Water Gaps- worth a voyage across the Atlantic
Water gaps are intriguing and iconic landforms that have long drawn humans to them. We are all familiar with streams and rivers flowing in valleys; a water gap is dramatically different- it’s a place where a river cuts though a ridge or mountain range. Thomas Jefferson discusses the Potomac River water gap in his Notes on the State of Virginia (1785), declaring in an often-quoted passage: “This scene is worth a voyage across the Atlantic.”
View of the Potomac Watergap from Jefferson Rock by Ferdinand Richardt (1857).
View is to the east, with Harpers Ferry in the foreground. (from- http://www.histarch.uiuc.edu/harper/1859render.html)
For me, the prose that comes earlier in the same paragraph is even more vivid.
The passage of the Patowmac through the Blue ridge is perhaps one of the most stupendous scenes in nature. You stand on a very high point of land. On your right comes up the Shenandoah, having ranged along the foot of the mountain an hundred miles to seek a vent. On your left approaches the Patowmac, in quest of a passage also. In the moment of their junction they rush together against the mountain, rend it asunder, and pass off to the sea.
Topographic map of the Potomac River water gap. Note the river cutting through two ridges.
The Appalachian Mountains are flush with water gaps (e.g. Delaware water gap, Cumberland gap), but water gaps are common features in many mountain ranges worldwide. Water gaps are important as they typically form a route of conveyance through steep and mountainous country and they’ve long been utilized as routes for wagon trails, railroads, and highways. The Potomac River water gap through the Blue Ridge Mountains has been a pivotal place in American history since Colonial times.
Southwest from Harpers Ferry the Blue Ridge Mountains form an unbroken topographic barrier for 240 km (150 miles). The next water gap is near Lexington, Virginia, where the James River has carved a 10-km (6 mile) gorge through the Blue Ridge, a range with peaks over 1,200 meters (4,000 feet) in elevation. This is an impressive gap- see for yourself by playing this Google Earth tour through the James River water gap (kmz).
How can a stream cut a path across a mountain ridge or range that lies in its course?
At the Potomac River water gap Jefferson opined:
…this scene hurries our senses into the opinion, that this earth has been created in time, that the mountains were formed first, that the rivers began to flow afterwards, that in this place particularly they have been dammed up by the Blue ridge of mountains, and have formed an ocean which filled the whole valley; that continuing to rise they have at length broken over at this spot, and have torn the mountain down from its summit to its base.
TJ is certainly entitled to his opinion, but he’s not the only one to wax poetic on this topic.
For another take on the landscape consider John Denver’s famous lyrics in the song Country Roads:
Almost heaven, West Virginia
Blue Ridge Mountains, Shenandoah River
Life is old there, older than the trees
Younger than the mountains, blowing like a breeze
So according to John Denver the trees are younger than the life, and both the trees and the life are younger than the mountains (i.e. old mountains).
But what about the Shenandoah River? Is the Shenandoah River older or younger than the Blue Ridge Mountains? In Jefferson’s landscape model, the mountains formed first, creating a topographic barrier that was later breached by the river carving out a water gap. But there is another possibility: what if the rivers were there first and the mountains formed later? In this model, the rivers are older and maintain their courses while the mountains are uplifted. These rivers would be antecedent streams that pre-date the current topography through which they flow.
What do you think? Are the Blue Ridge Mountains older than the rivers (Potomac/Shenandoah and James systems) that flow through these impressive water gaps? Or do these rivers pre-date the formation of the Blue Ridge and Appalachian Mountains?
Let me know and we’ll return to this question in a follow up post.
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I have been waiting for the “answer” (or at least more information) ever since I was assigned this topic for either your or Greg’s class.
I would have to side with your first description. It would be too ‘convenient’ for mountains to form around a river. Rather, it makes far more sense for rivers to carve through mountains over many millions of years.
Great blog Chuck, I finally understand why you are always pushing for us students to read them. Truly stupendous work on your part, you are a true revolutionary in your field.
I think the mountains must be older or else the rivers would have nothing to travel through
To answer this question, it would seem that a geological sample taken from the mountain peaks would shed light on the timeline. Assuming oceanic fossils (fossil limestone) are found, then it would follow that continental uplift caused Earth’s crust to rise from the ocean. From this perspective, however, it would seem that the creation of the mountain range and the creation of the river could have been simultaneous. That is, if a certain amount of the sea was uplifted on top of the uplifted crust as surface water and within the crust as groundwater, then theoretically a stream could have been created during uplift that would from then on be supplemented through precipitation and groundwater. This would still allow the rivers to carve into the sides of the mountains (erosion) that would support the modern claim that the mountains predate the rivers.
