has fold belts of similar age that are topographically now almost flat. What you have posted seems irrelevant to the question. Is it your claim that the Appalachians have been eroding, without any uplift, since the Pennsylvanian, and what we see can be explained as merely the remnant of a very old uplift? on Wed, 04 May 2005 14:38:07 GMT, John Harshman <jharshman.diespamdie@pacbell.net> sez: ` George wrote: ` > ` > http://academic.emporia.edu/aberjame/struc_geo/appalach/appalach.htm ` > ` > The Appalachian Mountains represent a series of Paleozoic orogenies along the ` > eastern margin of North America. The Appalachians extend from Alabama through ` > Maine in the United States, and continue across the southeastern provinces of ` > Canada to Newfoundland. More than 10,000 m of Paleozoic sedimentary and volcanic ` > strata are contained in the exposed portions of the Appalachian Mountains, about ` > 10 times the thickness of equivalent strata in the mid-continent region. ` > ` > The tectonic history of the Appalachian Mountains is divided traditionally into ` > three main orogenic phases during the Paleozoic. Each phase begins with ` > accumulation of thick marine sediments and volcanic deposits, culminates with ` > deformation and uplift of mountains, and finally ends with tectonic quiessence. ` > A particular consequence of orogeny is production of sediment in great volumes ` > as uplifted mountains undergo rapid erosion. Thus, each phase is marked by ` > buildup of a so-called "delta" filling shallow seas on the continental side of ` > the orogeny. These deltas are actually clastic fans that include sediments ` > deposited in many environments--terrestrial, coastal, near-shore, and off-shore ` > settings. Much of what is known about the timing, location, and magnitude of ` > mountain uplift is deciphered from the record of sediment accumulation. ` > ` > Taconic Orogeny: -- First important tectonic activity took place during the ` > Ordovician. Thrusting and folding was mainly in the northern portion. Uplifted ` > mountains shed sediment to the west, and the "Queenstown delta" built up near ` > Albany, New York. ` > Acadian Orogeny: -- Major orogeny of the northern Appalachians occurred in the ` > Devonian, centered in New England and southern New York. Strong folding, ` > thrusting, metamorphism and granite intrusion took place. Sediments accumulated ` > in the "Catskill delta" across southern New York and northern Pennsylvania. ` > ` > ` > Allegheny Orogeny: -- Major orogeny of the southern Appalachians culminated in ` > the Pennsylvanian. Thrusting, folding, metamorphism and intrusion happened from ` > Pennsylvania southward to Alabama. A clastic "delta" spread over western ` > Pennsylvania, West Virginia, Kentucky and Tennessee. ` > ` > All three orogenies are interpreted in terms of collisions during closing of the ` > Iapetus Ocean between North America, Europe and Africa (Gondwana). Seafloor ` > spreading, subduction-zone volcanism, and accretionary wedges were all part of a ` > long, drawn-out sequence of events. Both continental and oceanic terranes were ` > involved with collisions at different times and places. The Taconic Orogeny ` > represented terrane collisions with North America. The Acadian Orogeny took ` > place between North America and Europe, and is contemporaneous with Caledonian ` > Orogeny of the British Isles, Greenland, and Scandinavia. Finally the Allegheny ` > Orogeny involved the collision between Africa (a portion of Gondwana) and the ` > North American-European continent. The ultimate result was joining of several ` > large land masses to create Pangaea by the end of the Paleozoic. ` > ` > You heard it from a geologist. I hope this helps. ` Not really, since the reference makes no attempt to explain why the ` mountains are currently mountains. The Canadian Shield has fold belts of ` similar age that are topographically now almost flat. What you have ` posted seems irrelevant to the question. Is it your claim that the ` Appalachians have been eroding, without any uplift, since the ` Pennsylvanian, and what we see can be explained as merely the remnant of ` a very old uplift? Isn't it correct to note that the Atlas mountains in North Africa are the same formation as the Appalachians, and so this isn't an isolated condition. If you want to invoke recent (post-atlantic) uplift, you will have to postulate a parallel process in Africa. Isn't it simpler to assume they were just really big mountains, too near the equator to be planed off (as happened to the shield) by glaciers, so they've just persisted. After all, compared to recent mountains, they're not particularly big. Out west we'd call 'em hills. Not a geologist, but will consider playing one on TV, for the right price.  