It is easy to see why many people would think the river cut through an older set of mountains. However I actually believe that the rivers are older than the Blue Ridge Mountains. If you look at places where water has cut away land (for example the Grand Canyon) there is evidence of changes in weather and river discharge.These Virginian gaps are much more uniform which might hint at a younger age for the mountains. I believe if the mountains were older, the rivers would have changed direction to circumnavigate them instead of going right through with a large gap. At the end of the day though, both processes most likely worked together to create these land formations.
I would say that the rivers are older than the mountains around them. As the land around the river was uplifted it just continued to cut through those layers, too.
I agree with Michelle and Jefferson’s argument. The rivers would not be able to cut across the mountains if they were older because they would not exist yet. So, logically the mountains outdate the streams.
It’s hard for me to picture how mountains could have formed around a river, but then again, I’m certainly a geography novice. I’m inclined to go with Jefferson here and say the river cut through the topography after it was already formed, but I look forward to you shedding more light on the subject.
If the rivers were older than the mountains that they run through, then when the land rose to form the mountains, the water would have been pushed back and eventually the river would dry up. The mountains are older than the rivers and the rivers slowly cut through them.
I think that the rivers flowing through the water gaps are older than the Appalachian Mountains. The tectonic uplift of the mountains was a slow process, as they rose the pre-existing river could have cut through each of the new layers of rock, creating the clear gap between areas of elevated topography seen today.
Let’s try this again…I had my post written and then the power went out. So as I was saying, it would only make sense that the mountains are older than the rivers. What are the chances that the mountains would be pushed up in a formation that would go around the rivers. It would make much more sense for the mountains to be formed, and then rivers slowly begin to meander through the mountain formations. Sorry this is short, but I’m trying to finish before the power goes out again. 🙂 everyone stay safe!
To me it seems that mountains would form before a river, since it would appear more reasonable for water to cut through rock rather than the other way around.
A few thoughts on how to more accurately explore the topic:
• Taking samples of the mountain rock and fossils from the bottom of the river in order to narrow the dates for each substance.
• Looking at the point bar and cutbanks of the water, one could see if any patterns occur. This could help date the deposition and erosion patterns, in addition to explaining the overall flow of the water.
• Locating the shifting tectonic plates below Earth’s surface could explain the location for both water and rock – allowing one to explore the past “markings” left on Earth still visible today.
It is very likely that both processes contributed to the formation of the river. To measure the degree to which each one did, a skilled geologist could take a sample from the top of the mountains and find evidence of water having flowed through the years. If the river cut the mountains down, there should be evidence of that along the rock formations. Furthermore, by examining the point bar and cutbanks of the river, the geologist could determine the river’s flow pattern over the course of many years.
I believe that the mountains formed first (as the rocks on the East Coast are “cold, old, and dense” after all). This opinion comes from the idea that if the river were around longer, the paths of the river flows would be interrupted by the uplifting of mountains around them, something that would be evident in studying the river beds and surrounding areas. However, the rivers are also quite old, and perhaps were once much larger and faster-flowing in order to create such large and significant river gaps. Perhaps as the mountains continued to uplift later on, the river flow was slowed and weakened, causing less erosion. So, while the mountains may have been there first, both mountain growth and stream flow contributed to the current system within the mountains.
Guess: The gap was probably not established until the cessation of Appalachian orogenesis and the onset of Atlantic rifting. I can imagine an axial drainage system existing in a high elevation Blue Ridge plateau, in a similar configuration to those of the Himalayas today (eg Brahmaputra). Once the orogen underwent regional SE/NW extension, younger rivers on the east flank of the mountains probably started incising headward into the Blue Ridge anticlinorium. Eventually, these rivers might have broken through the topographic barrier on the east side of the Blue Ridge anticlinorium, capturing the original axial drainages and establishing the current configuration. Wouldn’t be surprised if the James River west of Apple Orchard Mountain originally flowed southward, or if the Maury originally flowed northward. Seems like one of these branches must have been reversed.
I would think that the watergaps themselves would be younger than the mountains through which they cut. I would side with Jefferson’s view here. However, I think it is possible that the land surrounding the water could have been uplifted by tectonic processes as we..
I definitely think the river came first. Water doesn’t just come out of thin air, forming bodies of water takes a lot of time and a huge mass like this doesn’t just flow out of nowhere after a mountain range forms. I would say that logically, the river was there and as the tectonic plates created a mountain range very slowly for millions of years the river gradually found the crevices between ranges. I also really like John Denver’s song… so, logically, I have to go against TJ on this one.
I think the Shenandoah formed after the Blue Ridge Mountains. Somehow that just makes more logical sense than there being a river and mountains forming around it. If the mountains were formed after, wouldn’t the river have been pushed up and around with them?
I think the mountains have to older. The rivers through them seem to bend around and vary in width which makes me believe that they did so in response to topography already present. The Google Earth tour was really cool and you could definitely see that the mountains were probably continuous at one point until the rivers wore them down.
I think that the river was there before the mountains. As the land was uplifted the river continued to cut down through the mountains forming a large valley due to its flood plain. The river’s path/shape/size has obviously been changed by the mountains, creating a river that is unrecognizable compared to its ancient ancestor, but it is not impossible to imagine the Shenandoah river without the Blue Ridge Mountains.
I believe that the mountains must have formed first. The rivers could have been formed as a result of the snowmelt runoff due to the high elevation of the Blue Ridge Mountains. The snowmelt could also be used as an input source for the river today. I also think that the Atlantic Ocean and other water sources pushed their way into the continent and meandered around the already present mountains. Lastly, and most importantly, TJ and John Denver were pretty smart guys so if they think the mountains are older- it must be true!
Mountains are older. Rivers are younger. Read a historical geology book. Dr. Chuck ain’t as clever as he thinks.
I’m no geologist, but I would say that any orogeny capable of creating the Blue Ridge Mountains (in all their original splendor) is going to severely mess up the existing topography of the land, including bodies of water. I think the mountains themselves provide the necessary elevation for a fast flowing river – perhaps just fast and powerful enough to cut through a mountain.
I don’t think TJ was too far off. While the mountains most likely did not dam up the river into a “sea,” it’s possible that the streams flowing through the mountains found a weak spot: a particular cross section of the mountain range that was lower in elevation and more easily weathered/eroded. Over millions of years the river would do the rest of the work of tearing down that section of the mountain to create a beautiful water gap.
It seems to me that TJ and a lot of the previous comments have the right idea: the mountains must be older than the river that carves through it. It seems to be as though the mountains would have formed through the tectonic uplift processes and then the rivers would have eroded through later. If the mountains weren’t there first, the rivers would have nothing to cut through. Also, the Blue Ridge mountains are heavily worn down overall, so it makes sense that the river would develop later and erode the mountains. But clearly, I’m no expert, so I’ll look forward to your answer.
The first time I read this, I thought it was obvious that the mountains were older and the river simply cut a path through the mountains after they had formed. But as we learned when we were talking about drainage basins, water flows from a higher elevation to a lower elevation, so how would this river have the capability to flow over a mountain for an extended period of time before it carved out a path that was all downhill? It probably wouldn’t. That being said, the gradient on both sides of this river gap is quite steep, leading me to question why mountains could just appear directly beside a preexisting river. The way I see this occurring is that the rivers must be older than the mountains. In this scenario, the rivers would be running through this area as the tectonic plates came together to uplift this land. As the land was slowly uplifted over a lengthy period of time, the river continued to run its course and continued to carve and deepen its path as the land around and underneath was being uplifted. So as the river continued to erode and carve its trajectory, the land around it was not being eroded at nearly the same rate as it was uplifted to form mountains directly adjacent to the river.
The formation of the river could have benefited from a number of factors thanks to the preexisting mountains (i.e. snowmelt runoff, higher levels of ground flow from increased elevation) whereas the formation of mountains after a river had already been established could only complicate the river’s course by gradually moving its terrain uphill. The most important clue to me is the high level of erosion that the Blue Ridge Mountains have experienced—this makes it seem logical that preexisting mountains began to be worn down by later-developing rivers over the course of millions of years.
The Potomac/Shenandoah and James river systems are as old, if not older, than the Blue Ridge and Appalachian Mountains surrounding them. The rivers were able to keep pace with the mountain building process by creating the water gaps that allowed for the continuation of their flow to the Atlantic ocean.