Signature========================================================================== vincent@triumf[munge].ca Pete Vincent Disclaimer: all I know I learned from reading Usenet. > on Wed, 04 May 2005 14:38:07 GMT, John Harshman > <jharshman.diespamdie@pacbell.net> sez: [quoted text clipped - 98 lines] > recent mountains, they're not particularly big. Out west we'd call > 'em hills. The Appalachians continue into New Brunswick and Newfoundland in Canada and across to northern Ireland, Scotland, and Norway (this area was connected to North America over 200 million years ago). The Atlas Mountains formed under different circumstances, and much later in time. >>>>>Could anyone recommend a website that shows how strata build up over time, >>>>>and how fossils that were way underground, originally under the ocean, end [quoted text clipped - 92 lines] > eroding, without any uplift, since the Pennsylvanian, and what we see can be > explained as merely the remnant of a very old uplift? If the question was about the folding, my post answers that question. If the question is about current topographic relief, there is evidence of more recent uplift, on the order of 150 million years ago. And some researchers think that slow uplift is ongoing. There is no evidence that I am aware of that links the massive folds in the Appalachian Mountains to recent uplift. There is plenty of evidence that those massive folds and thrust zones are related to plate movements during the three orogenys I mention above. >>>>>>Could anyone recommend a website that shows how strata build up over time, >>>>>>and how fossils that were way underground, originally under the ocean, end [quoted text clipped - 94 lines] > > If the question was about the folding, my post answers that question. It wasn't. > If the > question is about current topographic relief, there is evidence of more recent [quoted text clipped - 3 lines] > evidence that those massive folds and thrust zones are related to plate > movements during the three orogenys I mention above. Which, as I mentioned before, is all irrelevent to the question at hand, i.e. the current topographic relief. >>>>>>>Could anyone recommend a website that shows how strata build up over >>>>>>>time, and how fossils that were way underground, originally under the [quoted text clipped - 108 lines] > Which, as I mentioned before, is all irrelevent to the question at hand, i.e. > the current topographic relief. Really? All of it? Even this part? "If the question is about current topographic relief, there is evidence of more recent uplift, on the order of 150 million years ago. And some researchers think that slow uplift is ongoing." [snip] >>Which, as I mentioned before, is all irrelevent to the question at hand, i.e. >>the current topographic relief. > > Really? All of it? Even this part? "If the question is about current > topographic relief, there is evidence of more recent uplift, on the order of 150 > million years ago. And some researchers think that slow uplift is ongoing." No, not that part, quite. I still find it hard to believe in current topographic relief that results from anything that ended 150ma. What about you? So the relevance, as far as I can see, is limited to "And some researchers think that slow uplift is ongoing". Not much to sink the teeth into there, but it's a start. Let's ask the question again: Why are the Appalachians topographic mountains at present? If you could provide some details on various theories, and citations or links to support them, that would be helpful. I found two theories in a quick web search: 1. Differential resistance to erosion in different parts of the fold belts produces the ridge and valley province. 2. Episodes of uplift, the latest in the Miocene, resulting from isostatic rebound (presumable following major erosional episodes clearing away the overburden). What else you got? > [snip] > [quoted text clipped - 17 lines] > 1. Differential resistance to erosion in different parts of the fold belts > produces the ridge and valley province. Yes, that explains the geomorphology or the ridge and valley province. However, in order to get those ridges and valleys, you have to start with a much higher elevation, and erode it down to the current levels. So the ridge and valley provinces are evidence of lower elevation in recent times, not higher elevations. You have to realize that the structure of the Appalachians indicate that they were much higher than they currently are. The Ridge and Valley Province are a result of extensive overthrusting and crustal shortening during the continental collisions that occurred during the Allegheny orogeny and the consequent erosion. Huge sediment deposition along the flanks due to erosion also bears this out. And your comparison with the Canadian shield isn't relevant since the Canadian shield has undergone millions of years of intensive glaciation and peneplanation, something which the bulk of the Appalachains didn't undergo. > 2. Episodes of uplift, the latest in the Miocene, resulting from isostatic > rebound (presumable following major erosional episodes clearing away the > overburden). I've