Being that the rivers gaps are quite steep, the Potomac, Shenandoah, and James River systems are older than the Blue Ridge Mountains. It would be impossible to have such steep, eroded water gaps without the river being there first and the mountain slowing rising around it due to tectonic uplift.
The Blue Ridge Mountains are one of the oldest ranges in the world, which would leave rivers a realistic amount of time to carve out such big gaps. Rivers also need slopes to flow. Without some sort of significant elevation in that area water would not be moving in the form of a river.
If I were a betting man I’d agree with the very high John Denver and say the mountains are older than the rivers.
I would have to agree with TJ and John Denver; the mountains are much older than the water gap which now slides through them. I don’t think that it is possible that the rivers came first, because that would imply that they are dominant and are able to effect greatly the shape of mountains and growth of mountains. While rivers can carve into mountains some, rivers are controlled by mountains in their journey towards lower elevation and eventually their movement towards the ocean.
While TJ was not correct in assuming that a school needed to be built to “improve” upon the schooling techniques of William & Mary, I believe he was correct in assuming that the Blue Ridge Mountains came before the Shenandoah.
After doing some further reading, I can infer that mountains are indeed older. For rivers to form, they need a point of origin and this point origin must be at higher elevations. As said above, even though rivers can erode the land and cut through mountains, they need a mountain, or at least higher elevation, to form.
I’d say that the mountains are older, barring evidence of the rivers’ paths before the mountains supposedly uplifted and changed the flow.
I propose a third hypothesis, the multiple stream hypothesis. The mountains formed first, but there was a third stream starting at the top of where the gap now exists. This third stream eroded down the mountain and cut a gorge, which was then used by the Potomac and Shenandoah as a pressure release.
I believe the waterways developed alongside the mountains. They would each have to shape one another.
Chuck, Being from Nevada and the Great Basin. Posed with the question which is older: The mountains or the Rivers? I think it is definitely the Rivers, for the water to be trapped in the Great basin then the mountains had to be formed after the rivers.
I believe that the mountains formed first, alongside Thomas Jefferson, it seems unlikely that mountains would form around a river. How would plate tectonics “know” where to uplift in order to ensure a river gap? The mountains formed first and then the river carved its path across the mountains, forming the river gap. Some argue that the river would have circumnavigated around the mountains, but I think that the river’s power is underestimated, it slowly carved out a path for itself through the weakest gaps in the mountain chain in a similar process to the Grand Canyon.
If the Blue Ridge Mountains rose due to uplifting processes after the creation of the Potomac and Shenandoah, then the mountain’s composition should show signs of the river (biology, aquatic material, more mud and ‘softer’ elements, etc.).
If the rivers came after the formation of the mountains though, we would expect to find erosion of the mountain sides and resultant ‘mountain-side’ sediment deposits further down the river.
I think the river came second – for the river to survive random uplifting without suffering any cutoffs would be remarkable. Too remarkable. Mountains first, rivers second.
I think it makes more sense that the rivers formed after the mountains because it seems more likely that a water way would cut into a mountain than to have mountains grow on both sides of the river. That wouldn’t make much sense to me.
I’ve bounced back and forth between both ideas but I think it’s most likely that the river formed first. Mountains aren’t created all of the sudden — they’re formed by tectonic uplift over millions of years, only raising centimeters or inches at a time, right? If the land gradually raised like this with only a few centimeters at a time, it would make perfect sense for the river — that already existed — to wear down and erode any of the uplift that was occurring beneath it in that same time period. Therefore the area around it would continue to raise in height while the river would continue to wear down the area immediately around it due to the extremely low growth rates.
On the other side, if they mountains came first, how on Earth would the water go down both sides of the mountain continuously for a long enough time period to erode an entire hole in it? And why in only that specific location of the mountain range when most of the peaks are presumably of similar height? It just makes much less sense when you think about the time span over which the erosion occurred.
I agree with the idea that the mountains came first. The mountains are very old which would give the rivers plenty of time to erode the water gaps. The way rivers flow through mountains makes me think that the mountains helped determine their course.
As water flows from a higher gradient to a lower gradient and the eastern coastal U.S. is of a low gradient I would have to think that the mountains predate the river. Otherwise, the river would have flowed without a change in gradient, which doesn’t make much sense.
I’m going to have to agree with Jefferson, Chuck. I really think the mountains formed first, and the rivers later cut through them. I find it hard to believe we have antecedent rivers here, that the mountains just formed around them.