read that the present Appalachian Mtns. are the result of Cenozoic uplift and erosion. According to one hypothesis, by the end of the Mesozoic the fold mtns that formed as a result of the allegheny orogeny at the end of the Permian had been worn down to a smooth plain. According to what I've read, uplift in the Cenozoic (Paleocene) may be the result of North. America overriding two hot spots. One of the hot spots would be located just east of Bermuda today and would be responsible for uplifting the central Appalachians near N. Carolina. The second hot spot would have lifted the northern Appalachians in New England. Unfortunately, that doesn't provide an explanation for supposed uplift along the entire length of the chain (since any uplift would appear to be have been relatively uniform based on current sectional profiles), which would be necessary to explain the current topography as being recent. Secondly, the volume of Cenozoic volcanics within the Appalachians is very small. Notable exposures are sparse. In addition, the vast majority of faults in the chain are Mesozoic-aged and older with relatively little significant movement in recent times in evidence. Current strain models I've seen do not indicate significant regional strain, except in specific, localized areas, such as Charleston, SC, and along the St. Lawrence River. I'd like to see seismic data that clearly shows anomalous crust at depth that would indicate volumes of Cenozoic intrusions underlying and/or intruding into the much older rock in sufficient quantity to have uplifted the bulk of the chain to its current elevation and higher. I've not seen any data that demonstrates that this is the case. There are no large-scale magnetic anomalies that could be attributed to the presence of such volumes of recently intruded rock. What I have seen are studies that indicate that much of the basement rock there is very old (Mesoproterozoic). > What else you got? Look at any historic seismic map series of the appalachians. It is relatively quiet there. Not much seismic activity going on there, and little evidence of significant recent seismic activity. Just a few scattered 2s through 4s, and even those are relatively rare. Of course there have been a handful of significant historic jolts (most notably in Charleston, SC). Current activity is primarily to the west of the mountains. The idea about current uplift stems primarily from calculations of erosion rates, which even the authors have admitted has been highly variable over the geologic history of the region, and even from location to location in the region. Add to that uncertainty the presence of highly resistent beds holding up the mountains, and you have potential ingredients for relatively long-lasting mountains. Entire sections of the southern Appalachians, for instance, are composed of pure quartzite, the strongest rock known, and the most highly resistant to weathering. Finally, there are vast clastic deposits west and northwest of the center line of the chain that are directly associated with various erosional episodes of the once much higher and larger Appalachians. I'm not completely dismissing the idea that uplift is occurring at present in the Appalachians. That would be foolish. I am suggesting that before decades of research into the thinking about the Appalachians gets thrown completely out, someone had better line up their ducks in a better row, because I don't think that they are there yet. On another note, I've studied and collected specimens in the Blue Ridge Mountains for 20 years, and the Appalachian Plateau for even longer. While that certainly doesn't qualify me as "emminent" in this area by any stretch, I have a lot of field experience there, and have looked at a lot of rock and a lot of structures. It has been a pet project of mine for much of my professional career. So I hope you can understand my enthusiasm in discussing this topic. > In addition, the vast majority of faults in the chain are Mesozoic-aged and > older with relatively little significant movement in recent times in evidence. [quoted text clipped - 8 lines] > such volumes of recently intruded rock. What I have seen are studies that > indicate that much of the basement rock there is very old (Mesoproterozoic). Oops. That didn't quite come out right. What I meant to say is "Current strain models I've seen do not indicate significant regional strain. Specific, localized areas, such as Charleston, SC, and in the mid-St. Lawrence River valley do indicate moderate strain." >>[snip] >> [quoted text clipped - 23 lines] > provinces are evidence of lower elevation in recent times, not higher > elevations. Yes, this is obvious. I don't know why you make this point. > You have to realize that the structure of the Appalachians indicate > that they were much higher than they currently are. Yes, that's obvious, and again I don't know why you make that point. > The Ridge and Valley > Province are a result of extensive overthrusting and crustal shortening during [quoted text clipped - 4 lines] > glaciation and peneplanation, something which the bulk of the Appalachains > didn't undergo. All of this doesn't necessarily have anything to do with the current topographic relief, nor is it in any way at issue. >>2. Episodes of uplift, the latest in the Miocene, resulting from isostatic >>rebound (presumable following major erosional episodes clearing away the [quoted text clipped - 14 lines] > volume of Cenozoic volcanics within the Appalachians is very small. Notable > exposures are sparse. And there's a third explanation. It does seem farfetched, for the reasons you give. > In addition, the vast majority of faults in the chain are Mesozoic-aged and > older with relatively little significant movement in recent times in evidence. [quoted text clipped - 27 lines] > chain that are directly associated with various erosional episodes of the once > much higher and larger Appalachians. My understanding is that these clastic deposits are all seriously ancient. Are any of them Mesozoic or Cenozoic in age? But you do present a fourth theory here, which is that the Appalachians are so resistant to erosion that they have been maintained for very long periods in the absence of significant uplift. > I'm not completely dismissing the idea that uplift is occurring at present in > the Appalachians. That would be foolish. I am suggesting that before decades [quoted text clipped - 7 lines] > professional career. So I hope you can understand my enthusiasm in discussing > this topic. It's an interesting topic. > And there's a third explanation. It does seem farfetched, for the reasons you > give. Well? Don't hold your tongue. Spill the beans, so to speak. >> In addition, the vast majority of faults in the chain are Mesozoic-aged and >> older with relatively little significant movement in recent times in [quoted text clipped - 31 lines] > My understanding is that these clastic deposits are all seriously ancient. Are > any of them Mesozoic or Cenozoic in age? Well, yes, in fact there are vast cenozoic-aged deposits. You just have to know where to look for them (i.e., the many river valleys - current and abandoned - within and leading outward from the Appalachians) all the way to the Atlantic on one side, and mostly the Gulf of Mexico on the other. > But you do present a fourth theory here, which is that the Appalachians are so > resistant to erosion that they have been maintained for very long periods in > the absence of significant uplift. Fourth theory? It has been a part of the interpretation of the Appalachians for a very long time. >> I'm not completely dismissing the idea that uplift is occurring at present in >> the Appalachians. That would be foolish. I am suggesting that before [quoted text clipped - 9 lines] > > It's an interesting topic. Yes, it is. I've never gotten enough of just walking skyline drive at Shenondoah National Park. I think it is one of the little-known treasures of our National Park system. Stunning beauty, and remarkable geology. Go there just once, and you will be hooked on it for life. That is where I did my undergraduate field training (basic training and AIT for geologists all rolled into one). One thing that came out of that experience, and explains why the Appalachians are so difficult is that these mountains are upthrust, faulted, folded, severely eroded, in many instances turned upside down, inside out, intruded, metamorphosed at least three times, squeezed, pulled apart, and invariably covered in dense brush, making mapping a huge challenge. It has been said that if you can map these mountains, you can map anything. It is absolutely true. I've been to nearly all the states of the lower 48, and no other place I've seen prepares you for field geology quite like they do. They are quite remarkable. Oh, dear. Am I repeating myself? :-) >>And there's a third explanation. It does seem farfetched, for the reasons you >>give. > > Well? Don't hold your tongue. Spill the beans, so to speak. You already spilled them. When I said "there" I didn't mean "there exists"; I was referring to what you wrote just above my comment, where you have an explanation that you consider implausible. >>>In addition, the vast majority of faults in the chain are Mesozoic-aged and >>>older with relatively little significant movement in recent times in [quoted text clipped - 36 lines] > within and leading outward from the Appalachians) all the way to the Atlantic on > one side, and mostly the Gulf of Mexico on the other. Next question: Are these deposits continuous enough in age that we can infer constant erosion of existing mountains throughout the Mesozoic and Cenozoic, or could an alternative of several episodes of uplift and erosion explain them as well or better? Can one calculate how high the original mountains would have had to be (when uplift ended) in order for all uplift to have occurred