Chuck, I really think the mountains formed first. I know that in some caves, large crevices are formed from antecedent rivers, but I just don;t see that happening here. I feel like the mountains were formed by converging plates and that the river was later formed by erosion, weathering, and precipitation. Then, with the river, the land became more fertile and vegetation was able to spring up on the mountain. Also, the river seems to be meandering. i am a lowly GEO 110 student, and not sure if that has anything to do with our question, but just an observation.
Chuck, sounds like you need a magic school bus so you can just take your students on a field trip back in time to see for themselves. Is there an app for that?
If we assume John Deutschendorf (“Denver”) endorses the lyrics in ‘the Flower that Shattered the Stone’- “As the river runs freely, the mountain does rise”- then we can throw him on team antecedent streams (and question his musical taste).
Now it’s apparent that the question is not at all what actually came first, but rather, TJ vs. JD? Personally, that is a throw down I would like to see- mano y mano.
Keep tackling the big issues!
Just checking in …
This may be a wildcard, but could glacial movement have anything to do with the Blue Ridge Mountain’s creation? If that were the case, I would assume that the mountains were produced first and the river came afterward.
I would have to agree with Jefferson that the mountains came first. This is just a guess, but I do know that the Appalachian Mountains are really, really old. I also can’t imagine how the mountains would have formed around the river. Or is it possible that the water source was present before the creation of the mountains, and then became the river as it was forced to adjust to the newly created mountains?
Either way, both TJ and John Denver were right about the Blue Ridge Mountains being beautiful–but we SWVA natives are awfully proud of our mountains!
I think the river is older, Chuck! What are the chances that two ridges formed perfectly so that water could flow between them? All you need is a little erosion and tectonic activity to form those ridges around a river.
I think the rivers are older. If they were younger, we’d see marine fossils on the other side of the Blue Ridge, because the water would’ve dammed up like TJ said then broken through, and a large internal body of water would’ve existed until there was enough erosion to empty the body. Plus, if the mountains were older, than the rivers would flow away from them, not through them. Unless there was 3 sides which are higher up, forming a bowl with the Blue Ridge as one of the sides (and there isn’t, looking towards the West), then there would be no reason for the water to cut through the mountains rather than going any other direction. The river cut through the mountains as they formed, because the river is older.
I think it makes more sense that the mountains would be older. The idea of the mountains forming around the rivers doesn’t make sense to me, because when the plates pushed together and created the mountains, gravity would have most likely driven the water downwards into a valley, not just pushed the ridges of mountains around water. Plus the fact that the mountains are so rounded indicates that they are old, the Blue Ridge mountains are older than the Rockies. Therefore some serious erosion has already taken place on the Blue Ridge mountains and it would be unlikely that through all of that the river would maintain its exact course.
I can see both sides of this discussion, however I find myself siding with Mr. Jefferson (and I’m not just saying that because I’m a Charlottesville native.) For these small (in comparison to the rest of the mountain range) water gaps to form, they would have to have obstacles in their way and the mountains that are already present become these obstacles. If it was the other way around, the mountains would sure be able to form the water ways in a multitude of different directions and we wouldn’t end up with such a small gap within a gigantic mountain range.
Chuck, I think the mountains formed first.There could be no way for the rivers to have formed first then cut through the mountains later.
I also believe that the mountains had to have formed first, then the river cut through them later.
It makes since that the mountains formed first and the river then cut through them, but we’ve studied rock formations where the inside rock was actually older and the surrounding rock formed around it, so perhaps it could work the same way with the mountains forming around the river.
At first glance it seems that the mountains MUST have preceeded the river; after all, how could a river cut through a mountain range that wasn’t yet there? With a little more thought though, it seems to me that the river must have been present before the mountains.
The river would flow, and as the mountains sloooooowwwwwllllyyyy rose, the river cut out a gap roughly along its old path. All of this is over geologic time, so as the mountains gradually rose, the river continued to flow roughly as it had been doing, cutting out the ground as it rose up from below. The result of this process would be the sharp banks that we see in this particular spot. I think of it as the inverse of pushing a piece of wood, from above, onto a table saw. The geologic process (of the mountains forming) would continue on either side, while the river removed cut away at the narrow strip of its flow.
Of course, geo isn’t my forte, so I look forward to hearing your explanation.
The river must be older. If the mountains had formed first, how likely is it that two separate rivers would form on exactly opposite sides of the mountain and later join into one?
The mountains had to be formed first, and the river flows around them.