in the Paleozoic, or however old you think that would be? >>But you do present a fourth theory here, which is that the Appalachians are so >>resistant to erosion that they have been maintained for very long periods in >>the absence of significant uplift. > > Fourth theory? It has been a part of the interpretation of the Appalachians for > a very long time. When enumerating theories, it's not necessary to list them in order of age. >>>I'm not completely dismissing the idea that uplift is occurring at present in >>>the Appalachians. That would be foolish. I am suggesting that before [quoted text clipped - 24 lines] > other place I've seen prepares you for field geology quite like they do. They > are quite remarkable. Oh, dear. Am I repeating myself? :-) I dunno. California geologists say the same thing about this state, which has been accreted, displaced along transform faults, block-faulted, thrust-faulted (and in some cases turned upside down), much of it at quite small scales. Perhaps every geologist wants their own personal study area to be the hairiest of all. >>>And there's a third explanation. It does seem farfetched, for the reasons you >>>give. [quoted text clipped - 4 lines] > was referring to what you wrote just above my comment, where you have an > explanation that you consider implausible. Ok. As Emily Latella would say, "Nevermind". >>>>In addition, the vast majority of faults in the chain are Mesozoic-aged and >>>>older with relatively little significant movement in recent times in [quoted text clipped - 44 lines] > have had to be (when uplift ended) in order for all uplift to have occurred in > the Paleozoic, or however old you think that would be? Hmmm. I'm not a specialist of the cenozoic, but if I had to speculate, I would suspect that they are episodic simply because that is what is seen elsewhere in the rock record. The trick would be to determine why. For instance, are they episodic due to uplift and erosion, or due to changes in climate - floods, droughts, ice ages, interglacials, etc. As for calculating how high the original mountains would have to be, that is done by looking at structural features, since they are not so subject to erosion, at least in the sense that the strutures we see in the mountains are preserved. For instance, take a look at this cross-section of the Blue Ridge Mountains: http://csmres.jmu.edu/geollab/vageol/vahist/blurdgdiv.html The modern Blue Ridge is an overturned anticline. That is, the rocks have been arched up into a fold, and then shoved over toward the west (left) so that the rocks on the western flank are now no longer right side up (follow the red dashed line). Notice the red dotted line. This line traces, in space, the inferred aerial extent of that particular formation, extrapolated between outcrops. By doing this repeatedly for all formations, one can build a reconstruction of an outline of the original loftiness of the mountains, and their structure prior to erosion. Here is a .pdf file the provides more information on the geologic history of the appalachians: http://www.geography.wisc.edu/classes/geog329/Appalachians_web2.pdf >>>It's an interesting topic. >> [quoted text clipped - 19 lines] > Perhaps every geologist wants their own personal study area to be the hairiest > of all. Well, having been to Califonia on several occasions, I must say that it too is fubar. >>>>And there's a third explanation. It does seem farfetched, for the reasons you >>>>give. [quoted text clipped - 80 lines] > of the original loftiness of the mountains, and their structure prior to > erosion. Unfortunately, that doesn't tell you how high they were when uplift ended, just how much was removed, total. > Here is a .pdf file the provides more information on the geologic history of the > appalachians: > > http://www.geography.wisc.edu/classes/geog329/Appalachians_web2.pdf Thanks. I see there was block faulting as recently as 150ma, which would have created lots of relief, and that this document tentatively goes with the isostatic rebound theory, at least in the Blue Ridge. >>>>It's an interesting topic. >>> [quoted text clipped - 22 lines] > Well, having been to Califonia on several occasions, I must say that it too is > fubar. >>>>>And there's a third explanation. It does seem farfetched, for the reasons >>>>>you give. [quoted text clipped - 83 lines] > Unfortunately, that doesn't tell you how high they were when uplift ended, > just how much was removed, total. No, what it does is let you extrapolate back in time to see what they would look like had erosion not occurred. It gives you the total possible size of the mountains. >> Here is a .pdf file the provides more information on the geologic history of >> the appalachians: [quoted text clipped - 4 lines] > created lots of relief, and that this document tentatively goes with the > isostatic rebound theory, at least in the Blue Ridge. If you are referring to the diagram that shows the Jurassic configuration of the mountains, the faults shown are pre-existing faults from the triassic. Having said that, it does appear to show isostatic rebound either in the Cretaceous or the tertiary. It doesn't seem clear from the layout of the diagrams. What seems clear from the last diagram of that series is that there has been uplift due ot rebound in recent times. The following link at the U.S.G.S. web site indicates that the uplift has occurred during the cenozoic, or within the last 60 million years. This is consistant with the fact that uplift is considered to be responsible for the formation of the Red River Gorge, in Eastern Kentucky, which has been dated to 60-70 million years ago. http://www2.nature.nps.gov/geology/usgsnps/province/appalach.html >>>>>>And there's a third explanation. It does seem farfetched, for the reasons >>>>>>you give. [quoted text clipped - 89 lines] > like had erosion not occurred. It gives you the total possible size of the > mountains. Right, and that's interesting. That's just not the question at hand. Of course it doesn't give you the total possible size of the mountains, since nothing like that size would ever have been achieved in practice. >>>Here is a .pdf file the provides more information on the geologic history of >>>the appalachians: [quoted text clipped - 7 lines] > If you are referring to the diagram that shows the Jurassic configuration of the > mountains, the faults shown are pre-existing faults from the triassic. Yeah, I see that. > Having > said that, it does appear to show isostatic rebound either in the Cretaceous or > the tertiary. Actually, they suggest that it's ongoing now, and also happened in the Cretaceous. Whether there has been continuous uplift is unclear, but at least two claimed periods are clear. > It doesn't seem clear from the layout of the diagrams. What > seems clear from the last diagram of that series is that there has been uplift [quoted text clipped - 5 lines] > > http://www2.nature.nps.gov/geology/usgsnps/province/appalach.html >>>>>>>And there's a third explanation. It does seem farfetched, for the reasons >>>>>>>you give. [quoted text clipped - 107 lines] > course it doesn't give you the total possible size of the mountains, > since nothing like that size would ever have been achieved in practice. I don't why you think that is the case. The structure is there. I see no reason why it wouldn't be the case. My understanding is that it has been estimated that the Appalachians were at their peak as high as the alps. I know od no physical laws that would prevent that from being the case. >>>>Here is a .pdf file the provides more information on the geologic history of >>>>the appalachians: [quoted text clipped - 19 lines] > Cretaceous. Whether there has been continuous uplift is unclear, but at > least two claimed periods are clear. That would be consistant with my earlier suggestion that it is a phenomenon of the last 150 million years. My question is how recent has any of this uplift been? I've seen nothing that suggests that it has been occurring in recent times. I've seen no reports of evidence of significant seismic movement along any of the major faults in the last 10,000 years (at least none that would account for any significant uplift). Along some of the major faults in Kentucky, for instance, surveys have been conducted to determine movement along the Kentucky River fault,and has only found minor movement in the last 10,000 years. I don't know of any similar studies that have been done along the major Appaalchian faults (except for the study at Pine Mountain). That's not to say that none have been done. I just don't know about them if they have. I recognize that there are a number of faults in the Appalachians that are seismically active, but their current activity certainly can't account for any major uplift. Obviously, further studies are needed in this area. >>>>>>>>And there's a third explanation. It does seem farfetched, for the reasons >>>>>>>>you give. [quoted text clipped - 112 lines] > estimated that the Appalachians were at their peak as high as the alps. I know > od no physical laws that would prevent that from being the case. Sure, the Appalachians were as high as the Alps. But that doesn't mean their height at any time equalled the height of piling up all the sediments now deposited around them. Unless uplift is instantaneous, the mountains were eroding as they grew. In the average mountain, the amount of material cycled through it during its rise would be several times the height of that mountain at any given time. Same with any really large fold; we don't expect that the anticlines would escape erosion during their uplift, and thus there would never have been a point at which all the strata now found in adjacent synclines would have been sitting neatly atop the anticline. A strutural dome doesn't ever have to have been a topographic dome. And so on. That's all I'm saying. >>>>>Here is a .pdf