I am not determined with the answer because I think, for me, there are reasons for both of the explanations.
River probably formed after the mountain because in geology time, rivers are usually younger than mountains, and it is unimaginable that the mountain formed after the river which just cut through them. Just like the mountains were formed for the river to pass, and that is not what’s the nature gonna be.
However, the river can formed before the mountain; or at least there are certain possibility of that. Because there is no high mountains in this area so zero possibility of melting snow to form the river, and there are no other small creeks around which merged together to form the river. So, in this case, the water of the river is from underground water that flow out from underneath, which means there can be water flowing in this area a long time ago. Probably the river
is older than the mountain. When the mountain was forming slowly, the
water just cut through it constantly that make the gaps now we see.
Since they are among the oldest mountains in the world, the river probably cut the gaps through afterwards.
While both the river and the mountains are old, I’d have to say that the mountains are most likely younger than the river. While it would make sense for the mountains to be older, as the rivers could have formed after and flowed around the mountains, there had to have been water flowing from somewhere even before the mountains formed. The course of the rivers may have been altered due to the formation of the mountains, but I think the original rivers existed before the mountains.
I think that the mountains came before the river because the mountains take longer to create and they don’t just become created around rivers.
I think it’s interesting how people used to think that the streams around Virginia were beautiful, while nowadays they are grossly polluted to an extent where some find them hideous and disgusting.
Since the James River cuts across the Blue Ridge with steep water gaps and gorges, along with its meandering streams makes me inclined to say that the river formed before the mountains, making it an antecedent stream. I also think that rivers most likely have to do with land formation and since the split of Pangaea, before the continental plates collided, the river was formed.
It makes more sense for the mountains to have been formed before the river because otherwise, tectonic activity would have deformed the river along with the mountains. However, the rivers have taken the path of least resistance through the mountains often forming somewhat meandering streams.
I think the mountains were first to form and the river followed. This seems like the most logical way. As was mentioned above, plate tectonics aren’t shaped by the water surrounding the area. They could have uplifted land right through the river. The mountains must have existed first and the river carved through the mountain forming a water gap.
I think that the rivers predate the mountains. I think that when we think of mountain creation, we think of it as happening relatively quickly, but we have to keep in mind that they raise very slowly, maybe a few centimeters a year. At this rate, the rivers would be able to carve out gaps to flow through, and the amount of erosion they are causing is likely greater than the growth per year of the mountains. So, the rivers formed first but because the mountains took millions of years to form, the rivers quite easily flowed and cut through them.
The rivers are born usually close to the mountains or hills where aágua rain that penetrates the soil is rock underground and ficamaprisionadas. Ultimately seeping through the rocks to get brotarno soil, forming water sources. Are the springs. Most riosbrasileiros are formed that modo.Fontes water can also sprout from the ground under águaaprisionada in aquifers subterrâneos.No beginning, is only a trickle. Soon will be joining other filetespara form a stream, a stream and finally a rio.Os always looking rivers run lower regions and end numlago mar.Rios or can be born of water resulting from melting. It’s queacontece with the Amazon River which rises in the Andes.
It seems much more likely that the Blue Ridge Mountains predate the rivers. The rivers would likely have been seriously disrupted if the mountains had formed later rather than earlier. The tectonics necessary for mountain-creation would have had dramatic affects on the river that don’t seem to be present. Thus, I’d have to agree with TJ that the mountains formed first, creating a barrier that the rivers eventually broke through to create the water gap. On an unrelated note, I loved the Google Maps tour. Nice narration, Chuck!
I believe that predominantly, the rivers predated their current position in water gaps.Tectonic activity may have led to faulting and the creation of the Appalachians/Blue Ridge Mountains, which then channeled the movement of water already flowing on the plate’s surface. However, once the new ‘pathways’ had developed, the flow of water between mountains likely eroded its surroundings, which led to even more meandering and directional shifts.
I believe the Blue Ridge Mountains predate the James River. I would think that if the river was older than the mountains, the formation of the mountains would have caused the river to shift its course, rather than continue to flow through the mountains. Additionally, the James does not cut through the Allegheny Mountains to the west, which would have likely formed around the same time as the Blue Ridge Mountains, making it less likely that the James is older than either.