file the provides more information on the geologic history of >>>>>the appalachians: [quoted text clipped - 23 lines] > the last 150 million years. My question is how recent has any of this uplift > been? Yes, that's the big question. "Phenomenon of the last 150 million years" and "phenomenon that ended 150 million years ago" are two quite different things. > I've seen nothing that suggests that it has been occurring in recent > times. I've seen no reports of evidence of significant seismic movement along [quoted text clipped - 8 lines] > seismically active, but their current activity certainly can't account for any > major uplift. Obviously, further studies are needed in this area. Well, the pdf does make the tentative claim for current uplift. But that's hardly necessary. One of the previously mentioned sites has several episodes of uplift, the last one being Miocene. That would be good enough to explain the current mountains. >> I don't why you think that is the case. The structure is there. I see no >> reason why it wouldn't be the case. My understanding is that it has been [quoted text clipped - 13 lines] > neatly atop the anticline. A strutural dome doesn't ever have to have > been a topographic dome. And so on. That's all I'm saying. I understand that. I didn't mean to suggest that the height would equal the height if you piled all the sediment back where it originated. Obviously mountain-building is an ongoing process, as is erosion. > That would be consistant with my earlier suggestion that it is a phenomenon of > the last 150 million years. My question is how recent has any of this uplift [quoted text clipped - 10 lines] > seismically active, but their current activity certainly can't account for any > major uplift. Obviously, further studies are needed in this area. Just a little note from farther north in the Appalachians. In Earth Science class I learned that because the local peaks in Orange County, New York and adjacent areas (Catskills, Shawangunk ridge, Hudson Highlands) pretty much are all of a height, the area likely was a peniplane sometime in the past, followed by uplift and erosion. A significant amount of that erosion, but not all of it, was during the last glaciation, so most (if not all) the uplift predated the glaciation. >> That would be consistant with my earlier suggestion that it is a phenomenon >> of the last 150 million years. My question is how recent has any of this [quoted text clipped - 18 lines] > all of it, was during the last glaciation, so most (if not all) the uplift > predated the glaciation. That would make sense in your case, and would also support isostatic rebound happening due to the melting of the glaciers. The southern appalachians didn't experience extensive glaciation, if at all, so things are a little more complex down here. >>Just a little note from farther north in the Appalachians. In Earth Science >>class I learned that because the local peaks in Orange County, New York and [quoted text clipped - 8 lines] > experience extensive glaciation, if at all, so things are a little more complex > down here. There doesn't seem to be much isostatic rebound locally, at least not from the last glaciation. We're near the southern limit of glaciation. The peaks I mentioned are 2-3000 feet elevation, while the glacial valleys are much lower elevation. My family's farm near Pine Bush, which had some kame and kettle topography on it, was at 4-500 feet elevation, and was within sight of the highest part of the Shawangunk Ridge (at 2500 feet elevation. >>>Just a little note from farther north in the Appalachians. In Earth Science >>>class I learned that because the local peaks in Orange County, New York and [quoted text clipped - 14 lines] > topography on it, was at 4-500 feet elevation, and was within sight of the > highest part of the Shawangunk Ridge (at 2500 feet elevation. Sorry. I misunderstood what you were saying. Nevermind. >>>>>Could anyone recommend a website that shows how strata build up over time, >>>>>and how fossils that were way underground, originally under the ocean, end [quoted text clipped - 92 lines] > eroding, without any uplift, since the Pennsylvanian, and what we see can be > explained as merely the remnant of a very old uplift? You must also realize that the Canadian shield has been glaciated many times and so is essentially now very flat to gently rolling. That kind of widespread, intensive weathering hasn't taken place in most of the Appalachians. Do note that now that the Pleistocene glaciers are gone from the shield that it is experiencing isostatic rebound, and so is currently uplifting. thank you so much everyone who responded >>>> Could anyone recommend a website that shows how strata build up over >>>> time, and how fossils that were way underground, originally under the [quoted text clipped - 88 lines] > > You heard it from a geologist. I hope this helps. |
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