Hey Professor Bailey,
From looking at the water gap at Harper’s Ferry via Google Earth, I would say that the landforms lean heavily towards Jefferson’s interpretation. The strongest evidence for this lies in the sheer northern cliff-face that abuts the Potomac River water gap. The way it’s structured appears such that it has been a cutbank for the river for thousands of years, but that the mountain did in fact come first. The high reach of the cutbank suggests that the rivers were at a higher elevation at some point, and through the process of erosion, have worn their way into the two dips in the land on either side of Harper’s Ferry. The fact that this cliff-face appears to be made of igneous rock rather than sedimentary (at least from what I can see in the photos) suggests that the mountain existed before the water was flowing through the area.
I’m Interested to know the answer!
Hey Chuck!
I would have to agree with TJ considering the role of plate tectonics. Mountains are formed by the converging of plate tectonics and I think if the river predated the mountains, the river would have taken on a much different shape. It seems more logical for the river to cut through the mountains, which caused erosion and transformed the shape of the mountains.
It seems that the mountains would have come first due to plate tectonics and the tectonic uplift from which the mountains form. If the river had already been there, nothing would stop the mountains from emerging above the river and dispersing the water elsewhere. As much of a scholar as John Denver was, I’m going to take my chances with TJ here.
I agree with the idea that the mountains came first. There could be no way for the rivers to have formed first then cut through the mountains later.
The Blue Ridge mountains would have to come first, since rivers that run through them could not have formed on their own, especially in the directions that they flow through.
How is it possible that this river flowed over a mountain, carving out a downhill path? It is obvious from our lessons on drainage basins that water flows from a higher elevation to a lower one. So how did this river do this? The answer is that the rivers are much older than the mountains it is surrounded by. As the river was running, the tectonic plates clashed to bring about this uplift of land. As the land around it was shapeshifting and moving, the river kept flowing and carving out its path.
I agree with everyone’s comments about the mountains being older than the river. It simply makes no sense that mountains would purposefully (or luckily) grow around the river. Therefore, i feel fairly confident in saying that the river must have come later, cutting through the mountains for millions of years.
I believe that the rivers would be older than the mountains in this case. Tectonic plate movement to cause the uplift that creates the mountains would have been a slow process that would take hundreds of thousands of years. The river could have developed this water gap simultaneously with this process as it already existed and slowly cut its way through the basin of the mountain before it had fully developed.
I’d have to agree that the river is older than the mountains. It doesn’t make sense that the river simply cut through the mountains rather than adjusting its path around them. It seems more likely that as the mountains formed, the preexisting river cut away at the mountain little by little as they uplifted on either side.
I think that the mountains came first. Rivers often form because precipitation from higher elevations eventually flows down to lower elevations. This couldn’t have happened if the river existed before the mountains.
Though the power of rivers to cut through geographic landmasses is certainly possible, just look at the erosion that occurs in a meandering river as flowing water deposits sand from the cut bank onto the point bar. Mountains however, are another story. Mountains probably came first created by converging plates. When the plates collided, the water on these ( once flat) plates was redirected downslope. As water from all the mountains collected in the valleys, it was carried further due to the slope toward the ocean, creating flowing rivers.
I agree with TJ that the mountains came before the rivers and that the rivers meandering through the mountains were formed from rain that got caught in the lower elevations or valley between the mountains. I think it would be very unlikely that if the rivers were there first that the mountains would uplift , leaving the rivers untouched.
I agree with Thomas Jefferson. I think the mountains came before the river because a large part of the rivers water is the accumulation of water runoff from higher elevations, meaning the mountains would have to be there first.
I think that the mountains must have come first and then the rivers carved their way through the mountains. It just seems to make more sense to me in the way I think the world works, the thing I don’t understand is how exactly the rivers would have found enough force to actually carve their way through such big and dense mountains. The only thing I could think of is maybe glaciers, like we were talking about today in class. Maybe the ice carved out the water gaps but I’m not really sure. All I know is it seems to make more sense for the rivers to have come second. The elevation of the mountains would allow for water to collect in the water gap easily and it wouldn’t make sense for mountains to just spring up everywhere except for where there was a river.
According to the Northern Va Community College, the Blue Ridge Mountains are about 1 billion years old. According to the Ann Woodlief from VCU the James River is about 600 million years old. Looks like the mountains preceded the river.
I say that the mountains were there first but that the river lended to the processes of erosion, deposition, and weathering to give make a path for itself.
I believe that rivers are older than the mountains.
According to my theory, the land was uplifted first. Then precipitation caused erosion and weathering of loose sediments and soft rocks. After millions of years, the loose parts of the land were eroded away and a path cut through. Because of the lower elevation of the path, water kept running into it and accumulated to form a river. The riverbed continued to be eroded, and thus its elevation continues to decrease to create a large local relief. Finally, the materials resistant to erosion and weathering remained to form mountains.
I think that the first approach to this question is more accurate than the latter. This is because it is logical that the water would flow between the mountains rather than the mountains forming around a river. When the mountains formed there was most likely space between them. That area could have been eroded down over time, and swelled with water during a rainstorm one day and formed a river. Or perhaps, snow melted off the mountains and formed the river during a snowstorm one winter. The possibilities of just how the river formed are endless, but it seems as though it formed after the mountains.
I would have to agree with TJ and say that the mountains came before the river. The Blue Ridge were carved by tectonic processes millions of years ago while the river would later form from precipitation runoff. And in order for a river to flow, it needs to go from a point of high elevation to a point of low elevation; this would not be possible if there were no mountains present to cause notable changes in elevation.
I am hard pressed to imagine rivers persistently flowing into a mountain until it eroded. Water takes the path of least resistance, that is, flows downhill from whatever point it’s at, and that would never be right into a mountain. It would make sense if as mountains were rising, the channeled flow remained where it was and simply eroded an emergent mountain as it was forming, creating the gap.
I would have to lean towards mountains being older. Once a water droplet crashes to Earth it will flow downhill of whatever it lands on. The consistent repetition of rain flowing downhill, down these mountain sides and into a collection area have eroded away the mountains. After years upon years of water flowing downhill, the water adds up and forms a river which continues to snake its way through the mountain range with every new rain shower. Therefore I stand by the river carving out the mountains that the other way around as I find it hard to imagine mountains popping up around an already formed river while still allowing the river to keep its shape, etc. The gap was carved by the water rather than formed by the uplift of land around a particular pattern of water flow.
The mountains should be older. Rivers change all the time, and at the forming of the mountains there may not have been any of any significance in the region. But the mountains have been there for millions of years. If the rivers flow between them, it is because they cut a path through them well after they were formed.
I think the river is older. I don’t think of the James or the Shenandoah as being particularly old rivers, but I know from taking summer camp groups out to West Virginia that the New River is fantastically old by most estimates, running up an age in the hundreds of millions of years (subject, obviously, to cycles: glaciation during the Ice Age being one detrimental factor to the river at one time. I’m not totally sure of the mechanisms that could allow rivers to remain in place while surrounding land is pushed upward into mountain ranges, but it seems likely that that’s the case here.
I think the Blue Ridge Mountains are older than the rivers flowing through these water gaps? Although the quantity of river flow is not very significant, I guess the water penetrated into the underground also corrodes the mountain over time. Moreover, the elevation of that part of mountain is not as high as those peaks. I think the river curving out the mountain is highly plausible.
It seems to me that Jefferson is correct in this case. The mountains are most likely older than the river because if the mountains are first formed, it is very logical that water would accumulate in the watershed areas created by the rising terrain features and eventually form a river.
I think that the river is older. If the river had formerd after the mountain, than perhaps water gaps would be much smaller because the river would have formed in regard to the mountains topography, perhaps making its flow relatively “smooth” compared to the current examples of water gaps. As it is, there are water gaps because the river was uprooted by the uplifting of the mountains.
I’d say that the mountains were formed first. In order for there to be a flowing water, there must be some type of topographic relief, and these mountains provide that. There are good points for both sides of this argument, but I think the mountains forming millions of years ago, prior to the river, is the most likely situation
It makes sense to me that the mountains are older, and the river was formed after.
The river is likely older than the mountains. The strength of the river, which has also prevented it from drying out throughout the years, eroded the surrounding land near the river bed and underneath the river to create mountains during periods of uplift.
With what we’ve learned about plate tectonics, I think that it is more likely that the mountains formed around the river.
To my understanding, a water gap is an opening between mountain ranges typically caused by water’s erosion of layers of rock before and after periods of uplift. A visible water gap typically means that the river is older than the mountain range around it. The river usually forms when the surrounding land is at low elevations. After a period of uplift, there is increased erosion along the riverbed. This is a process which exposes deeper layers of rock. As the river gets large enough, it continues to erode the rising land causing what is known as a water gap!
Didn’t I hear of canoes wrapped around rocks near Harpers Ferry?
This is a crazy popular post. We apparently need some help here Chuck!